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IP Application Services

Understanding and Troubleshooting HSRP Problems in Catalyst Switch Networks

Document ID: 10583

Updated: May 05, 2009

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Contents

Introduction

Because of the nature of the Hot Standby Router Protocol (HSRP), specific network problems can lead to HSRP instability. This document covers common issues and ways to troubleshoot HSRP problems. Most HSRP-related problems are not true HSRP issues. Instead, they are network problems that affect the behavior of HSRP.

This document covers these most-common issues that relate to HSRP:

  • Router report of a duplicate HSRP standby IP address

  • Constant HSRP state changes (active, standby, speak)

  • Missing HSRP peers

  • Switch error messages that relate to HSRP

  • Excessive network unicast flooding to the HSRP configuration

Note: This document details how to troubleshoot HSRP in Catalyst switch environments. The document contains many references to software versions and network topology design. Nevertheless, the sole purpose of this document is to facilitate and guide engineers on who to troubleshoot HSRP. This document is not intended to be a design guide, software-recommendation document, or a best practices document.

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software and hardware versions.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Understand HSRP

Background Information

Businesses and consumers that rely on intranet and Internet services for their mission-critical communications require and expect their networks and applications to be continuously available to them. Customers can satisfy their demands for near-100 percent network uptime if they leverage the HSRP in Cisco IOS® Software. HSRP, which is unique to Cisco platforms, provides network redundancy for IP networks in a manner that ensures that user traffic immediately and transparently recovers from first-hop failures in network edge devices or access circuits.

Two or more routers can act as a single, virtual router if they share an IP address and a MAC (Layer 2 [L2]) address. The address is necessary for host workstation default gateway redundancy. Most host workstations do not contain routing tables and use only a single next hop IP and MAC address. This address is known as a default gateway. With HSRP, members of the virtual router group continually exchange status messages. One router can assume the routing responsibility of another if a router goes out of commission for either planned or unplanned reasons. Hosts are configured with a single default gateway and continue to forward IP packets to a consistent IP and MAC address. The changeover of devices that do the routing is transparent to the end workstations.

Note: You can configure host workstations that run Microsoft OS for multiple default gateways. But, the multiple default gateways are not dynamic. The OS only uses one single default gateway at a time. The system only selects an additional configured default gateway at boot time if the first configured default gateway is determined unreachable by Internet Control Management Protocol (ICMP).

Basic Operation

A set of routers that run HSRP works in concert to present the illusion of a single default gateway router to the hosts on the LAN. This set of routers is known as an HSRP group or standby group. A single router that is elected from the group is responsible for the forwarding of the packets that hosts send to the virtual router. This router is known as the active router. Another router is elected as the standby router. If the active router fails, the standby assumes the packet forwarding duties. Although an arbitrary number of routers may run HSRP, only the active router forwards the packets that are sent to the virtual router IP address.

In order to minimize network traffic, only the active and the standby routers send periodic HSRP messages after the protocol has completed the election process. Additional routers in the HSRP group remain in the Listen state. If the active router fails, the standby router takes over as the active router. If the standby router fails or becomes the active router, another router is elected as the standby router.

Each standby group emulates a single virtual router (default gateway). For each group, a single well-known MAC and IP address is allocated to that group. Multiple standby groups can coexist and overlap on a LAN, and individual routers can participate in multiple groups. In this case, the router maintains a separate state and timers for each group.

HSRP Terms

Term Definition
Active router The router that currently forwards packets for the virtual router
Standby router The primary backup router
Standby group The set of routers that participate in HSRP and jointly emulate a virtual router
Hello time The interval between successive HSRP hello messages from a given router
Hold time The interval between the receipt of a hello message and the presumption that the sending router has failed

HSRP Addressing

HSRP Router Communication

Routers that run HSRP communicate HSRP information between each other through HSRP hello packets. These packets are sent to the destination IP multicast address 224.0.0.2 on User Datagram Protocol (UDP) port 1985. IP multicast address 224.0.0.2 is a reserved multicast address that is used to communicate to all routers. The active router sources hello packets from its configured IP address and the HSRP virtual MAC address. The standby router sources hellos from its configured IP address and the burned-in MAC address (BIA). This use of source addressing is necessary so that HSRP routers can correctly identify each other.

In most cases, when you configure routers to be part of an HSRP group, the routers listen for the HSRP MAC address for that group as well as their own BIA. The only exception to this behavior is for Cisco 2500, 4000, and 4500 routers. These routers have Ethernet hardware that only recognizes a single MAC address. Therefore, these routers use the HSRP MAC address when they serve as the active router. The routers use their BIA when they serve as the standby router.

HSRP Standby IP Address Communication on All Media Except Token Ring

Because host workstations are configured with their default gateway as the HSRP standby IP address, hosts must communicate with the MAC address that is associated with the HSRP standby IP address. This MAC address is a virtual MAC address that is composed of 0000.0c07.ac**. The ** is the HSRP group number in hexadecimal, based on the respective interface. For example, HSRP group 1 uses the HSRP virtual MAC address of 0000.0c07.ac01. Hosts on the adjoining LAN segment use the normal Address Resolution Protocol (ARP) process in order to resolve the associated MAC addresses.

HSRP Standby IP Address Communication on Token Ring Media

Token Ring interfaces use functional addresses for the HSRP MAC address. Functional addresses are the only general multicast mechanism available. There is a limited number of Token Ring functional addresses available, and many of these addresses are reserved for other functions. These three addresses are the only addresses available for use with HSRP:

c000.0001.0000 (group 0)
c000.0002.0000 (group 1)
c000.0004.0000 (group 2)

Therefore, you can configure only three HSRP groups on Token Ring interfaces, unless you configure the standby use-bia parameter.

ICMP Redirects

HSRP peer routers that protect a subnet are able to provide access to all other subnets in the network. This is the basis of HSRP. Therefore, which router becomes the active HSRP router is irrelevant. In Cisco IOS software releases earlier than Cisco IOS Software Release 12.1(3)T, ICMP redirects are automatically disabled on an interface when HSRP is used on that interface. Without this configuration, the hosts can be redirected away from the HSRP virtual IP address and toward an interface IP and MAC address of a single router. Redundancy is lost.

Cisco IOS Software Release 12.1(3)T introduces a method to allow ICMP redirects with HSRP. This method filters outbound ICMP redirect messages through HSRP. The next hop IP address is changed to an HSRP virtual address. The gateway IP address in the outbound ICMP redirect message is compared to a list of HSRP active routers that are present on that network. If the router that corresponds to the gateway IP address is an active router for an HSRP group, the gateway IP address is replaced with that group virtual IP address. This solution allows hosts to learn optimal routes to remote networks and, at the same time, maintain the resilience that HSRP provides.

HSRP Functionality Matrix

Refer to the Cisco IOS Release and HSRP Functionality Matrix section of Hot Standby Router Protocol Features and Functionality in order to learn about the features and Cisco IOS Software releases that support HSRP.

HSRP Features

Refer to Hot Standby Router Protocol Features and Functionality for information on most of the HSRP features. This document provides information on these HSRP features:

  • Preemption

  • Interface tracking

  • Use of a BIA

  • Multiple HSRP groups

  • Configurable MAC addresses

  • Syslog support

  • HSRP debugging

  • Enhanced HSRP debugging

  • Authentication

  • IP redundancy

  • Simple Network Management Protocol (SNMP) MIB

  • HSRP for Multiprotocol Label Switching (MPLS)

Note: You can use your browser Find feature in order to locate these sections within the document.

Packet Format

This table shows the format of the data portion of the UDP HSRP frame:

Version Op Code State Hellotime
Holdtime Priority Group Reserved
Authentication Data
Authentication Data
Virtual IP Address

This table describes each of the fields in the HSRP packet:

Packet Field Description
Op Code (1 octet) The Op Code describes the type of message that the packet contains. Possible values are: 0 - hello, 1 - coup, and 2 - resign. Hello messages are sent to indicate that a router runs HSRP and is able to become the active router. Coup messages are sent when a router wishes to become the active router. Resign messages are sent when a router no longer wishes to be the active router.
State (1 octet) Each router in the standby group implements a state machine. The state field describes the current state of the router that sends the message. These are details on the individual states: 0 - initial, 1 - learn, 2 - listen, 4 - speak, 8 - standby, and 16 - active.
Hellotime (1 octet) This field is only meaningful in hello messages. It contains the approximate period between the hello messages that the router sends. The time is given in seconds.
Holdtime (1 octet) This field is only meaningful in hello messages. It contains the amount of time that the routers wait for a hello message before they initiate a state change.
Priority (1 octet) This field is used to elect the active and standby routers. In a comparison of the priorities of two routers, the router with the highest value becomes the active router. The tie breaker is the router with the higher IP address.
Group (1 octet) This field identifies the standby group.
Authentication Data (8 octets) This field contains a cleartext, eight-character password.
Virtual IP Address (4 octets) If the virtual IP address is not configured on a router, the address can be learned from the hello message from the active router. An address is only learned if no HSRP standby IP address has been configured, and the hello message is authenticated (if authentication is configured).

HSRP States

State Definition
Initial This is the state at the start. This state indicates that HSRP does not run. This state is entered through a configuration change or when an interface first becomes available.
Learn The router has not determined the virtual IP address and has not yet seen an authenticated hello message from the active router. In this state, the router still waits to hear from the active router.
Listen The router knows the virtual IP address, but the router is neither the active router nor the standby router. It listens for hello messages from those routers.
Speak The router sends periodic hello messages and actively participates in the election of the active and/or standby router. A router cannot enter speak state unless the router has the virtual IP address.
Standby The router is a candidate to become the next active router and sends periodic hello messages. With the exclusion of transient conditions, there is, at most, one router in the group in standby state.
Active The router currently forwards packets that are sent to the group virtual MAC address. The router sends periodic hello messages. With the exclusion of transient conditions, there must be, at most, one router in active state in the group.

HSRP Timers

Each router only uses three timers in HSRP. The timers time hello messages. The HSRP converges, when a failure occurs, depend on how the HSRP hello and hold timers are configured. By default, these timers are set to 3 and 10 seconds, respectively, which means that a hello packet is sent between the HSRP standby group devices every 3 seconds, and the standby device becomes active when a hello packet has not been received for 10 seconds. You can lower these timer settings to speed up the failover or preemption, but, to avoid increased CPU usage and unnecessary standby state flapping, do not set the hello timer below one (1) second or the hold timer below 4 seconds. Note that, if you use the HSRP tracking mechanism and the tracked link fails, the failover or preemption occurs immediately, regardless of the hello and hold timers. When a timer expires, the router transitions to a new HSRP state. The timers can be changed with this command: standby [group-number] timers hellotime holdtime . For example, standby 1 timers 5 15.

This table provides more information on these timers:

Timer Description
Active timer This timer is used to monitor the active router. This timer starts any time an active router receives a hello packet. This timer expires in accordance with the hold time value that is set in the related field of the HSRP hello message.
Standby timer This timer is used in order to monitor the standby router. The timer starts any time the standby router receives a hello packet. This timer expires in accordance with the hold time value that is set in the respective hello packet.
Hello timer This timer is used to clock hello packets. All HSRP routers in any HSRP state generate a hello packet when this hello timer expires.

HSRP Events

This table provides the events in the HSRP finite state machine:

Key Events
1 HSRP is configured on an enabled interface.
2 HSRP is disabled on an interface or the interface is disabled.
3 Active timer expiry The active timer is set to the hold time when the last hello message is seen from the active router.
4 Standby timer expiry The standby timer is set to the hold time when the last hello message is seen from the standby router.
5 Hello timer expiry The periodic timer for the send of hello messages is expired.
6 Receipt of a hello message of higher priority from a router in speak state
7 Receipt of a hello message of higher priority from the active router
8 Receipt of a hello message of lower priority from the active router
9 Receipt of a resign message from the active router
10 Receipt of a coup message from a higher priority router
11 Receipt of a hello message of higher priority from the standby router
12 Receipt of a hello message of lower priority from the standby router

HSRP Actions

This table specifies the actions to be taken as part of the state machine:

Initial Action
A Start active timer—If this action occurrs as the result of the receipt of an authenticated hello message from the active router, the active timer is set to the hold time field in the hello message. Otherwise, the active timer is set to the current hold time value that is in use by this router. The active timer then starts.
B Start standby timer—If this action occurrs as the result of the receipt of an authenticated hello message from the standby router, the standby timer is set to the hold time field in the hello message. Otherwise, the standby timer is set to the current hold time value that is in use by this router. The standby timer then starts.
C Stop active timer—The active timer stops.
D Stop standby timer—The standby timer stops.
E Learn parameters—This action is taken when an authenticated message is received from the active router. If the virtual IP address for this group is not manually configured, the virtual IP address can be learned from the message. The router can learn hello time and hold time values from the message.
F Send hello message—The router sends a hello message with its current state, hello time, and hold time.
G Send coup message—The router sends a coup message in order to inform the active router that there is a higher-priority router available.
H Send resign message—The router sends a resign message in order to allow another router to become the active router.
I Send gratuitous ARP message—The router broadcasts an ARP response packet that advertises the group virtual IP and MAC addresses. The packet is sent with the virtual MAC address as the source MAC address in the link layer header, as well as within the ARP packet.

HSRP State Table

The diagram in this section shows the state transitions of the HSRP state machine. Each time that an event occurs, the associated action results, and the router transitions to the next HSRP state. In the diagram, numbers designate events, and letters designate the associated action. The table in the section HSRP Events defines the numbers, and the table in the section HSRP Actions defines the letters. Use this diagram only as a reference. The diagram is detailed and is not necessary for general troubleshooting purposes.

62a.gif

Packet Flow

62b.gif

Device MAC Address IP Address Subnet Mask Default Gateway
PC1 0000.0c00.0001 10.1.1.10 255.255.255.0 10.1.1.1
PC2 0000.0c00.1110 10.1.2.10 255.255.255.0 10.1.2.1

Router A Configuration (Active Router)

interface ethernet 0
    ip address 10.1.1.2 255.255.255.0
    mac-address 4000.0000.0010
    standby 1 ip 10.1.1.1
    standby 1 priority 200
interface ethernet 1
    ip address 10.1.2.2 255.255.255.0
    mac-address 4000.0000.0011
    standby 1 ip 10.1.2.1
    standby 1 priority 200

Router B Configuration (Standby Router)

interface ethernet 0
    ip address 10.1.1.3 255.255.225.0    
    mac-address 4000.0000.0020
    standby 1 ip 10.1.1.1
interface ethernet 1
     ip address 10.1.2.3 255.255.255.0
     mac-address 4000.0000.0021
     standby 1 ip 10.1.2.1

Note: These examples configure static MAC addresses for illustration purposes only. Do not configure static MAC addresses unless you are required to do so.

You must understand the concept behind packet flow when you obtain sniffer traces in order to troubleshoot HSRP problems. Router A uses the priority of 200 and becomes the active router on both interfaces. In the example in this section, packets from the router that are destined for a host workstation have the source MAC address of the router physical MAC address (BIA). Packets from the host machines that are destined for the HSRP IP address have the destination MAC address of the HSRP virtual MAC address. Note that the MAC addresses are not the same for each flow between the router and the host.

This table shows the respective MAC and IP address information per flow on the basis of a sniffer trace that is taken from Switch X.

Packet Flow Source MAC Destination MAC Source IP Destination IP
Packets from PC1 that are destined for PC2 PC1 (0000.0c00.0001) HSRP virtual MAC address of Router A interface Ethernet 0 (0000.0c07.ac01) 10.1.1.10 10.1.2.10
Packets that return through Router A from PC2 and are destined for PC1 Router A Ethernet 0 BIA (4000.0000.0010) PC1 (0000.0c00.0001) 10.1.2.10 10.1.1.10
Packets from PC1 that are destined for HSRP standby IP address (ICMP, Telnet) PC1 (0000.0c00.0001) HSRP virtual MAC address of Router A interface Ethernet 0 (0000.0c07.ac01) 10.1.1.10 10.1.1.1
Packets that are destined for the actual IP address of the active router (ICMP, Telnet) PC1 (0000.0c00.0001) Router A Ethernet 0 BIA (4000.0000.0010) 10.1.1.10 10.1.1.2
Packets that are destined for the actual IP address of the standby router (ICMP, Telnet) PC1 (0000.0c00.0001) Router B Ethernet 0 BIA (4000.0000.0020) 10.1.1.10 10.1.1.3

Troubleshoot HSRP Case Studies

Case Study #1: HSRP Standby IP Address Is Reported as a Duplicate IP Address

These error messages can appear:

Oct 12 13:15:41: %STANDBY-3-DUPADDR: Duplicate address 10.25.0.1 
  on Vlan25, sourced by 0000.0c07.ac19 
Oct 13 16:25:41: %STANDBY-3-DUPADDR: Duplicate address 10.25.0.1 
  on Vlan25, sourced by 0000.0c07.ac19 
Oct 15 22:31:02: %STANDBY-3-DUPADDR: Duplicate address 10.25.0.1 
  on Vlan25, sourced by 0000.0c07.ac19 
Oct 15 22:41:01: %STANDBY-3-DUPADDR: Duplicate address 10.25.0.1 
  on Vlan25, sourced by 0000.0c07.ac19 

These error messages do not necessarily indicate an HSRP problem. Rather, the error messages indicate a possible Spanning Tree Protocol (STP) loop or router/switch configuration issue. The error messages are just symptoms of another problem.

In addition, these error messages do not prevent the proper operation of HSRP. The duplicate HSRP packet is ignored. These error messages are throttled at 30-second intervals. But, slow network performance and packet loss can result from the network instability that causes the STANDBY-3-DUPADDR error messages of the HSRP address.

These error messages can appear:

Oct 15 22:41:01: %STANDBY-3-DUPADDR: Duplicate address 10.25.0.1 
  on Vlan25, sourced by 0000.0c07.ac19 

These messages specifically indicate that the router received a data packet that was sourced from the HSRP IP address on VLAN 25 with the MAC addresses 0000.0c07.ac19. Since the HSRP MAC address is 0000.0c07.ac19, either the router in question received its own packet back or both routers in the HSRP group went into the active state. Because the router received its own packet, the problem most likely is with the network rather than the router. A variety of problems can cause this behavior. Among the possible network problems that cause the error messages are:

  • Momentary STP loops

  • EtherChannel configuration issues

  • Duplicated frames

When you troubleshoot these error messages, see the troubleshooting steps in the HSRP Troubleshooting Modules for CatOS Switches section of this document. All the troubleshooting modules are applicable to this section, which includes modules on configuration. In addition, note any errors in the switch log and reference additional case studies as necessary.

You can use an access list in order to prevent the active router from receiving its own multicast hello packet. But, this is only a workaround for the error messages and actually hides the symptom of the problem. The workaround is to apply an extended inbound access list to the HSRP interfaces. The access list blocks all traffic that is sourced from the physical IP address and that is destined to all routers multicast address 224.0.0.2.

access-list 101 deny ip host 172.16.12.3 host 224.0.0.2 
access-list 101 permit ip any any 
  
interface ethernet 0 
  ip address 172.16.12.3 255.255.255.0 
  standby 1 ip 172.16.12.1 
  ip access-group 101 in

Case Study #2: HSRP State Continuously Changes (Active, Standby, Speak) or %HSRP-6-STATECHANGE

These error messages can appear:

Jan 9 08:00:42.623: %STANDBY-6-STATECHANGE: Standby: 49: 
  Vlan149 state Standby -> Active
Jan 9 08:00:56.011: %STANDBY-6-STATECHANGE: Standby: 49: 
  Vlan149 state Active -> Speak
Jan 9 08:01:03.011: %STANDBY-6-STATECHANGE: Standby: 49: 
  Vlan149 state Speak -> Standby
Jan 9 08:01:29.427: %STANDBY-6-STATECHANGE: Standby: 49: 
  Vlan149 state Standby -> Active
Jan 9 08:01:36.808: %STANDBY-6-STATECHANGE: Standby: 49: 
  Vlan149 state Active -> Speak
Jan 9 08:01:43.808: %STANDBY-6-STATECHANGE: Standby: 49: 
  Vlan149 state Speak -> Standby

These error messages describe a situation in which a standby HSRP router did not receive three successive HSRP hello packets from its HSRP peer. The output shows that the standby router moves from the standby state to the active state. Shortly thereafter, the router returns to the standby state. Unless this error message occurs during the initial installation, an HSRP issue probably does not cause the error message. The error messages signify the loss of HSRP hellos between the peers. When you troubleshoot this issue, you must verify the communication between the HSRP peers. A random, momentary loss of data communication between the peers is the most common problem that results in these messages. HSRP state changes are often due to High CPU Utilization. If the error message is due to high CPU utilization, put a sniffer on the network and the trace the system that causes the high CPU utilization.

There are several possible causes for the loss of HSRP packets between the peers. The most common problems are physical layer problems, excessive network traffic caused by spanning tree issues or excessive traffic caused by each Vlan. As with Case Study #1, all the troubleshooting modules are applicable to the resolution of HSRP state changes, particularly the Layer 3 HSRP Debugging.

If the loss of HSRP packets between peers is due to excessive traffic caused by each VLAN as mentioned, you can tune or increase the SPD and hold the queue size to overcome the input queue drop problem.

In order to increase the Selective Packet Discard (SPD) size, go to the configuration mode and execute these commands on the Cat6500 switches:

(config)# ip spd queue max-threshold 600

!--- Hidden Command

(config)# ip spd queue min-threshold 500

!--- Hidden Command

Note: Refer to Understanding Selective Packet Discard (SPD) for more information on the SPD.

In order to increase the hold queue size, go to the VLAN interface mode and execute this command.:

(config-if)# hold-queue 500 in

After you increase the SPD and hold queue size, you can clear the interface counters if you execute the 'clear counter interface'command.

Case Study #3: HSRP Does Not Recognize Peer

The router output in this section shows a router that is configured for HSRP but does not recognize its HSRP peers. In order for this to occur, the router must fail to receive HSRP hellos from the neighbor router. When you troubleshoot this issue, see the Verify Physical Layer Connectivity section and the Verify HSRP Router Configuration section of this document. If the physical layer connectivity is correct, check for the mismatched VTP modes.

Vlan8 - Group 8
Local state is Active, priority 110, may preempt
Hellotime 3 holdtime 10
Next hello sent in 00:00:01.168
Hot standby IP address is 10.1.2.2 configured
Active router is local
Standby router is unknown expired
Standby virtual mac address is 0000.0c07.ac08
5 state changes, last state change 00:05:03

Case Study #4: HSRP State Changes and Switch Reports SYS-4-P2_WARN: 1/Host <mac_address> Is Flapping Between Port <port_1> and Port <port_2> in Syslog

These error messages can appear:

2001 Jan 03 14:18:43 %SYS-4-P2_WARN: 1/Host 00:00:0c:14:9d:08 
  is flapping between port 2/4 and port 2/3

In software version 5.5.2 and later for the Catalyst 4500/4000 and 2948G, the switch reports a host MAC address that moves if the host MAC address moves twice within 15 seconds. A common cause is an STP loop. The switch discards packets from this host for about 15 seconds in an effort to minimize the impact of an STP loop. If the MAC address move between two ports that is reported is the HSRP virtual MAC address, the problem is most likely an issue in which both HSRP routers go into the active state.

If the MAC address that is reported is not the HSRP virtual MAC address, the issue can indicate the loop, duplication, or reflection of packets in the network. These types of conditions can contribute to HSRP problems. The most common causes for the move of MAC addresses are spanning tree problems or physical layer problems.

When you troubleshoot this error message, complete these steps:

Note: Also, complete the steps in the HSRP Troubleshooting Modules for CatOS Switches section of this document.

  1. Determine the correct source (port) of the MAC address that the error message reports.

  2. Disconnect the port that must not source the host MAC address and check for HSRP stability.

  3. Document the STP topology on each VLAN and check for STP failure.

  4. Verify the port channel configuration.

    An incorrect port channel configuration can result in the flap of error messages by the host MAC address. This is because of the load-balancing nature of port channeling.

Case Study #5: HSRP State Changes and Switch Reports RTD-1-ADDR_FLAP in Syslog

These error messages can appear:

*Mar 9 14:51:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 21 addrs per min 
*Mar 9 14:52:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 22 addrs per min 
*Mar 9 14:53:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 20 addrs per min 
*Mar 9 14:54:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 20 addrs per min 
*Mar 9 14:55:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 21 addrs per min 
*Mar 9 14:56:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 22 addrs per min 
*Mar 9 14:57:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 
  relearning 21 addrs per min

These error message signify that a MAC address moves consistently between different ports. These error messages are only applicable on the Catalyst 2900XL and 3500XL switches. The messages can indicate that two or more HSRP routers have become active. The messages can indicate the source of an STP loop, duplicated frames, or reflected packets.

In order to gather more information about the error messages, issue this debug command:

switch#debug ethernet-controller address

Ethernet Controller Addresses debugging is on l

*Mar 9 08:06:06: Add address 0000.0c07.ac02, on port 35 vlan 2 
*Mar 9 08:06:06: 0000.0c07.ac02 has moved from port 6 to port 35 in vlan 2  
*Mar 9 08:06:07: Add address 0000.0c07.ac02, on port 6 vlan 2 
*Mar 9 08:06:07: 0000.0c07.ac02 has moved from port 35 to port 6 in vlan 2 
*Mar 9 08:06:08: Add address 0000.0c07.ac02, on port 35 vlan 2 
*Mar 9 08:06:08: 0000.0c07.ac02 has moved from port 6 to port 35 in vlan 2 
*Mar 9 08:06:10: Add address 0000.0c07.ac02, on port 6 vlan 2 
*Mar 9 08:06:10: 0000.0c07.ac02 has moved from port 35 to port 6 in vlan 2 
*Mar 9 08:06:11: Add address 0000.0c07.ac02, on port 35 vlan 2 
*Mar 9 08:06:11: 0000.0c07.ac02 has moved from port 6 to port 35 in vlan 2  
*Mar 9 08:06:12: %RTD-1-ADDR_FLAP: Fast Ethernet 0/7 relearning 20 addrs per min 
*Mar 9 08:06:13: Add address 0000.0c07.ac02, on port 6 vlan 2 
*Mar 9 08:06:13: 0000.0c07.ac02 has moved from port 35 to port 6 in vlan 2 

The ports that the debug command references are off by one. For example, port 0 is Fast Ethernet 0/1. The error messages indicate the flap of a MAC address between ports 5 and 34 on the respective switch.

Note: The message RTD-1-ADDR_FLAP can be incorrect. Refer to these Cisco bug IDs in order to rule out this possibility:

The most common causes for the move of MAC addresses are spanning tree problems or physical layer problems.

When you troubleshoot this error message, complete these steps:

Note: Also, complete the steps in the HSRP Troubleshooting Modules for CatOS Switches section of this document.

  1. Determine the correct source (port) of the host MAC address.

  2. Disconnect the port that should not source the host MAC address.

  3. Document the STP topology on a per-VLAN basis and check for STP failure.

  4. Verify the port channeling configuration.

    An incorrect port channel configuration can result in the flap of error messages by the host MAC address. This is because of the load-balancing nature of port channeling.

Case Study #6: HSRP State Changes and Switch Reports MLS-4-MOVEOVERFLOW:Too many moves, stop MLS for 5 sec(20000000) in Syslog

These error messages can appear:

05/13/2000,08:55:10:MLS-4-MOVEOVERFLOW:Too many moves, stop MLS for 5 sec(20000000) 
05/13/2000,08:55:15:MLS-4:Resume MLS after detecting too many moves

These messages indicate that the switch learns the same MAC address on two different ports. This message is only reported on Catalyst 5500/5000 switches. Issue these commands in order to gather additional information about the problem:

Note: The commands that this section mentions are not documented. You must enter them completely. The show mls notification command provides a table address (TA) value. The show looktable TA-value command returns a possible MAC address that you can trace to the root of the problem.

Switch (enable) show mls notification 

1: (0004e8e6-000202ce) Noti Chg TA e8e6 OI 2ce (12/15) V 1 

!--- This is the mod/port and VLAN. The MAC address is 
!--- seen on this module 12, port 15 in VLAN 1.

2: (0004e8e6-000202cd) Noti Chg TA e8e6 OI 2cd (12/14) V 1

!--- This is the mod/port and VLAN. The next is seen on 
!--- module 12, port 14 in VLAN 1.

Write down the four-digit/letter combination that appears after Chg TA in this command output. The show looktable command gives the MAC address that causes the MLS TOO MANY MOVES error message:

150S_CR(S2)> (enable) show looktable e8e6

Table address: 0xe8e6, Hash: 0x1d1c, Page: 6 
Entry Data[3-0]: 0x000002cd 0x00800108 0x0008c790 0x215d0005, Entry Map [00] 

Router-Xtag QOS SwGrp3 Port-Index 
0 0 0x0 0x2cd 

Fab AgeByte C-Mask L-Mask Static SwSc HwSc EnSc AL Trap R-Mac 
0 0x01 0x0000 0x0000 0 0 0 0 0 0 0 

MacAge Pri-In Modify Notify IPX-Sw IPX-Hw IPX-En Valid SwGrp2 Parity2 
0 0 1 0 0 0 0 1 0x0 0 

Entry-Mac-Address FID SwGrp1 Parity1 
00-08-c7-90-21-5d 1 0x0 1

The entry MAC address 00-08-c7-90-21-5d is the MAC address that flaps between ports. You must know the MAC address in order to find the offending device. If the entry MAC address is the virtual HSRP MAC address, the issue can be that both HSRP routers have gone into the active state.

The most common causes for the move of MAC addresses are spanning tree problems or physical layer problems.

When you troubleshoot this error message, complete these steps:

Note: Also complete the steps in the HSRP Troubleshooting Modules for CatOS Switches section of this document.

  1. Determine the correct source (port) of the host MAC address.

  2. Disconnect the port that should not source the host MAC address.

  3. Document the STP topology on a per-VLAN basis and check for STP failure.

  4. Verify the port channeling configuration.

    An incorrect port channel configuration can result in the flap of error messages by the host MAC address. This is because of the load-balancing nature of port channeling.

  5. Disable PortFast on all of the ports that connect to devices other than a PC or IP phone in order to avoid bridging loops.

Case Study #7: HSRP Intermittent State Changes on Multicast Stub Network

There is a common cause for HSRP anomalous state changes for an HSRP router that is part of a multicast stub network. This common cause deals with the non-Reverse Path Forwarding (RPF) traffic that the non-designated router (DR) sees. This is the router that does not forward the multicast traffic stream.

IP multicast uses one router to forward data onto a LAN in redundant topologies. If multiple routers have interfaces onto a LAN or VLAN, only one router forwards the data. There is no load balancing for multicast traffic on LANs. All multicast traffic is always visible by every router on a LAN. This is also the case if Cisco Group Management Protocol (CGMP) or Internet Group Management Protocol (IGMP) snooping is configured. Both routers need to see the multicast traffic in order to make a forwarding decision.

This diagram provides an example. The red lines indicate multicast feed.

62c.gif

The redundant router, which is the router that does not forward the multicast traffic stream, sees this data on the outbound interface for the LAN. The redundant router must drop this traffic because the traffic arrived on the wrong interface and, therefore, fails the RPF check. This traffic is referred to as non-RPF traffic because it is reflected backward against the flow from the source. For this non-RPF traffic, there is usually no (*,G) or (S,G) state in the redundant router. Therefore, no hardware or software shortcuts can be created in order to drop the packet. The processor must examine each multicast packet individually. This requirement can cause the CPU on these routers to spike or run at a very high processing rate. Often, a high rate of multicast traffic on the redundant router causes HSRP to lose hello packets from its peer and change states.

Therefore, enable hardware access lists on Catalyst 6500 and 8500 routers that do not handle non-RPF traffic efficiently by default. The access lists prevent the CPU from processing the non-RPF traffic.

Note: Do not attempt to work around this problem with a disablement of the IP Protocol Independent Multicast (PIM) on the redundant router interfaces. This configuration can have an undesirable impact on the redundant router.

On the 6500/8500 routers, there is an access list engine that enables filtering to take place at wire rate. You can use this feature to handle non-RPF traffic for sparse mode groups efficiently.

In software versions 6.2.1 and later, the system software automatically enables filtering so that the non-DR does not receive unnecessary non-RPF traffic. In earlier software versions, you need to configure access lists manually. In order to implement this solution for software versions that are earlier than 6.2.1, place an access list on the inbound interface of the stub network. The access list filters multicast traffic that did not originate from the stub network. The access list is pushed down to the hardware in the switch. This access list ensures that the CPU never sees the packet and allows the hardware to drop the non-RPF traffic.

For example, assume that you have two routers with two VLANs in common. You can expand this number of VLANs to as many VLANs as necessary. Router A is HSRP primary for VLAN 1 and secondary for VLAN 2. Router B is secondary for VLAN 1 and primary for VLAN 2. Give either Router A or Router B a higher IP address in order to make that router the DR. Be sure that only one router is the DR for all segments, as this example shows:

Router A
     VLAN1 Physical IP Address
     A.B.C.3

Router B
     VLAN1 Physical IP Address
     A.B.C.2
     VLAN1 HSRP Address
     A.B.C.1

Router A
     VLAN2 Physical IP Address
     A.B.D.3

Router B
     VLAN2 Physical IP Address
     A.B.D.2
     VLAN2 HSRP Address
     A.B.D.1

Place this access list on the non-DR router:

access-list 100 permit ip A.B.C.0 0.0.0.255 any
access-list 100 permit ip A.B.D.0 0.0.0.255 any
access-list 100 permit ip any 224.0.0.0 0.0.0.255
access-list 100 permit ip any 224.0.1.0 0.0.0.255
access-list 100 deny ip any 224.0.0.0 15.255.255.255

You should have one permit for each subnet that the two routers share. Other permits allow auto-rendezvous point (RP) and reserved groups to operate correctly.

Issue these additional commands in order to apply the access control lists (ACLs) to each VLAN interface on the non-DR:

Note: You must run Catalyst software 5.4(3) or later in order for the ACLs to work in hybrid configuration.

Note: The redundant router designs that this document discusses are externally redundant, which means that there are two physical 6500 routers. Do not use this workaround for internal redundancy, in which two route processors are in one box.

Case Study #8: Asymmetric Routing and HSRP (Excessive Flooding of Unicast Traffic in Network with Routers That Run HSRP)

With asymmetric routing, transmit and receive packets follow different paths between a host and the peer with which it communicates. This packet flow is a result of the configuration of load balancing between HSRP routers, based on HSRP priority, which set the HSRP to active or standby. This type of packet flow in a switching environment can result in excessive unknown unicast flooding. Also, Multilayer Switching (MLS) entries can be absent. Unknown unicast flooding occurs when the switch floods a unicast packet out of all ports. The switch floods the packet because there is no entry for the destination MAC address. This behavior does not break connectivity because packets are still forwarded. But, the behavior does account for the flood of extra packets on host ports. This case studies the behavior of asymmetric routing and why unicast flooding results.

Symptoms of asymmetric routing include:

  • Excessive unicast packet flooding

  • Absent MLS entry for flows

  • Sniffer trace that shows that packets on the host port are not destined for the host

  • Increased network latency with L2-based packet rewrite engines, such as server load balancers, web cache devices, and network appliances

    Examples include the Cisco LocalDirector and Cisco Cache Engine.

  • Dropped packets on connected hosts and workstations that cannot handle the additional unicast-flooding traffic load

Note: The default ARP cache aging time on a router is four hours. The default aging time of the switch content-addressable memory (CAM) entry is five minutes. The ARP aging time of the host workstations is not significant for this discussion. but, the example sets the ARP aging time to four hours.

This diagram illustrates this issue. This topology example includes Catalyst 6500s with Multilayer Switch Feature Cards (MSFCs) in each switch. Although this example uses MSFCs, you can use any router instead of the MSFC. Example routers that you can use include the Route Switch Module (RSM), Gigabit Switch Router (GSR), and Cisco 7500. The hosts are directly connected to ports on the switch. The switches are interconnected through a trunk which carries traffic for VLAN 1 and VLAN 2.

62d.gif

These outputs are excerpts from the show standby command configuration from each MSFC:

MSFC1

interface Vlan 1 
   mac-address 0003.6bf1.2a01 
    ip address 10.1.1.2 255.255.255.0 
    no ip redirects 
    standby 1 ip 10.1.1.1 
    standby 1 priority 110 
	
interface Vlan 2 
    mac-address 0003.6bf1.2a01 
    ip address 10.1.2.2 255.255.255.0 
    no ip redirects 
    standby 2 ip 10.1.2.1 
	
MSFC1#show standby
Vlan1 - Group 1
Local state is Active, priority 110
Hellotime 3 holdtime 10
Next hello sent in 00:00:00.696
Hot standby IP address is 10.1.1.1 configured
Active router is local
Standby router is 10.1.1.3 expires in 00:00:07
Standby virtual mac address is 0000.0c07.ac01
2 state changes, last state change 00:20:40
Vlan2 - Group 2
Local state is Standby, priority 100
Hellotime 3 holdtime 10
Next hello sent in 00:00:00.776
Hot standby IP address is 10.1.2.1 configured
Active router is 10.1.2.3 expires in 00:00:09, priority 110
Standby router is local
4 state changes, last state change 00:00:51
MSFC1#exit
Console> (enable)

MSFC2

interface Vlan 1 
    mac-address 0003.6bf1.2a02 
    ip address 10.1.1.3 255.255.255.0 
    no ip redirects 
    standby 1 ip 10.1.1.1 
    
interface Vlan 2 
    mac-address 0003.6bf1.2a02 
    ip address 10.1.2.3 255.255.255.0 
    no ip redirects 
    standby 2 ip 10.1.2.1 
    standby 2 priority 110  
	 
MSFC2#show standby
Vlan1 - Group 1
Local state is Standby, priority 100
Hellotime 3 holdtime 10
Next hello sent in 00:00:01.242
Hot standby IP address is 10.1.1.1 configured
Active router is 10.1.1.2 expires in 00:00:09, priority 110
Standby router is local
7 state changes, last state change 00:01:17 
Vlan2 - Group 2
Local state is Active, priority 110
Hellotime 3 holdtime 10
Next hello sent in 00:00:00.924
Hot standby IP address is 10.1.2.1 configured
Active router is local
Standby router is 10.1.2.2 expires in 00:00:09
Standby virtual mac address is 0000.0c07.ac02
2 state changes, last state change 00:40:08
MSFC2#exit

Note: On MSFC1, VLAN 1 is in the HSRP active state, and VLAN 2 is in the HSRP standby state. On MSFC2, VLAN 2 is in the HSRP active state, and VLAN 1 is in the HSRP standby state. The default gateway of each host is the respective standby IP address.

  1. Initially, all caches are empty. Host A uses MSFC1 as its default gateway. Host B uses MSFC2.

    ARP and MAC Address Tables Before Ping Is Initiated

    Host A ARP Table Switch 1 MAC Address Table MAC VLAN Port MSFC1 ARP Table MSFC2 ARP Table Switch 2 MAC Address Table MAC VLAN Port Host B ARP Table
      0003.6bf1.2a01 1 15/1     0003.6bf1.2a02 1 15/1  
      0003.6bf1.2a01 2 15/1     0003.6bf1.2a02 2 15/1  
      0000.0c07.ac01 1 15/1     0000.0c07.ac01 1 1/1  
      0000.0c07.ac02 2 1/1     0000.0c07.ac02 2 15/1  
      0003.6bf1.2a02 1 1/1     0003.6bf1.2a01 1 1/1  
      0003.6bf1.2a02 2 1/1     0003.6bf1.2a01 2 1/1  

    Note: For brevity, the Switch 1 MAC address for the router HSRP and MAC address are not included in the other tables that appear in this section.

  2. Host A pings host B, which means that host A sends an ICMP echo packet. Because each host resides on a separate VLAN, host A forwards its packets that are destined for host B to its default gateway. In order for that process to occur, host A must send an ARP in order to resolve its default gateway MAC address, 10.1.1.1.

    ARP and MAC Address Tables After Host A Sends ARP for Default Gateway

    Host A ARP Table Switch 1 MAC Address Table MAC VLAN Port MSFC1 ARP Table MSFC2 ARP Table Switch 2 MAC Address Table MAC VLAN Port Host B ARP Table
    10.1.1.1 : 0000.0c07.ac01 0000.0c00.0001 1 2/1 10.1.1.10 : 0000.0c00.0001    

  3. MSFC1 receives the packet, rewrites the packet, and forwards the packet to host B. In order to rewrite the packet, MSFC1 sends an ARP request for host B because the host resides off a directly connected interface. MSFC2 has yet to receive any packets in this flow. When MSFC1 receives the ARP reply from host B, both switches learn the source port that is associated with host B.

    ARP and MAC Address Tables After Host A Sends Packet to Default Gateway and MSFC1 Sends ARP for Host B

    Host A ARP Table Switch 1 MAC Address Table MAC VLAN Port MSFC1 ARP Table MSFC2 ARP Table Switch 2 MAC Address Table MAC VLAN Port Host B ARP Table
    10.1.1.1 : 0000.0c07.ac01 0000.0c00.0001 1 2/1 10.1.1.10 : 0000.0c00.0001   0000.0c00.0002 2 2/1 10.1.2.2 : 0003.6bf1.2a01
      0000.0c00.0002 2 1/1 10.1.2.10 : 0000.0c00.0002      

  4. Host B receives the echo packet from host A, through MSFC1. Host B must now send an echo reply to host A. Since host A resides on a different VLAN, host B forwards the reply through its default gateway, MSFC2. In order to forward the packet throughMSFC2, host B must send an ARP for its default gateway IP address, 10.1.2.1.

    ARP and MAC Address Tables After Host B Sends ARP for Its Default Gateway

    Host A ARP Table Switch 1 MAC Address Table MAC VLAN Port MSFC1 ARP Table MSFC2 ARP Table Switch 2 MAC Address Table MAC VLAN Port Host B ARP Table
    10.1.1.1 : 0000.0c07.ac01 0000.0c00.0001 1 2/1 10.1.1.10 : 0000.0c00.0001 10.1.2.10 0000.0c00.0002 0000.0c00.0002 2 2/1 10.1.2.2 (0003.6bf1.2a01)
      0000.0c00.0002 2 1/1 10.1.2.10 : 0000.0c00.0001     10.1.2.1 (0000.0c07.ac02)

  5. Host B now forwards the echo reply packet to MSFC2. MSFC2 sends an ARP request for host A because it is directly connected on VLAN 1. Switch 2 populates its MAC address table with the MAC address of host B.

    ARP and MAC Address Tables After Echo Packet Has Been Received by Host A

    Host A ARP Table Switch 1 MAC Address Table MAC VLAN Port MSFC1 ARP Table MSFC2 ARP Table Switch 2 MAC Address Table MAC VLAN Port Host B ARP Table
    10.1.1.1 : 0000.0c07.ac01 0000.0c00.0001 1 2/1 10.1.1.10 : 0000.0c00.0001 10.1.2.10 0000.0c00.0002 0000.0c00.0002 2 2/1 10.1.2.2 ( 0003.6bf1.2a01)
    10.1.1.3 : 0003.6bf1.2a0 0000.0c00.0002 2 1/1 10.1.2.10 : 0000.0c00.0001 10.1.1.10 0000.0c00.0001 0000.0c00.00001 1 1/1 10.1.2.1 (0000.0c07.ac02)

  6. The echo reply reaches host A and the flow is complete.

Consequences of Asymmetric Routing

Consider the case of the continuous ping of host B by host A. Remember that host A sends the echo packet to MSFC1, and host B sends the echo reply to MSFC2, which is in an asymmetric routing state. The only time that Switch 1 learns the source MAC of host B is when host B replies to an ARP request from MSFC1. This is because host B uses MSFC2 as its default gateway and does not send packets to MSFC1 and, consequently, Switch 1. Since the ARP timeout is four hours by default, Switch 1 ages the MAC address of host B after five minutes by default. Switch 2 ages host A after fiveminutes. As a result, Switch 1 must treat any packet with a destination MAC of host B as an unknown unicast. The switch floods the packet that comes from host A and is destined for host B out all ports. In addition, because there is no MAC address entry host B in Switch 1, there is no MLS entry as well.

ARP and MAC Address Tables After 5 Minutes of Continuous Ping of Host B by Host A

Host A ARP Table Switch 1 MAC Address Table MAC VLAN Port MSFC1 ARP Table MSFC2 ARP Table Switch 2 MAC Address Table MAC VLAN Port Host B ARP Table
10.1.1.1 : 0000.0c07.ac01 0000.0c00.0001 1 2/1 10.1.1.10 : 0000.0c00.0001 10.1.2.10 0000.0c00.0002 0000.0c00.0002 2 2/1 10.1.2.2 : 0003.6bf1.2a01
10.1.1.3 : 0003.6bf1.2a0   10.1.2.10 : 0000.0c00.0001 10.1.1.10 0000.0c00.0001   10.1.2.1 : 0000.0c07.ac01

The echo reply packets that come from host B experience the same issue after the MAC address entry for host A ages on Switch 2. Host B forwards the echo reply to MSFC2, which in turn routes the packet and sends it out on VLAN 1. The switch does not have an entry host A in the MAC address table and must flood the packet out all ports in VLAN 1.

Asymmetric routing issues do not break connectivity. But, asymmetric routing can cause excessive unicast flooding and MLS entries that are missing. There are three configuration changes that can remedy this situation:

  • Adjust the MAC aging time on the respective switches to 14,400 seconds (four hours) or longer.

  • Change the ARP timeout on the routers to five minutes (300 seconds).

  • Change the MAC aging time and ARP timeout to the same timeout value.

The preferable method is to change the MAC aging time to 14,400 seconds. These are the configuration guidelines:

Case Study #9: HSRP Virtual IP Address Is Reported as a Different IP Address

The STANDBY-3-DIFFVIP1 error message occurs when there is interVLAN leakage because of bridging loops in the switch.

If you get this error message and there is interVLAN leakage because of bridging loops in the switch, complete these steps in order to resolve the error:

  1. Identify the path that the packets should take between end nodes.

    If there is a router on this path, complete these steps:

    1. Troubleshoot the path from the first switch to the router.

    2. Troubleshoot the path from the router to the second switch.

  2. Connect to each switch on the path and check the status of the ports that are used on the path between end nodes.

For more information on this error message and other HSRP error messages, refer to the STANDBY Messages section of Cisco IOS System Error Messages, Volume 2 of 2.

Case Study #10: HSRP Causes MAC Violation on a Secure Port

When port security is configured on the switch ports that are connected to the HSRP enabled routers, it causes a MAC violation, since you cannot have the same secure MAC address on more than one interface. A security violation occurs on a secure port in one of these situations:

  • The maximum number of secure MAC addresses is added to the address table, and a station whose MAC address is not in the address table attempts to access the interface.

  • An address that is learned or configured on one secure interface is seen on another secure interface in the same VLAN.

By default, a port security violation causes the switch interface to become error-disabled and to shutdown immediately, which blocks the HSRP status messages between the routers.

Workaround

  • Issue the standby use-bia command on the routers. This forces the routers to use a burned-in address for HSRP instead of the virtual MAC address.

  • Disable port security on the switch ports that connect to the HSRP enabled routers.

Case Study #11: %Interface Hardware Cannot Support Multiple Groups

If multiple HSRP groups are created on the interface, this error message is received:

%Interface hardware cannot support multiple groups

This error message is received due to the hardware limitation on some Routers or switches. It is not possible to overcome the limitation by any software methods. The problem is that each HSRP group uses one additional MAC address on interface, so the Ethernet MAC chip must support multiple programmable MAC addresses in order to enable several HSRP groups.

The workaround is to use the standby use-bia interface configuration command, which uses the Burned-In Address (BIA) of the interface as its virtual MAC address, instead of the preassigned MAC address.

HSRP Troubleshooting Modules for CatOS Switches

A. Verify HSRP Router Configuration

1. Verify Unique Router Interface IP Address

Verify that each HSRP router has a unique IP address for each subnet on a per-interface basis. Also, verify that each interface has the line protocol up. In order to quickly verify the current state of each interface, issue the show ip interface brief command. Here is an example:

Router_1#show ip interface brief
Interface                  IP-Address       OK?  Method    Status      Protocol 
Vlan1                    192.168.1.1    YES   manual        up             up 
Vlan10                   192.168.10.1    YES   manual        up             up 
Vlan11                   192.168.11.1    YES   manual        up             up 

Router_2#show ip interface brief 
Interface                  IP-Address       OK?  Method   Status       Protocol 
Vlan1                    192.168.1.2    YES   manual      up               up 
Vlan10                   192.168.10.2   YES   manual      up               up 
Vlan11                   192.168.11.2   YES   manual      up               up 

2. Verify Standby (HSRP) IP Addresses and Standby Group Numbers

Verify that the configured standby (HSRP) IP addresses and standby group numbers match each HSRP-participating router. A mismatch of standby groups or HSRP standby addresses can cause HSRP problems. The show standby command details the standby group and standby IP address configuration of each interface. Here is an example:

Router_1#show standby
Vlan10 - Group 10
  Local state is Active, priority 110, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:00.216
  Hot standby IP address is 192.168.10.100 configured
  Active router is local
  Standby router is 192.168.10.2 expires in 00:00:08
  Standby virtual mac address is 0000.0c07.ac0a
  8 state changes, last state change 00:18:04
  
Vlan11 - Group 11
  Local state is Active, priority 110, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:01.848
  Hot standby IP address is 192.168.11.100 configured
  Active router is local
  Standby router is 192.168.11.2 expires in 00:00:08
  Standby virtual mac address is 0000.0c07.ac0b
  2 state changes, last state change 00:04:45


Router_2#show standby
Vlan10 - Group 10
  Local state is Standby, priority 109, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:01.710
  Hot standby IP address is 192.168.10.100 configured
  Active router is 192.168.10.1 expires in 00:00:09, priority 110
  Standby router is local
  Standby virtual mac address is 0000.0c07.ac0a
  9 state changes, last state change 00:20:22
  
Vlan11 - Group 11
  Local state is Standby, priority 109, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:02.506
  Hot standby IP address is 192.168.11.100 configured
  Active router is 192.168.11.1 expires in 00:00:09, priority 110
  Standby router is local
  Standby virtual mac address is 0000.0c07.ac0b
  4 state changes, last state change 00:07:07

3. Verify That Standby (HSRP) IP Address Is Different per Interface

Verify that the standby (HSRP) IP address is unique from the configured IP address on each interface. The show standby command is a quick reference in order to view this information. Here is an example:

Router_1#show standby 
Vlan10 - Group 10
  Local state is Active, priority 110, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:00.216
  Hot standby IP address is 192.168.10.100 configured
  Active router is local
  Standby router is 192.168.10.2 expires in 00:00:08
  Standby virtual mac address is 0000.0c07.ac0a
  8 state changes, last state change 00:18:04
  
Vlan11 - Group 11
  Local state is Active, priority 110, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:01.848
  Hot standby IP address is 192.168.11.100 configured
  Active router is local
  Standby router is 192.168.11.2 expires in 00:00:08
  Standby virtual mac address is 0000.0c07.ac0b
  2 state changes, last state change 00:04:45

Router_2#show standby
Vlan10 - Group 10
  Local state is Standby, priority 109, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:01.710
  Hot standby IP address is 192.168.10.100 configured
  Active router is 192.168.10.1 expires in 00:00:09, priority 110
  Standby router is local
  Standby virtual mac address is 0000.0c07.ac0a
  9 state changes, last state change 00:20:22
  
Vlan11 - Group 11
  Local state is Standby, priority 109, may preempt
  Hellotime 3 holdtime 10
  Next hello sent in 00:00:02.506
  Hot standby IP address is 192.168.11.100 configured
  Active router is 192.168.11.1 expires in 00:00:09, priority 110
  Standby router is local
  Standby virtual mac address is 0000.0c07.ac0b
  4 state changes, last state change 00:07:07

4. When to Use the standy use-bia Command

Unless HSRP is configured on a Token Ring interface, only use the standby use-bia command in special circumstances. This command tells the router to use its BIA instead of the virtual HSRP MAC address for the HSRP group. On a Token Ring network, if source-route bridging (SRB) is in use, the standby use-bia command allows the new active router to update the host Routing Information Field (RIF) cache with a gratuitous ARP. But, not all of the host implementations handle the gratuitous ARP correctly. Another caveat for the standby use-bia command involves proxy ARP. A standby router cannot cover for the lost proxy ARP database of the failed active router.

5. Verify Access List Configuration

Verify that the access lists that are configured on all of the HSRP peers do not filter any HSRP addresses that are configured on their interfaces. Specifically, verify the multicast address that is used in order to send traffic to all of the routers on a subnet (224.0.0.2). Also, verify that the UDP traffic that is destined for the HSRP port 1985 is not filtered. HSRP uses this address and port to send hello packets between peers. Issue the show access-lists command as a quick reference to note the access lists that are configured on the router. Here is an example:

Router_1#show access-lists
Standard IP access list 77
    deny   167.19.0.0, wildcard bits 0.0.255.255 
    permit any
Extended IP access list 144
    deny pim 238.0.10.0 0.0.0.255 any 
    permit ip any any (58 matches)

6. Review Unique Router Configurations (MSM and 4232-L3)

Note: The Multilayer Switch Module (MSM) for the Catalyst 6500/6000 and the 4232-L3 blade for the Catalyst 4000 have unique configurations. When you troubleshoot HSRP issues, verify the configuration of, not only the 4232-L3 or MSM, but also the adjoining switch port configuration. If you neglect to configure the adjoining switch ports correctly, HSRP instability and other connectivity issues can result. The HSRP duplicated IP address error message is the most common message that is associated with incorrect configuration of these hardware modules.

Refer to these documents for more information:

7. Additional HSRP Sample Configurations

Refer to these documents:

B. Verify Catalyst Fast EtherChannel and Trunking Configuration

1. Verify Trunking Configuration

If a trunk is used in order to connect the HSRP routers, verify the trunking configurations on the routers and switches. There are five possible trunking modes:

  • on

  • desirable

  • auto

  • off

  • nonegotiate

Verify that the trunking modes that are configured provide the desired trunking method. Refer to Configuring Ethernet VLAN Trunks for a table that details the possible configuration modes.

Use the desirable configuration for switch-to-switch connections when you troubleshoot HSRP issues. This configuration can isolate issues where switch ports are unable to establish trunks correctly. Set a router-to-switch configuration as nonegotiate because most Cisco IOS routers do not support negotiation of a trunk.

For IEEE 802.1Q (dot1q) trunking mode, verify that both sides of the trunk are configured to use the same native VLAN. Because Cisco products do not tag the native VLAN by default, a mismatch of native VLAN configurations results in no connectivity on mismatched VLANs. Lastly, verify that the trunk is configured to carry the VLANs that are configured on the router, and verify that the VLANs are not pruned and in the STP state for router-connected ports. Issue the show trunk mod/port command for a quick reference that shows this information. Here is an example:

Switch_1> (enable) show trunk 2/11
Port      Mode         Encapsulation  Status        Native vlan 
--------  -----------  -------------  ------------  ----------- 
 2/11     desirable    isl            trunking      1 
 
Port      Vlans allowed on trunk 
--------  --------------------------------------------------------------------- 
 2/11     1-1005 
 
Port      Vlans allowed and active in management domain 
--------  --------------------------------------------------------------------- 
 2/11     1-2 
 
Port      Vlans in spanning tree forwarding state and not pruned 
--------  --------------------------------------------------------------------- 
2/11     1-2 

Switch_2> (enable) show trunk 2/10
Port      Mode         Encapsulation  Status        Native vlan 
--------  -----------  -------------  ------------  ----------- 
 2/10     desirable    isl            trunking      1 
 
Port      Vlans allowed on trunk 
--------  --------------------------------------------------------------------- 
 2/10     1-1005 
 
Port      Vlans allowed and active in management domain 
--------  --------------------------------------------------------------------- 
 2/10     1-2 
 
Port      Vlans in spanning tree forwarding state and not pruned 
--------  --------------------------------------------------------------------- 
2/10     1-2 

Switch_1> (enable) show trunk 2/11 
Port      Mode         Encapsulation  Status        Native vlan 
--------  -----------  -------------  ------------  ----------- 
 2/11     nonegotiate isl            trunking      1 
 
Port      Vlans allowed on trunk 
--------  --------------------------------------------------------------------- 
 2/11     1-1005 
 
Port      Vlans allowed and active in management domain 
--------  --------------------------------------------------------------------- 
 2/11     1-2 
 
Port      Vlans in spanning tree forwarding state and not pruned 
--------  --------------------------------------------------------------------- 
 2/11     1-2 

Switch_1> (enable) show trunk 2/11 
Port      Mode         Encapsulation  Status        Native vlan 
--------  -----------  -------------  ------------  ----------- 
 2/11     nonegotiate  dot1q          trunking      1 
 
Port      Vlans allowed on trunk 
--------  --------------------------------------------------------------------- 
 2/11     1-1005 
 
Port      Vlans allowed and active in management domain 
--------  --------------------------------------------------------------------- 
 2/11     1-2 
 
Port      Vlans in spanning tree forwarding state and not pruned 
--------  --------------------------------------------------------------------- 
 2/11     1-2 

2. Verify Fast EtherChannel (Port Channeling) Configuration

If a port channel is used in order to connect the HSRP routers, verify the EtherChannel configuration on both routers and switches. Configure a switch-to-switch port channel as desirable on at least one side. The other side can be in any of these modes:

  • on

  • desirable

  • auto

Here is an example:

Switch_1> (enable) show port channel
Port  Status     Channel              Admin Ch
                 Mode                 Group Id
----- ---------- -------------------- ----- -----
 1/1  connected  desirable silent       16   769
 1/2  connected  desirable silent       16   769
----- ---------- -------------------- ----- ----- 
Port  Device-ID                       Port-ID                   Platform
----- ------------------------------- ------------------------- ----------------
 1/1  SCA031700TR                     1/1                       WS-C6509
 1/2  SCA031700TR                     1/2                       WS-C6509
----- ------------------------------- ------------------------- ---------------- 
Switch_2> (enable) show port channel
Port  Status     Channel              Admin Ch
                 Mode                 Group Id
----- ---------- -------------------- ----- -----
 1/1  connected  desirable silent        29   769
 1/2  connected  desirable silent        29   769
----- ---------- -------------------- ----- ----- 
Port  Device-ID                       Port-ID                   Platform
----- ------------------------------- ------------------------- ----------------
 1/1  TBA03501066                     1/1                       WS-C6506
 1/2  TBA03501066                     1/2                       WS-C6506
----- ------------------------------- ------------------------- ----------------

3 Additional Channeling and Trunking Sample Configurations

Refer to these documents:

4. Investigate Switch MAC Address Forwarding Table

Verify that the MAC address table entries exist on the switch for the HSRP routers for the HSRP virtual MAC address and the physical BIAs. The show standby command on the router provides the virtual MAC address. The show interface command provides the physical BIA. Here are sample outputs:

Router_1#show standby 
Vlan1 - Group 1 
  Local state is Active, priority 100 
  Hellotime 3 holdtime 10 
  Next hello sent in 00:00:01.820 
  Hot standby IP address is 10.1.1.254 configured 
  Active router is local 
  Standby router is 10.1.1.2 expires in 00:00:07 
  Standby virtual mac address is 0000.0c07.ac01
  2 state changes, last state change 00:50:15 
Vlan2 - Group 2 
  Local state is Active, priority 200, may preempt 
  Hellotime 3 holdtime 10 
  Next hello sent in 00:00:00.724 
  Hot standby IP address is 10.2.1.254 configured 
  Active router is local 
  Standby router is 10.2.1.2 expires in 00:00:09 
  Standby virtual mac address is 0000.0c07.ac02
  6 state changes, last state change 00:07:59
Switch_1> (enable) show cam 00-00-0c-07-ac-01 
* = Static Entry + = Permanent Entry # = System Entry R = Router Entry X = Port Security 
Entry
VLAN  Dest MAC/Route Des    [CoS]  Destination Ports or VCs / [Protocol Type] 
----  ------------------    -----  ------------------------------------------- 
1     00-00-0c-07-ac-01  R          15/1 [ALL]
Total Matching CAM Entries Displayed = 1 
Switch_1> (enable) show cam 00-00-0c-07-ac-02 
* = Static Entry  + = Permanent Entry  # = System Entry R = Router Entry X = Port Security 
Entry
VLAN  Dest MAC/Route Des    [CoS]  Destination Ports or VCs / [Protocol Type] 
----  ------------------    -----  ------------------------------------------- 
2     00-00-0c-07-ac-02  R          15/1 [ALL] 
Total Matching CAM Entries Displayed = 1

Be sure to check the CAM aging time in order to determine how quickly the entries are aged. If the time equals the configured value for STP forward delay, which is 15 seconds by default, there is a strong possibility that there is an STP loop in the network. Here is sample command output:

Switch_1> (enable) show cam agingtime 
VLAN    1 aging time = 300 sec 
VLAN    2 aging time = 300 sec
VLAN 1003 aging time = 300 sec 
VLAN 1005 aging time = 300 sec 
  
Switch_2> (enable) show cam agingtime 
VLAN    1 aging time = 300 sec 
VLAN    2 aging time = 300 sec 
VLAN 1003 aging time = 300 sec 
VLAN 1005 aging time = 300 sec

C. Verify Physical Layer Connectivity

If more than one router in an HSRP group becomes active, those routers do not consistently receive the hello packets from fellow HSRP peers. Physical layer problems can prevent the consistent pass of traffic between peers and cause this scenario. Be sure to verify physical connectivity and IP connectivity between HSRP peers when you troubleshoot HSRP. Issue the show standby command in order to verify connectivity. Here is an example:

Router_1#show standby
Vlan10 - Group 10
  Local state is Active, priority 110, may preempt
  Hellotime 3 holdtime 10
  Hot standby IP address is 192.168.10.100 configured
  Active router is local
  Standby router is unknown expired
  Standby virtual mac address is 0000.0c07.ac0a
  12 state changes, last state change 00:00:48
  
Vlan11 - Group 11
  Local state is Active, priority 110, may preempt
  Hellotime 3 holdtime 10
  Hot standby IP address is 192.168.11.100 configured
 Active router is local
  Standby router is unknown expired
  Standby virtual mac address is 0000.0c07.ac0b
  6 state changes, last state change 00:00:48 
Router_2#show standby
Vlan10 - Group 10
  Local state is Active, priority 109, may preempt
  Hellotime 3 holdtime 10
  Hot standby IP address is 192.168.10.100 configured
  Active router is local
  Standby router is unknown expired
  Standby virtual mac address is 0000.0c07.ac0a
  15 state changes, last state change 00:01:18
  
Vlan11 - Group 11
  Local state is Active, priority 109, may preempt
  Hellotime 3 holdtime 10
  Hot standby IP address is 192.168.11.100 configured
  Active router is local
  Standby router is unknown expired
  Standby virtual mac address is 0000.0c07.ac0b
  10 state changes, last state change 00:01:18

1. Check Interface Status

Check the interfaces. Verify that all HSRP-configured interfaces are up/up, as this example shows:

Router_1#show ip interface brief 
Interface                  IP-Address      OK? Method Status                     Protocol 
Vlan1                      10.1.1.1            YES manual administratively down   down 
Vlan2                      10.2.1.1            YES manual up                      up
  
Router_2#show ip interface brief 
Interface                  IP-Address      OK? Method Status              Protocol 
Vlan1                      10.1.1.2            YES manual up                 up  
Vlan2                      10.2.1.2            YES manual down               down  

If any interfaces are administratively down/down, enter the configuration mode on the router and issue the no shutdown interface-specific command. Here is an example:

Router_1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z. 
Router_1(config)# interface vlan 1 
Router_1(config-if)# no shutdown 
Router_1(config-if)# ^Z 

Router_1#show ip interface brief 
Interface                  IP-Address      OK? Method Status                   Protocol 
Vlan1                      10.1.1.1            YES manual up                     down 
Vlan2                      10.2.1.1            YES manual up                      up  

If any interfaces are down/down or up/down, review the log for any interface change notifications. For Cisco IOS Software-based switches, these messages appear for link up/down situations:

%LINK-3-UPDOWN: Interface "interface", changed state to up
%LINK-3-UPDOWN: Interface "interface", changed state to down
  
Router_1#show log 
3d04h: %STANDBY-6-STATECHANGE: Standby: 0: Vlan2 state Active-> Speak 
3d04h: %LINK-5-CHANGED: Interface Vlan2, changed state to down 
3d04h: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan2, changed state to down 

Inspect the ports, cables, and any transceivers or other devices that are between the HSRP peers. Has anyone removed or loosened any connections? Are there any interfaces that lose a link repeatedly? Are the proper cable types used? Check the interfaces for any errors, as this example shows:

Router_1#show interface vlan2 
Vlan2 is down, line protocol is down 
  Hardware is Cat5k RP Virtual Ethernet, address is 0030.f2c9.5638 (bia 0030.f2c9.5638) 
  Internet address is 10.2.1.1/24 
  MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, 
     reliability 255/255, txload 1/255, rxload 1/255 
  Encapsulation ARPA, loopback not set 
  ARP type: ARPA, ARP Timeout 04:00:00 
  Last input 00:00:00, output never, output hang never 
  Last clearing of "show interface" counters never 
  Queueing strategy: fifo 
  Output queue 0/40, 0 drops; input queue 0/75, 0 drops 
  5 minute input rate 0 bits/sec, 0 packets/sec 
  5 minute output rate 0 bits/sec, 0 packets/sec 
     155314 packets input, 8259895 bytes, 0 no buffer 
     Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 
     0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
     8185 packets output, 647322 bytes, 0 underruns 
     0 output errors, 3 interface resets 
     0 output buffer failures, 0 output buffers swapped out 

2. Link Change and Port Errors

Check the switch ports link changes and other errors. Issue these commands and review the output:

These commands help you determine if there is a problem with connectivity between switches and other devices.

These messages are normal for link up/down situations:

PAGP-5-PORTTOSTP:Port [dec]/[dec] joined bridge port [dec]/[chars] 
PAGP-5-PORTFROMSTP: Port [dec]/[dec] left bridge port [dec]/[chars] 

Switch_1> (enable) show logging buffer 
2001 Jan 08 20:37:24 %PAGP-5-PORTTOSTP:Port 2/1 joined bridge port 2/1 
2001 Jan 08 20:37:25 %PAGP-5-PORTTOSTP:Port 2/2 joined bridge port 2/2 
2001 Jan 08 20:37:25 %PAGP-5-PORTTOSTP:Port 2/3 joined bridge port 2/3 
2001 Jan 08 20:37:25 %PAGP-5-PORTTOSTP:Port 2/11 joined bridge port 2/11 
2001 Jan 08 20:46:39 %PAGP-5-PORTTOSTP:Port 2/12 joined bridge port 2/12 
2001 Jan 08 20:46:29 %PAGP-5-PORTFROMSTP:Port 2/11 left bridge port 2/11 
2001 Jan 08 20:46:29 %PAGP-5-PORTFROMSTP:Port 2/12 left bridge port 2/12 
2001 Jan 08 20:47:05 %DTP-5-TRUNKPORTON:Port 2/11 has become isl trunk 
2001 Jan 08 20:52:15 %PAGP-5-PORTTOSTP:Port 2/11 joined bridge port 2/11 
2001 Jan 08 22:18:24 %DTP-5-TRUNKPORTON:Port 2/12 has become isl trunk 
2001 Jan 08 22:18:34 %PAGP-5-PORTTOSTP:Port 2/12 joined bridge port 2/12

Issue the show port command in order to determine the general health of a port. Here is an example:

Switch_1> (enable) show port status 2/11
Port  Name               Status     Vlan       Level  Duplex Speed Type 
----- ------------------ ---------- ---------- ------ ------ ----- ------------ 
2/11                    connected  trunk      normal a-full a-100 10/100BaseTX

Is the port status connected, notconnect, or errdisable? If the status is notconnect, check that the cable is plugged in on both sides. Check that the proper cable is used. If the status is errdisable, review the counters for excessive errors. Refer to Recovering From errDisable Port State on the CatOS Platforms for more information.

For what VLAN is this port configured? Be sure that the other side of the connection is configured for the same VLAN. If the link is configured to be a trunk, be sure that both sides of the trunk carry the same VLANs.

What is the speed and duplex configuration? If the setting is preceded by a-, the port is configured to autonegotiate the speed and duplex. Otherwise, the network administrator has predetermined this configuration. For configuration of the speed and duplex for a link, the settings on both sides of the link must match. If one switch port is configured for autonegotiation, the other side of the link must also be configured for autonegotiation. If one side is hard coded to a specific speed and duplex, the other side must be hard coded as well. If you leave one side to autonegotiate while the other side is hard coded, you break the autonegotiation process.

Switch_1> (enable) show port counters 2/11 
Port  Align-Err  FCS-Err    Xmit-Err   Rcv-Err    UnderSize 
----- ---------- ---------- ---------- ---------- --------- 
 2/11  0          0          0          0         0 
 
Port  Single-Col Multi-Coll Late-Coll  Excess-Col Carri-Sen Runts     Giants 
----- ---------- ---------- ---------- ---------- --------- --------- --------- 
 2/11          0          0          0          0         0         0         - 
  
Last-Time-Cleared 
-------------------------- 
Fri Jan 5 2001, 13:30:45

Are there a lot of Align-Err, FCS-Err, or Runts? These indicate a speed or duplex mismatch between the port and the connecting device. Change the speed and duplex settings for that port in order to help correct these errors.

Issue the show mac command in order to verify that the port is passing traffic. The Rcv- and Xmit- columns indicate the number of unicast, multicast, and broadcast packets that are received and transmitted on a particular port. The bottom counters reveal how many packets are discarded or lost and whether these packets are a part of inbound or outbound traffic. Lrn-Discrd, In-Lost, and Out-Lost count the number of packets that are mistakenly forwarded or dropped due to insufficient buffers.

Switch_1> (enable) show mac 2/11 
Port     Rcv-Unicast          Rcv-Multicast        Rcv-Broadcast 
-------- -------------------- -------------------- -------------------- 
2/11                    9786                 9939                 2678
Port     Xmit-Unicast         Xmit-Multicast       Xmit-Broadcast 
-------- -------------------- -------------------- -------------------- 
 2/11                     587                55517                  148 

Port     Rcv-Octet            Xmit-Octet 
-------- -------------------- -------------------- 
 2/11                 2354136              7206386 
  
MAC      Dely-Exced MTU-Exced  In-Discard Lrn-Discrd In-Lost    Out-Lost 
-------- ---------- ---------- ---------- ---------- ---------- ---------- 
 2/11             0          -         13          0          0          0 
  
Last-Time-Cleared 
-------------------------- 
Fri Jan 5 2001, 13:30:45 

3. Verify IP Connectivity

Verify IP connectivity. Issue an IP ping from the associated router. This helps expose any momentary losses of connectivity. An extended ping is only available in enable mode. Here is sample command output:

router_1#ping 
Protocol [ip]: 
Target IP address: 10.2.1.2 
Repeat count [5]: 1000 
Datagram size [100]: 1500 
Timeout in seconds [2]: 
Extended commands [n]: 
Sweep range of sizes [n]: 
Type escape sequence to abort. 
Sending 1000, 1500-byte ICMP Echos to 10.2.1.2, timeout is 2 seconds: 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 
!!!!!!!!!!!!!!!!!!!! 
Success rate is 100 percent (1000/1000), round-trip min/avg/max = 4/4/20 ms 

Issue the ping from each HSRP router to its peers in order to determine the location of the failure in connectivity.

4. Check for Unidirectional Link

Check the switch for unidirectional links between HSRP peers. A unidirectional link occurs whenever traffic that a local device transmits over a link is received by the neighbor, but traffic that the neighbor transmits is not received by the local device. Later versions of CatOS have a feature that can detect a unidirectional link. This feature is known as UniDirectional Link Detection (UDLD) aggressive mode. Use of UDLD is only possible if both sides of the connection support the feature. UDLD aggressive mode operates at L2 to determine if a link is connected correctly and if traffic flows bidirectionally between the right neighbors. You can configure UDLD aggressive mode in CatOS release 5.4(3) and later for point-to-point connections on a port-by-port basis. Refer to Configuring UDLD for more information. Here are sample command outputs:

Note: Enablement of UDLD without enablement of UDLD aggressive mode only checks for incorrectly wired fiber cable. In this case, UDLD checks where the receive and transmit are crossed among multiple connections.

Switch_1> (enable) set udld enable 
UDLD enabled globally
 
Console> (enable) set udld aggressive-mode enable 1/1-2
Aggressive UDLD enabled on ports 1/1-2.
Console> (enable) show udld 
UDLD      : enabled   
Message Interval : 15 seconds 

Console> (enable) show udld port 1
UDLD : enabled
Message Interval : 15 seconds
Port Admin Status Aggressive Mode Link State
-------- ------------ --------------- ----------------
1/1 enabled enabled undetermined
1/2 enabled enabled undetermined

In CatOS 5.4.3 and earlier, in which the switch does not support UDLD, or if the link in question has a router on one end, you can enable Cisco Discovery Protocol (CDP). Enablement of CDP is another way to detect if a unidirectional link exists. If only one side of a link can see its neighbor device, replace the cable between the devices and check for faulty interfaces. Refer to Configuring CDP for more information.

Switch_1> (enable) show cdp 
CDP               : enabled
Message Interval  : 60 
Hold Time         : 180 
Switch_1> (enable) show cdp neighbors 
* - indicates vlan mismatch. 
# - indicates duplex mismatch. 
Port     Device-ID                       Port-ID                   Platform 
-------- ------------------------------- ------------------------- ------------ 
2/5     066560091(Switch_2)             2/9                       WS-C5505 
2/6     066560091(Switch_2)             2/10                      WS-C5505 
15/1     Router_1                       Vlan1                     cisco Cat5k-RSFC 

Switch_2> (enable) show cdp 
CDP               : enabled
Message Interval  : 60 
Hold Time         : 180 
Switch_2> (enable) show cdp neighbors 
* - indicates vlan mismatch. 
# - indicates duplex mismatch. 
Port     Device-ID                       Port-ID                   Platform 
-------- ------------------------------- ------------------------- ------------ 
2/9     066565061(Switch_1)             2/5                       WS-C5505 
2/10    066565061(Switch_1)             2/6                       WS-C5505 
15/1     Router_2                       Vlan1                     cisco Cat5k-RSFC               

5. Additional Physical Layer Troubleshooting References

Refer to these documents:

D. Layer 3 HSRP Debugging

If the HSRP state changes are frequent, use the HSRP debug commands in enable mode on the router in order to watch HSRP activity. This information helps you determine what HSRP packets are received and sent by the router. Gather this information if you create a service request with Cisco Technical Support. The debug output also shows HSRP state information, along with detailed HSRP hello packet accounts.

1. Standard HSRP Debugging

In Cisco IOS Software Release 12.1 and earlier, the HSRP debug command is simply debug standby . This information is useful where problems are intermittent and affect only a few interfaces. The debug enables you to determine if the HSRP router in question receives and transmits HSRP hello packets at specific intervals. If the router does not receive hello packets, you can infer that either the peer does not transmit the hello packets or the network drops the packets.

Command Purpose
debug standby Enables HSRP debugging

Here is sample command output:

Router_1#debug standby
 
HSRP debugging is on 

Router_1# 
4d01h: SB1: Vlan1 Hello  out 10.1.1.1 Active  pri 100 ip 10.1.1.254 
4d01h: SB1: Vlan1 Hello  in  10.1.1.2 Standby pri 100 ip 10.1.1.254 
4d01h: SB2: Vlan2 Hello  in  10.2.1.2 Standby pri 100 ip 10.2.1.254 
4d01h: SB2: Vlan2 Hello  out 10.2.1.1 Active  pri 100 ip 10.2.1.254   

2. Conditional HSRP Debugging (Limiting Output Based on Standby Group and/or VLAN)

Cisco IOS Software Release 12.0(3) introduced a debug condition to allow the output from the debug standby command to be filtered based on interface and group number. The command utilizes the debug condition paradigm that was introduced in Cisco IOS Software Release 12.0.

Command Purpose
debug condition standby interface_group Enables HSRP conditional debugging of the group (0–255)

The interface must be a valid interface that can support HSRP. The group can be any group, from 0 through 255. A debug condition can be set for groups that do not exist. This allows debugs to be captured during the initialization of a new group. Debug standby must be enabled in order to produce any debug output. If no standby debug conditions exist, debug output is produced for all groups on all interfaces. If at least one standby debug condition exists, standby debug output is filtered based on all of the standby debug conditions. Here is sample command output:

Router_1#debug condition standby vlan 2 2 
Condition 1 set 
Router_1# 
4d01h: Vl2 SB2 Debug: Condition 1, standby Vl2 SB2 triggered, count 1 
Router_1#debug standby 
HSRP debugging is on 
Router_1# 
4d01h: SB2: Vlan2 Hello  in  10.2.1.2 Standby pri 100 ip 10.2.1.254 
4d01h: SB2: Vlan2 Hello  out 10.2.1.1 Active  pri 100 ip 10.2.1.254 
4d01h: SB2: Vlan2 Hello  out 10.2.1.1 Active  pri 100 ip 10.2.1.254 
4d01h: SB2: Vlan2 Hello  in  10.2.1.2 Standby pri 100 ip 10.2.1.254   

3. Enhanced HSRP Debugging

Cisco IOS Software Release 12.1(1) added enhanced HSRP debugging. In order to help find useful information, enhanced HSRP debugging limits the noise from periodic hello messages and includes additional state information. This information is particularly useful when you work with a Cisco Technical Support engineer if you create a service request.

Command Purpose
debug standby Displays all HSRP errors, events, and packets
debug standby errors Displays HSRP errors
debug standby events [[all] | [hsrp | redundancy | track]] [detail] Displays HSRP events
debug standby packets [[all | terse] | [advertise | coup | hello | resign]] [detail] Displays HSRP packets

Here is sample command output:

Router_2#debug standby terse
HSRP: 
  HSRP Errors debugging is on 
  HSRP Events debugging is on 
  HSRP Packets debugging is on 
  (Coup, Resign) 
Router_2# 
00:39:50: SB2: Vlan2 Standby: c/Active timer expired (10.2.1.1) 
00:39:50: SB2: Vlan2 Standby -> Active 
00:39:50: %STANDBY-6-STATECHANGE: Standby: 2: Vlan2 state Standby -> Active 
00:40:30: SB2: Vlan2 Standby router is 10.2.1.1 
00:41:12: SB2: Vlan2 Active: d/Standby timer expired (10.2.1.1) 
00:42:09: SB2: Vlan2 Coup   in  10.2.1.1 Listen  pri 200 ip 10.2.1.254 
00:42:09: SB2: Vlan2 Active: j/Coup rcvd from higher pri router 
00:42:09: SB2: Vlan2 Active -> Speak 
00:42:09: %STANDBY-6-STATECHANGE: Standby: 2: Vlan2 state Active -> Speak 
00:42:09: SB2: Vlan2 Active router is 10.2.1.1 
00:42:19: SB2: Vlan2 Speak: d/Standby timer expired (unknown) 
00:42:19: SB2: Vlan2 Speak -> Standby 
00:42:19: %STANDBY-6-STATECHANGE: Standby: 2: Vlan2 state Speak  -> Standby

You can use interface and/or HSRP group conditional debugging in order to filter this debug output.

Command Purpose
debug condition interface interface Enables interface conditional debugging
debug condition standby interface_group Enables HSRP conditional debugging

In this example, the router joins a preexisting HSRP group:

SB1: Ethernet0/2 Init: a/HSRP enabled
SB1: Ethernet0/2 Active: b/HSRP disabled (interface down)
SB1: Ethernet0/2 Listen: c/Active timer expired (unknown)
SB1: Ethernet0/2 Active: d/Standby timer expired (10.0.0.3)
SB1: Ethernet0/2 Speak: f/Hello rcvd from higher pri Speak router
SB1: Ethernet0/2 Active: g/Hello rcvd from higher pri Active router
SB1: Ethernet0/2 Speak: h/Hello rcvd from lower pri Active router
SB1: Ethernet0/2 Standby: i/Resign rcvd
SB1: Ethernet0/2 Active: j/Coup rcvd from higher pri router
SB1: Ethernet0/2 Standby: k/Hello rcvd from higher pri Standby router
SB1: Ethernet0/2 Standby: l/Hello rcvd from lower pri Standby router
SB1: Ethernet0/2 Active: m/Standby mac address changed
SB1: Ethernet0/2 Active: n/Standby IP address configured

E. Spanning Tree Troubleshooting

STP loop conditions or instability in a network can prevent proper communication of HSRP peers. Because of this improper communication, each peer becomes an active router. STP loops can cause broadcast storms, duplicated frames, and MAC table inconsistency. All of these problems affect the entire network, and especially HSRP. HSRP error messages can be the first indication of an STP issue.

When you troubleshoot STP, you must understand the STP topology of the network on each VLAN. You must determine what switch is the root bridge and which ports on the switch are on blocking and forwarding. Because each VLAN has its own STP topology, this information is very important on each VLAN.

1. Verify Spanning Tree Configuration

Be sure that STP is configured on every switch and bridging device in the network. Take note of where each switch believes the root bridge is located. Also, note the values of these timers:

  • Root Max Age

  • Hello Time

  • Forward Delay

Issue the show spantree command in order to see all of this information. By default, the command shows this information for VLAN 1. But, you can also see other VLAN information if you supply the VLAN number with the command. This information is very useful when you troubleshoot STP issues.

These three timers that you note in the show spantree output are learned from the root bridge. These timers do not need to match the timers that are set on that specific bridge. But, be sure that the timers match the root bridge in the case that this switch becomes the root bridge at any point. This match of the timers to the root bridge helps maintain continuity and ease of administration. The match also prevents a switch with incorrect timers from crippling the network.

Note: Enable STP for all VLANs at all times, regardless of whether there are redundant links in the network. If you enable STP in nonredundant networks, you prevent a breakage. A breakage can occur if someone bridges switches together with hubs or other switches and accidentally creates a physical loop. STP is also very useful in the isolation of specific problems. If the enablement of STP affects the operation of something in the network, there can be an existing problem that you need to isolate.

Here is sample output of the show spantree command:

Switch_1> (enable) show spantree
VLAN 1 
Spanning tree enabled 
Spanning tree type          ieee 

Designated Root             00-01-64-34-90-00 
Designated Root Priority    98
Designated Root Cost        0 
Designated Root Port        1/0 
Root Max Age   20 sec       Hello Time 2  sec   Forward Delay 15 sec
 
Bridge ID MAC ADDR          00-01-64-34-90-00 
Bridge ID Priority          98 
Bridge Max Age 20 sec       Hello Time 2  sec   Forward Delay 15 sec 

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id 
------------------------ ---- ------------- ----- -------- ---------- ---------- 
1/1                       1    not-connected     4       32 disabled   0         
1/2                       1    not-connected     4       32 disabled   0         
2/1                       1    forwarding      100       32 disabled   0         
2/2                       1    not-connected   100       32 disabled   0         
2/3                       1    not-connected   100       32 disabled   0         
2/4                       1    not-connected   100       32 disabled   0         
2/5-6                     1    forwarding       12       32 disabled   803       
2/10                      1    not-connected   100       32 disabled   0         
2/11                      1    not-connected   100       32 disabled   0         
2/12                      1    not-connected   100       32 disabled   0         
15/1                      1    forwarding        5       32 disabled   0          

Switch_1> (enable) show spantree 2 
VLAN 2 
Spanning tree enabled 
Spanning tree type          ieee 

Designated Root             00-30-96-73-74-01 
Designated Root Priority    8192 
Designated Root Cost        12 
Designated Root Port        2/5-6 (agPort 13/35)    
Root Max Age   20 sec       Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-01-64-34-90-01 
Bridge ID Priority          16384 
Bridge Max Age 20 sec       Hello Time 2  sec   Forward Delay 15 sec 

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id 
------------------------ ---- ------------- ----- -------- ---------- ---------- 
2/5-6                     2    forwarding       12       32 disabled   803       
2/7                       2    not-connected   100       32 disabled   0         
2/8                       2    not-connected   100       32 disabled   0         
2/9                       2    not-connected   100       32 disabled   0         
15/1                      2    forwarding        5       32 disabled   0

Switch 1 is the root of VLAN 1 and believes that Switch 2 is the root of VLAN 2. Switch 2 concurs.

Switch_2> (enable) show spantree 
VLAN 1 
Spanning tree enabled 
Spanning tree type          ieee 

Designated Root             00-01-64-34-90-00 
Designated Root Priority    98 
Designated Root Cost        12 
Designated Root Port        2/9-10 (agPort 13/37)   
Root Max Age   20 sec       Hello Time 2  sec   Forward Delay 15 sec 

Bridge ID MAC ADDR          00-30-96-73-74-00 
Bridge ID Priority          16384 
Bridge Max Age 20 sec       Hello Time 2  sec   Forward Delay 15 sec 

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id 
------------------------ ---- ------------- ----- -------- ---------- ---------- 
1/1                       1    not-connected     4       32 disabled   0         
1/2                       1    not-connected     4       32 disabled   0         
2/6                       1    not-connected   100       32 disabled   0         
2/7                       1    not-connected   100       32 disabled   0         
2/8                       1    not-connected   100       32 disabled   0         
2/9-10                    1    forwarding       12       32 disabled   805       
2/11                      1    not-connected   100       32 disabled   0         
2/12                      1    not-connected   100       32 disabled   0         
15/1                      1    forwarding        5       32 disabled   0         

Switch_2> (enable) show spantree 2
VLAN 2 
Spanning tree enabled 
Spanning tree type          ieee 

Designated Root             00-30-96-73-74-01 
Designated Root Priority    8192 
Designated Root Cost        0 
Designated Root Port        1/0
Root Max Age   20 sec       Hello Time 2  sec   Forward Delay 15 sec

Bridge ID MAC ADDR          00-30-96-73-74-01 
Bridge ID Priority          8192 
Bridge Max Age 20 sec       Hello Time 2  sec   Forward Delay 15 sec 

Port                     Vlan Port-State    Cost  Priority Portfast   Channel_id 
------------------------ ---- ------------- ----- -------- ---------- ---------- 
2/1                       2    not-connected   100       32 disabled   0         
2/2                       2    not-connected   100       32 disabled   0         
2/3                       2    not-connected   100       32 disabled   0         
2/4                       2    not-connected   100       32 disabled   0         
2/5                       2    not-connected   100       32 disabled   0         
2/9-10                    2    forwarding       12       32 disabled   805       
15/1                      2    forwarding        5       32 disabled   0         

2. Spanning Tree Loop Conditions

In order for an STP loop to occur, there must be L2 physical redundancy in the network. An STP does not occur if there is no possibility of a physical loop condition. Symptoms of an STP loop condition are:

  • Total network outage

  • Loss of connectivity

  • The report by network equipment of high process and system utilization

The show system command helps you determine the system utilization of a particular switch. The show system command denotes these items:

  • Current traffic percentage

  • Peak traffic percentage

  • Date and time of the last peak

System utilization that is above 20 percent usually indicates a loop. Utilization above seven percent indicates a possible loop. But, these percentages are only approximations. The approximations vary somewhat with different hardware, such as Supervisor Engine I versus Supervisor Engine IIIG or Catalyst 4000 versus Catalyst 6000.

Here is sample output of the show system command:

Switch_1> (enable) show system 
PS1-Status PS2-Status Fan-Status Temp-Alarm Sys-Status Uptime d,h:m:s Logout 
---------- ---------- ---------- ---------- ---------- -------------- --------- 
ok         none       ok         off        ok         5,00:58:16     20 min 
PS1-Type     PS2-Type     Modem   Baud  Traffic Peak Peak-Time 
------------ ------------ ------- ----- ------- ---- ------------------------- 
WS-C5008B    none         disable  9600   0%     70% Tue Jan 9 2001, 16:50:52 
System Name              System Location          System Contact 
------------------------ ------------------------ ------------------------ 
Switch_1  

This output shows these items:

  • The current traffic percentage, 0%

  • The peak traffic percentage, 70%

  • The date and time of the last peak

The system utilization of 70 percent indicates a possible loop at the time that the show system command output shows.

A single VLAN that experiences an STP loop condition can congest a link and starve the other VLANs of bandwidth. The show mac command notes which ports transmit or receive an excessive number of packets. Excessive broadcast and multicast can indicate ports that are part of an STP loop. This example output of the show mac command shows a high number of multicast and broadcast packets on port 2/11. Investigate this port. As a general rule, suspect a link of an STP loop condition any time that multicast or broadcast exceeds the number of unicast packets.

Note: The switch also counts STP bridge protocol data units (BPDUs) that are received and transmitted as multicast frames. A port that is in the STP blocking state still transmits and receives STP BPDUs.

Switch_1> (enable) show mac 
Port     Rcv-Unicast          Rcv-Multicast        Rcv-Broadcast 
-------- -------------------- -------------------- -------------------- 
 1/1                        0                    0                    0 
 1/2                        0                    0                    0 
 2/1                   551277               296902              1025640 
 2/2                        0                    0                    0 
 2/3                        0                    0                    0 
 2/4                        0                    0                    0 
 2/5                        0                69541                    0 
 2/6                        0                44026                    0
 2/7                        0                    0                    0 
 2/8                        0                    0                    0 
 2/9                        0                    0                    0 
 2/10                       0                    0                    0
 2/11                   12836              5911986              1126018 
 2/12                 6993144            177795414             19063645 

Port     Xmit-Unicast         Xmit-Multicast       Xmit-Broadcast 
-------- -------------------- -------------------- -------------------- 
 1/1                        0                    0                    0 
 1/2                        0                    0                    0 
 2/1                   326122              1151895               431125 
 2/2                        0                    0                    0 
 2/3                        0                    0                    0 
 2/4                        0                    0                    0
 2/5                        0               157414                    0 
 2/6                       10               652821                    1 
 2/7                        0                    0                    0 
 2/8                        0                    0                    0 
 2/9                        0                    0                    0 
 2/10                       0                    0                    0 
 2/11                20969162            127255514             56002139 
 2/12                   13598              7378244                 3166

Port     Rcv-Octet            Xmit-Octet 
-------- -------------------- -------------------- 
1/1                        0                    0 
1/2                        0                    0 
2/1                544904490            295721712 
2/2                        0                    0 
2/3                        0                    0 
2/4                        0                    0 
2/5                  6997319             15860816 
2/6                  4787570            185054891 
2/7                        0                    0 
2/8                        0                    0 
2/9                        0                    0 
2/10                       0                    0 
2/11               560753237           8058589649 
2/12              6822964273            815810803 

MAC      Dely-Exced MTU-Exced  In-Discard Lrn-Discrd In-Lost    Out-Lost 
-------- ---------- ---------- ---------- ---------- ---------- ---------- 
1/1              0          0          0          0          0          0 
1/2              0          0          0          0          0          0 
2/1              0          0     718920          0          0          0 
2/2              0          0          0          0          0          0 
2/3              0          0          0          0          0          0 
2/4              0          0          0          0          0          0 
2/5              0          -          3          0          1          0 
2/6              0          -          0          0          0          0 
2/7              0          0          0          0          0          0 
2/8              0          0          0          0          0          0 
2/9              0          0          0          0          0          0 
2/10             0          0          0          0          0          0 
2/11             0          0         67          0          0          0 
2/12             0          0        869          0          3          0 

Issue the session command in order to see the ATM and router counters.

Last-Time-Cleared 
-------------------------- 
Fri Jan 5 2001, 13:30:45 

3. Toplogy Change Notification

Another command that is vital to the diagnosis of STP issues is the show spantree statistics command. This command tracks Topology Change Notification (TCN) messages back to the originator. These messages, sent as special BPDUs between switches, indicate that there has been a topology change on a switch. That switch sends a TCN out its root port. The TCN moves upstream to the root bridge. The root bridge then sends another special BPDU, a Topology Change Acknowledgement (TCA), out all of its ports. The root bridge sets the TCN bit in the configuration BPDU. This causes all nonroot bridges to set their MAC address table aging timer to the configuration STP forward delay.

In order to isolate this problem, access the root bridge for each VLAN and issue the show spantree statistics command for the switch-connected ports. The last topology change occurred entry gives the time that the last TCN was received. In this situation, you are too late to see who issued the TCNs that can have caused the possible STP loop. The topology change count entry gives you an idea about the number of TCNs that occur. During an STP loop, this counter can increment every minute. Refer to Spanning Tree Protocol Problems and Related Design Considerations for more information. This document contains more information on how to interpret the show spantree statistics command. Other useful information includes:

  • Port of the last TCN

  • Time of last TCN

  • Current count of TCNs

Here is sample command output:

Switch_1> (enable) show spantree statistics 2/5 1 
Port  2/5   VLAN 1 
SpanningTree enabled for vlanNo = 1 
                BPDU-related parameters 
port spanning tree                   enabled 
state                                forwarding 
port_id                              0x8323 
port number                          0x323 
path cost                            12 
message age (port/VLAN)              20(20) 
designated_root                      00-01-64-34-90-00 
designated_cost                      0 
designated_bridge                    00-01-64-34-90-00 
designated_port                      0x8323 
top_change_ack                       FALSE 
config_pending                       FALSE 
port_inconsistency                   none 
                PORT based information & statistics 
config bpdu's xmitted (port/VLAN)    29660(357027) 
config bpdu's received (port/VLAN)   2(215721) 
tcn bpdu's xmitted (port/VLAN)       0(521) 
tcn bpdu's received (port/VLAN)      2(203) 
forward trans count                  1 
scp failure count                    0 
                Status of Port Timers 
forward delay timer                  INACTIVE 
forward delay timer value            15 
message age timer                    INACTIVE 
message age timer value              0 
topology change timer                INACTIVE 
topology change timer value          35 
hold timer                           INACTIVE 
hold timer value                     1 
delay root port timer                INACTIVE 
delay root port timer value          0 
                VLAN based information & statistics 
spanningtree type                    ieee 
spanningtree multicast address       01-80-c2-00-00-00 
bridge priority                      98 
bridge mac address                   00-01-64-34-90-00 
bridge hello time                    2 sec 
bridge forward delay                 15(15) sec 
topology change initiator:           2/2 
last topology change occurred:        Wed Jan 10 2001, 18:16:02 
topology change                      FALSE 
topology change time                 35 
topology change detected             FALSE 
topology change count                80 
topology change last recvd. from     00-10-7b-08-fb-94 
                  Other port-specific info 
dynamic max age transitions          0 
port bpdu ok count                   0 
msg age expiry count                 0 
link loading                         1 
bpdu in processing                   FALSE 
num of similar bpdus to process      1 
received_inferior_bpdu               FALSE 
next state                           3 
src mac count:                       0 
total src mac count                  0 
curr_src_mac                         00-00-00-00-00-00 
next_src_mac                         00-00-00-00-00-00 
channel_src_mac                      00-10-7b-08-e1-74 
channel src count                    0 
channel ok count                     0 

This output shows that the last topology change occurred from device 00-10-7b-08-fb-94 off port 2/2. Next, issue the same show spantree statistics command from the 00-10-7b-08-fb-94 device. Here is an excerpt of the show spantree statistics output from the adjoining device:

VLAN based information & statistics 
spanningtree type                    ieee 
spanningtree multicast address       01-80-c2-00-00-00 
bridge priority                      98 
bridge mac address                   00-10-7b-08-fb-94 
bridge hello time                    2 sec 
bridge forward delay                 15(15) sec 
topology change initiator:           5/2 
last topology change occurred:        Wed Jan 10 2001, 18:16:02 
topology change                      FALSE 
topology change time                 35 
topology change detected             FALSE 
topology change count                80 
topology change last recvd. from     00-00-00-00-00-00

The output notes the MAC address with all zeroes, which means that this switch is the topology change initiator. Port 5/2 is the port that transitioned states, which is most likely because the port goesup and down. If this port is attached to a PC or a single host, be sure that STP PortFast is enabled on this port. STP PortFast suppresses STP TCNs when a port transitions states.

Refer to these documents for information about STP and how to troubleshoot link transitions that are associated with network interface cards (NICs):

4. Disconnected Blocked Ports

Because of the load-balancing nature of Fast EtherChannel (FEC) (port-channeling), FEC issues can contribute to both HSRP and STP problems. When you troubleshoot STP or HSRP, remove the configuration for any FEC connections. After the configuration changes are in place, issue the show spantree blockedports command on both switches. Ensure that at least one of the ports starts blocking on either side of the connection. Here is sample command output:

Switch_1> (enable) show spantree blockedports 
T = trunk 
g = group 
Ports       Vlans 
-----       ---------- 
 2/6  (T)   2 
Number of blocked ports (segments) in the system : 1 

Switch_2> (enable) show spantree blockedports 
T = trunk 
g = group 
Ports       Vlans 
-----       ---------- 
2/10 (T)   1 
Number of blocked ports (segments) in the system : 1   

Refer to these documents for information about Fast EtherChannel:

5. Broadcast Suppression

Enable broadcast suppression in order to help cut down the impact from a broadcast storm. A broadcast storm is one of the main side effects of an STP loop. Refer to Configuring Broadcast Suppression for more information. Here is sample command output:

Switch_1> (enable) set port broadcast 2/5 ? 
                    Packets per second 
                Percentage 
Switch_1> (enable) set port broadcast 2/5 10% 
Port(s) 2/1-12 broadcast traffic limited to 10%. 
Switch_1> (enable) show port broadcast 2/5 
Port     Broadcast-Limit Broadcast-Drop 
-------- --------------- -------------- 
 2/5                10 %              - 

6. Console and Telnet Access

Console or Telnet traffic to the switch often becomes too sluggish to properly track down an offending device during an STP loop. In order to force the network to recover instantly, remove all redundant physical links. After STP is allowed to reconverge on the new nonredundant topology, reattach one redundant link at a time. If the STP loop returns after you add one particular segment, you have identified the offending devices.

7. Spanning Tree Features: Portfast, UplinkFast, and BackboneFast

Verify that PortFast, UplinkFast, and BackboneFast are configured properly. When you troubleshoot STP issues, disable all advanced STP (UplinkFast and BackboneFast). In addition, verify that STP PortFast is only enabled on ports that are directly connected to nonbridging hosts. Nonbridging hosts include user workstations and routers without bridge groups. Do not enable PortFast on ports that are connected to hubs or other switches. Here is sample command output:

Switch_2> (enable) show port spantree     
Port(s)                  Vlan Port-State    Cost  Priority Portfast   Channel_id 
------------------------ ---- ------------- ----- -------- ---------- ---------- 
1/1                     1    not-connected     4       32 disabled   0         
1/2                     1    not-connected     4       32 disabled   0         
2/1                     2    not-connected   100       32 disabled   0         
2/2                     2    not-connected   100       32 disabled   0         
2/3                     2    not-connected   100       32 disabled   0         
2/4                     2    not-connected   100       32 disabled   0         
2/5                     2    not-connected   100       32 disabled   0         
2/6                     1    forwarding       19       32 disabled   0         
2/7                     1    not-connected   100       32 disabled   0         
2/8                     1    not-connected   100       32 disabled   0         
2/9                     1    blocking         19       32 disabled   0         
2/9                     2    forwarding       19       32 disabled   0         
2/9                     3    forwarding       19       32 disabled   0         
2/9                     1003 not-connected    19       32 disabled   0         
2/9                     1005 not-connected    19        4 disabled   0         
2/10                    1    blocking         19       32 disabled   0         
2/10                    2    forwarding       19       32 disabled   0         
2/10                    3    blocking         19       32 disabled   0         
2/10                    1003 not-connected    19       32 disabled   0         
2/10                    1005 not-connected    19        4 disabled   0
2/11                    2    forwarding      100       32 enabled    0         
2/12                    1    not-connected   100       32 disabled   0         
15/1                    1    forwarding        5       32 disabled   0         
15/1                    2    forwarding        5       32 disabled   0         

Only enable UplinkFast on leaf-node switches. Leaf-node switches are closet switches to which users directly connect. UplinkFast is an STP optimization that is meant only for uplink ports to the distribution or core layer of the network. Here is sample command output:

Switch_1> (enable) set spantree uplinkfast enable 
VLANs 1-1005 bridge priority set to 49152. 
The port cost and portvlancost of all ports set to above 3000. 
Station update rate set to 15 packets/100ms. 
uplinkfast all-protocols field set to off. 
uplinkfast enabled for bridge. 
  
Switch_1> (enable) show spantree uplinkfast 
Station update rate set to 15 packets/100ms. 
uplinkfast all-protocols field set to off. 
  
VLAN          port list 
----------------------------------------------- 
1             2/2(fwd) ,2/5-6 
2             2/5(fwd) ,2/6

Configure BackboneFast on all switches in the network. BackboneFast is an STP optimization that alters the Max Age timer at the receipt of an inferior BPDU that the designated bridge sends. Here is sample command output:

Switch_1> (enable) set spantree backbonefast enable 
Backbonefast enabled for all VLANs
Switch_1> (enable) show spantree backbonefast 
Backbonefast is enabled. 

Refer to Configuring Spanning Tree PortFast, UplinkFast, BackboneFast, and Loop Guard for more information about these CatOS features.

8. BPDU Guard

When you enable PortFast BPDU guard, a nontrunking, PortFast-enabled port is moved into an errdisable state at the receipt of a BPDU on that port. This feature helps you find ports that are incorrectly configured for PortFast. The feature also detects where devices may reflect packets or interject STP BPDUs into the network. When you troubleshoot STP issues, enable this feature on all ports. Here is an example on CatOS:

Switch_1>(enable) set spantree portfast bpdu-quard enable
Spantree PortFast bpdu-guard enabled on this switch.

9. VTP Pruning

When VTP Pruning is enabled in the network, it can cause the devices of an HSRP group to go active. This results in IP conflicts among the gateways and cause traffic issues. Make sure the VLAN of any HSRP group is not pruned by VTP in the network.

F. CGMP Leave Processing and HSRP Interoperability

HSRP communicates to the destination MAC address of 01-00-5e-00-00-02, which is the same destination MAC address that IGMP fast-leave processing uses. IGMP fast-leave processing is an IGMP Version 2 feature. With CGMP leave processing enabled on Cisco switches, all multicast traffic with the destination MAC address of 01-00-5e-00-00-02 is forwarded to the switch CPU. If the packet is not an IGMP message, the switch CPU regenerates the packet and sends the packet to all router ports. Because HSRP uses the same destination multicast address, all HSRP packets must first be sent to the switch CPU, which then regenerates and sends the packets to all router ports. Therefore, when you troubleshoot HSRP problems, disable CGMP leave processing between HSRP peers.

Note: The use of IGMP snooping on the Catalyst 6500 and 5500 with NetFlow Feature Card (NFFC) II does not have this issue.

In order to determine if CGMP leave processing is enabled on CatOS switches, issue the show cgmp leave command. Here is an example:

Switch> (enable) show cgmp leave
CGMP: disabled
CGMP leave: disabled
For Catalyst 2900XL/3500XL switches, issue the show cgmp state command: 

s-2924xl-27a#show cgmp state
CGMP is running.
CGMP Fast Leave is not running.
Default router timeout is 300 sec.

G. Divide and Conquer

If all other attempts to isolate or resolve HSRP fail, the "divide and conquer" method is the next approach. This method helps isolate the network and components that make up the network. Divide and conquer involves any one of the guidelines in this list:

Note: This list repeats some guidelines from other sections of this document.

  • Create a test VLAN for HSRP and isolated VLAN to switch with HSRP routers.

  • Disconnect all redundant ports.

  • Break FEC ports into single connected ports.

  • Reduce HSRP group members to only two members.

  • Prune trunk ports such that only necessary VLANs propagate across those ports.

  • Disconnect connected switches in the network until the problems cease.

H. High CPU with Asymmetric Traffic in HSRP

CPU usage might run high as traffic flows from a POS interface to a gigabit ethernet interface in an HSRP asymmetric environment. Packets become fragmented as POS MTU size is 4470 bytes and Gig MTU size is 1500 bytes. Fragmentation consumes more CPU.

In order to resolve this issue, execute one of these commands:


!--- On the gigabit interface

mtu 4770

or


!--- On the POS interface

ip tcp adjust-mss 1460

Known Issues

Number of HSRP Groups Supported for Catalyst 6500/6000 Series PFC2/MSFC2 and Catalyst 3550

The Policy Feature Card 2 (PFC2)/MSFC2 for the Catalyst 6500/6000 series supports a maximum of 16 unique HSRP groups. If you need more than 16 HSRP groups, you can reuse the same HSRP group numbers in different VLANs. For more information on HSRP group limitations for the Catalyst 6500/6000 series, refer to HSRP Group Limitation on Catalyst 6500/6000 Series Switches Frequently Asked Questions.

A similar limitation exists for the Catalyst 3550 series, which supports a maximum of 16 HSRP groups. This is a hardware limitation and there is no workaround.

HSRP State Flapping/Unstable When You Use Cisco 2620/2621, Cisco 3600 with Fast Ethernet, or PA-2FEISL

This problem can occur with Fast Ethernet interfaces at the disruption of network connectivity or at the addition of an HSRP router with higher priority to a network. When the HSRP state changes from active to speaking, the router resets the interface in order to remove the HSRP MAC address from the interfaces MAC address filter. Only specific hardware that is used on the Fast Ethernet interfaces for Cisco 2600s, 3600s, and 7500s have this issue. The router interface reset causes a link state change on Fast Ethernet interfaces, and the switch detects the change. If the switch runs STP, the change causes an STP transition. The STP takes 30 seconds to transition the port into the forwarding state. This time is twice the default forward delay time of 15 seconds. At the same time, the speaking router transitions to the standby state after 10 seconds, which is the HSRP hold time. STP is not forwarding yet, so no HSRP hello messages are received from the active router. This causes the standby router to become active after about 10 seconds. Both routers are now active. When the STP ports become forwarding, the lower-priority router changes from active to speaking, and the whole process repeats.

Platform Description Cisco Bug ID Fix Workaround
Cisco 2620/2621 Fast Ethernet interface starts to flap when HSRP is configured and the cable is unplugged. CSCdp57792 (registered customers only) A software upgrade; refer to the bug for revision details. Enables spanning tree PortFast on the connected switch port.
Cisco 2620/2621 HSRP state is flapping on 2600 with Fast Ethernet. CSCdr02376 (registered customers only) Cisco IOS Software Release 12.1.3 Enables spanning tree PortFast on the connected switch port.
Cisco 3600 with NM-1FE-TX1 HSRP state is flapping on 2600 and 3600 Fast Ethernet. CSCdr02376 (registered customers only) Cisco IOS Software Release 12.1.3 Enables spanning tree PortFast on the connected switch port.
Cisco 4500 with Fast Ethernet interface HSRP state is flapping on 4500 Fast Ethernet. CSCds16055 (registered customers only) Cisco IOS Software Release 12.1.5 Enables spanning tree PortFast on the connected switch port.
Cisco 7200/7500 with PA-2FEISL2 HSRP state is flapping on PA-2FEISL. CSCdm89593 (registered customers only) Cisco IOS Software Release 12.1.5 Enables spanning tree PortFast on the connected switch port.

1NM-1FE-TX = one-port Fast Ethernet (10/100BASE-TX interface) network module.

1 PA-2FEISL = two-port Fast Ethernet InterSwitch Link [ISL] port adapter.

An alternative workaround is to adjust the HSRP timers so that the STP forward delay is less than half of the default HSRP hold time. The default STP forward delay is 15 seconds, and the default HSRP hold time is 10 seconds.

When you use the track command under the HSRP process, Cisco recommends that you use a particular decrement value in order to avoid the HSRP flap.

Here is a sample configuration in an HSRP active router when you use the track command:

standby 1 ip 10.0.0.1
 standby 1 priority 105
 standby 1 preempt delay minimum 60
 standby 1 name TEST
 standby 1 track Multilink100 15

Where 15 is the decrement value when the multilink100 flaps.

HSRP Stuck in Initial or Active State on Cisco 2620/2621, Cisco 3600 with Fast Ethernet, or PA-2FEISL

Fast Ethernet interfaces on the Cisco 2600, 3600, and 7200 routers can experience these issues when HSRP is configured:

  • HSRP remains in active state when the interface goes down or is unplugged.

  • HSRP remains in initial state when the interface goes up.

  • Interface tracking does not work.

A timing-sense problem of interface up/down causes these HSRP issues. The timing problem is that there is a delay between the occurrence of the interface event and the update of the interface state of the router.

Platform Description Cisco Bug ID Fix Workaround
Cisco 2620/2621 HSRP gets stuck in initial state. CSCdp24680 (registered customers only) A software upgrade; refer to the bug for revision details. Issue the shutdown and no shutdown commands in order to reset the interface.
Cisco 3600 with NM-1FE-TX HSRP gets stuck in initial state on the NM-1FE-TX module in 3600. CSCdp24680 (registered customers only) A software upgrade; refer to the bug for revision details. Issue the shutdown and no shutdown commands in order to reset the interface.
Cisco 7200/7500 with PA-2FEISL HSRP gets stuck in initial state on the PA-2FEISL module in 7200/7500. CSCdr01156 (registered customers only) A software upgrade; refer to the bug for revision details. Issue the shutdown and no shutdown commands in order to reset the interface.

Unable to Ping HSRP Standby Address on Cisco 2500 and 4500 Series Routers

62e.gif

In this diagram, Router A represents a Cisco 2500 series router, and Router B represents a Cisco 4500 series router. If Router A pings the virtual IP address on LAN 1, 10.1.1.1, the router first sends out an ARP request. Router B responds with an ARP reply that contains the virtual MAC address. Router B ignores this ARP reply because the virtual MAC address is the same as the Router B E1 interface address.

There is a known restriction with the 10 MB Ethernet controller on the Cisco 2500 and 4500 series routers. The Ethernet controller only supports a single MAC address in its address filter. As a result, only one HSRP group can be configured in an interface. The HSRP MAC address is also used as the interface MAC address. This causes problems when the same HSRP group is configured on different Ethernets on the same router. The show standby command shows use of the MAC address as the HSRP MAC address.

There are two workarounds to this issue:

  • Configure different HSRP groups on different interfaces.

    Note: This workaround is recommended.

  • Issue the standby use-bia command on one or both interfaces.

MLS Flows Are Not Created for Devices That Use HSRP Standby IP Address as Default Gateway

MLS switching can fail when HSRP is enabled and you use Cisco IOS Software Release 12.1(4)E on one of these:

  • Supervisor Engine 1/MSFC1

  • Supervisor Engine 2/MSFC2

  • Supervisor Engine 1/MSFC2

The symptoms are different for each combination, as this list shows:

  • For Supervisor Engine 1/MSFC1 and Supervisor Engine 1/MSFC2 (which use Netflow-MLS)—The MLS shortcuts can fail to be created when traffic is sent to an HSRP MAC address. Any client that uses the HSRP standby IP address as the default gateway uses the HSRP MAC address.

  • For Supervisor Engine 2/MSFC2 (which uses Cisco Express Forwarding-MLS)—The Cisco Express Forwarding adjacency table can fail to be populated correctly on the switch.

Refer to Cisco bug ID CSCds89040 (registered customers only) . The fix is available with Cisco IOS Software Release 12.1(5a)E for the CatOS (c6msfc) images, and with Cisco IOS Software Release 12.1(5a)E1 for the Cisco IOS Software (c6sup) images.

Catalyst 2948G, 2980G, 4912G, 4003, and 4006 HSRP-CGMP Interoperability Issues

The Catalyst 4000 product line (2948G, 2980G, 4912G, 4003, and 4006) software has several issues that relate to HSRP and CGMP interoperability. All the issues are resolved in software versions 6.3.6 and 7.2.1.

Enablement of CGMP can cause problems with HSRP. This problem is resolved in software release 6.3(6). A router in HSRP standby status is changed to active status. When the status is restored, the router does not go back to standby status from active status. This problem is resolved in software release 6.3(6).

If you run HSRP and have enabled CGMP leave, McastRx use can show at 25 percent CPU usage. This problem occurs because CGMP leave and HSRP hello packets share the same destination MAC address. The problem is resolved in software release 6.3(6).

Related Information

Updated: May 05, 2009
Document ID: 10583