Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Configuration Guide - Configuring ARP on Cisco IOS XR Software for Cisco ASR 9000 Series Routers [Cisco ASR 9000 Series Aggregation Services Routers]

Table Of Contents

Configuring ARP on Cisco ASR 9000 Series Routers

Contents

Prerequisites for Configuring ARP on Cisco ASR 9000 Series Routers

Restrictions for Configuring ARP on Cisco ASR 9000 Series Routers

Information About Configuring ARP on Cisco ASR 9000 Series Routers

IP Addressing Overview

Address Resolution on a Single LAN

Address Resolution When Interconnected by a Router

ARP and Proxy ARP

ARP Cache Entries

Direct Attached Gateway Redundancy

Additional Guidelines

How to Configure ARP on Cisco ASR 9000 Series Routers

Defining a Static ARP Cache Entry

Enabling Proxy ARP

Configuring DAGR

Configuration Examples for ARP Configuration on Cisco IOS XR Software

Creating a Static ARP Cache Entry: Example

Enabling Proxy ARP: Example

Displaying the ARP Table: Example

Enabling DAGR and Configuring a DAGR Group: Example

Displaying the Operational State of DAGR Groups: Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Configuring ARP on Cisco ASR 9000 Series Routers

Address resolution is the process of mapping network addresses to Media Access Control (MAC) addresses. This process is accomplished using the Address Resolution Protocol (ARP). This module describes how to configure ARP processes on the Cisco ASR 9000 Series Aggregation Services Router.

Note For a complete description of the ARP commands listed in this module, refer to the Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Command Reference publication. To locate documentation of other commands that appear in this module, use the command reference master index, or search online.

Feature History for Configuring ARP on Cisco ASR 9000 Series Routers

Release

Modification

Release 3.7.2

This feature was introduced on Cisco ASR 9000 Series Routers.

Contents

•Prerequisites for Configuring ARP on Cisco ASR 9000 Series Routers

•Restrictions for Configuring ARP on Cisco ASR 9000 Series Routers

•Information About Configuring ARP on Cisco ASR 9000 Series Routers

•How to Configure ARP on Cisco ASR 9000 Series Routers

•Configuration Examples for ARP Configuration on Cisco IOS XR Software

•Additional References

Prerequisites for Configuring ARP on Cisco ASR 9000 Series Routers

You must be in a user group associated with a task group that includes the proper task IDs. The command reference guides include the task IDs required for each command. If you suspect user group assignment is preventing you from using a command, contact your AAA administrator for assistance.

Restrictions for Configuring ARP on Cisco ASR 9000 Series Routers

The following restrictions apply to configuring ARP on Cisco ASR 9000 Series Routers:

•Reverse Address Resolution Protocol (RARP) is not supported.

•ARP throttling is not supported.

Note ARP throttling is the rate limiting of ARP packets in Forwarding Information Base (FIB).

The following additional restrictions apply when configuring the Direct Attached Gateway Redundancy (DAGR) feature on Cisco ASR 9000 Series Routers:

•IPv6 is not supported.

•Ethernet bundles are not supported.

•Non-Ethernet interfaces are not supported.

•Hitless ARP Process Restart is not supported.

•Hitless RSP Failover is not supported.

Information About Configuring ARP on Cisco ASR 9000 Series Routers

To configure ARP on Cisco ASR 9000 Series Routers, you must understand the following concepts:

•IP Addressing Overview

•Address Resolution on a Single LAN

•Address Resolution When Interconnected by a Router

•ARP and Proxy ARP

•ARP Cache Entries

•Direct Attached Gateway Redundancy

IP Addressing Overview

A device in the IP can have both a local address (which uniquely identifies the device on its local segment or LAN) and a network address (which identifies the network to which the device belongs). The local address is more properly known as a data link address, because it is contained in the data link layer (Layer 2 of the OSI model) part of the packet header and is read by data-link devices (bridges and all device interfaces, for example). The more technically inclined person will refer to local addresses as MAC addresses, because the MAC sublayer within the data link layer processes addresses for the layer.

To communicate with a device on Ethernet, for example, Cisco IOS XR software first must determine the 48-bit MAC or local data-link address of that device. The process of determining the local data-link address from an IP address is called address resolution.

Address Resolution on a Single LAN

The following process describes address resolution when the source and destination devices are attached to the same LAN:

1. End System A broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System B.

2. The broadcast is received and processed by all devices on the LAN, including End System B.

3. Only End System B replies to the ARP request. It sends an ARP reply containing its MAC address to End System A.

4. End System A receives the reply and saves the MAC address of End System B in its ARP cache. (The ARP cache is where network addresses are associated with MAC addresses.)

5. Whenever End System A needs to communicate with End System B, it checks the ARP cache, finds the MAC address of System B, and sends the frame directly, without needing to first use an ARP request.

Address Resolution When Interconnected by a Router

The following process describes address resolution when the source and destination devices are attached to different LANs that are interconnected by a router (only if proxy-arp is turned on):

1. End System Y broadcasts an ARP request onto the LAN, attempting to learn the MAC address of End System Z.

2. The broadcast is received and processed by all devices on the LAN, including Router X.

3. Router X checks its routing table and finds that End System Z is located on a different LAN.

4. Router X therefore acts as a proxy for End System Z. It replies to the ARP request from End System Y, sending an ARP reply containing its own MAC address as if it belonged to End System Z.

5. End System Y receives the ARP reply and saves the MAC address of Router X in its ARP cache, in the entry for End System Z.

6. When End System Y needs to communicate with End System Z, it checks the ARP cache, finds the MAC address of Router X, and sends the frame directly, without using ARP requests.

7. Router X receives the traffic from End System Y and forwards it to End System Z on the other LAN.

ARP and Proxy ARP

Two forms of address resolution are supported by Cisco IOS XR software: Address Resolution Protocol (ARP) and proxy ARP, as defined in IETF RFC 826 and IETF RFC 1027, respectively.

ARP is used to associate IP addresses with media or MAC addresses. Taking an IP address as input, ARP determines the associated media address. After a media or MAC address is determined, the IP address or media address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network.

When proxy ARP is disabled, the networking device responds to ARP requests received on an interface only if one of the following conditions is met:

•The target IP address in the ARP request is the same as the interface IP address on which the request is received.

•The target IP address in the ARP request has a statically configured ARP alias.

When proxy ARP is enabled, the networking device also responds to ARP requests that meet all the following conditions:

•The target IP address is not on the same physical network (LAN) on which the request is received.

•The networking device has one or more routes to the target IP address.

•All of the routes to the target IP address go through interfaces other than the one on which the request is received.

ARP Cache Entries

ARP establishes correspondences between network addresses (an IP address, for example) and Ethernet hardware addresses. A record of each correspondence is kept in a cache for a predetermined amount of time and then discarded.

You can also add a static (permanent) entry to the ARP cache that persists until expressly removed.

Direct Attached Gateway Redundancy

Direct Attached Gateway Redundancy (DAGR) allows third-party redundancy schemes on connected devices to use gratuitous ARP as a failover signal, enabling the ARP process to advertise an new type of route in the Routing Information Base (RIB). These routes are distributed by Open Shortest Path First (OSPF).

Sometimes part of an IP network requires redundancy without routing protocols. A prime example is in the mobile environment, where devices such as base station controllers and multimedia gateways are deployed in redundant pairs, with aggressive failover requirements (subsecond or less), but typically do not have the capability to use native Layer 3 protocols such as OSPF or Intermediate System-to-Intermediate System (IS-IS) protocol to manage this redundancy. Instead, these devices assume they are connected to adjacent IP devices over an Ethernet switch, and manage their redundancy at Layer 2, using proprietary mechanisms similar to Virtual Router Redundancy Protocol (VRRP). This requires a resilient Ethernet switching capability, and depends on mechanisms such as MAC learning and MAC flooding.

DAGR is a feature that enables many of these devices to connect directly to Cisco ASR 9000 Series Routers without an intervening Ethernet switch. DAGR enables the subsecond failover requirements to be met using a Layer 3 solution. No MAC learning, flooding, or switching is required.

Note Since mobile devices' 1:1 Layer 2 redundancy mechanisms are proprietary, they do not necessarily conform to any standard. So although most IP mobile equipment is compatible with DAGR, interoperability does require qualification, due to the possibly proprietary nature of the Layer 2 mechanisms with which DAGR interfaces.

Additional Guidelines

The following are additional guidelines to consider when configuring DAGR:

•Up to 40 DAGR peers, which may be on the same or different interfaces, are supported per system.

•Failover is supported for DAGR routes within 500 ms of receipt of an ARP reply packet.

•On ARP process restart, DAGR groups are reinitialized.

How to Configure ARP on Cisco ASR 9000 Series Routers

This section contains instructions for the following tasks:

•Defining a Static ARP Cache Entry (optional)

•Enabling Proxy ARP (optional)

•Configuring DAGR (optional)

Defining a Static ARP Cache Entry

ARP and other address resolution protocols provide a dynamic mapping between IP addresses and media addresses. Because most hosts support dynamic address resolution, generally you do not need to specify static ARP cache entries. If you must define them, you can do so globally. Performing this task installs a permanent entry in the ARP cache. Cisco IOS XR software uses this entry to translate 32-bit IP addresses into 48-bit hardware addresses.

Optionally, you can specify that the software responds to ARP requests as if it were the owner of the specified IP address by making an alias entry in the ARP cache.

SUMMARY STEPS

1. configure

2. arp [vrf vrf-name] ip-address hardware-address encapsulation-type
or
arp [vrf vrf-name] ip-address hardware-address encapsulation-type [alias]

3. end
or
commit

DETAILED STEPS
Command or Action

Purpose

Step 1

configure
Example:

RP/0/RSP0/CPU0:router# configure

Enters global configuration mode.

Step 2

arp [vrf vrf-name] ip-address hardware-address encapsulation-type
or

arp [vrf vrf-name] ip-address hardware-address encapsulation-type alias
Example:

RP/0/RSP0/CPU0:router(config)# arp 192.168.7.19 0800.0900.1834 arpa
or

RP/0/RSP0/CPU0:router(config)# arp 192.168.7.19 0800.0900.1834 arpa alias

Creates a static ARP cache entry associating the specified 32-bit IP address with the specified 48-bit hardware address.

Note If an alias entry is created, then any interface to which the entry is attached will act as if it is the owner of the specified addresses, that is, it will respond to ARP request packets for this network layer address with the data link layer address in the entry.
Step 3

end
or

commit
Example:

RP/0/RSP0/CPU0:router(config)# end
or

RP/0/RSP0/CPU0:router(config)# commit
Saves configuration changes.

•When you issue the end command, the system prompts you to commit changes:
Uncommitted changes found, commit them before 
exiting(yes/no/cancel)? 
[cancel]:
–Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

–Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

–Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

•Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
Enabling Proxy ARP

Cisco IOS XR software uses proxy ARP (as defined in RFC 1027) to help hosts with no knowledge of routing determine the media addresses of hosts on other networks or subnets. For example, if the router receives an ARP request for a host that is not on the same interface as the ARP request sender, and if the router has all of its routes to that host through other interfaces, then it generates a proxy ARP reply packet giving its own local data-link address. The host that sent the ARP request then sends its packets to the router, which forwards them to the intended host. Proxy ARP is disabled by default; this task describes how to enable proxy ARP.

SUMMARY STEPS

1. configure

2. interface type number

3. proxy-arp

4. end
or
commit

DETAILED STEPS
Command or Action

Purpose

Step 1

configure
Example:

RP/0/RSP0/CPU0:router# configure

Enters global configuration mode.

Step 2

interface type number
Example:

RP/0/RSP0/CPU0:router(config)# interface MgmtEth 0/RSP0/CPU0/0

Enters interface configuration mode.

Step 3

proxy-arp
Example:

RP/RSP0/CPU0:router(config-if)# proxy-arp

Enables proxy ARP on the interface.
Step 4

end
or

commit
Example:

RP/0/RSP0/CPU0:router(config-if)# end
or

RP/0/RSP0/CPU0:router(config-if)# commit
Saves configuration changes.

•When you issue the end command, the system prompts you to commit changes:
Uncommitted changes found, commit them before 
exiting(yes/no/cancel)? 
[cancel]:
–Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

–Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

–Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

•Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
Configuring DAGR

Follow these steps to create a DAGR group on the Cisco ASR 9000 Series Router.

SUMMARY STEPS

1. configure

2. interface type instance

3. arp dagr

4. peer ipv4 address

5. route distance normal normal-distance priority priority-distance

6. route metric normal normal-metric priority priority-metric

7. timers query query-time standby standby-time

8. priority-timeout time

9. end
or
commit

10. show arp dagr [interface [IP-address]]

DETAILED STEPS
Command or Action

Purpose

Step 1

configure
Example:

RP/0/RSP0/CPU0:router# configure

Enters global configuration mode.

Step 2

interface type instance
Example:

RP/0/RSP0/CPU0:router(config)# interface gigabitethernet 0/2/0/0

Enters interface configuration mode and configures an interface.

Step 3

arp dagr
Example:

RP/0/RSP0/CPU0:router(config-if)# arp dagr

Enters DAGR configuration mode.

Step 4

peer ipv4 address
Example:

RP/0/RSP0/CPU0:router(config-if-dagr)# peer ipv4 10.0.0.100

Creates a new DAGR group for the virtual IP address.

Step 5

route distance normal normal-distance priority priority-distance
Example:

RP/0/RSP0/CPU0:router(config-if-dagr-peer)# route distance normal 140 priority 3

(Optional) Configures route distance for the DAGR group.

Step 6

route metric normal normal-metric priority priority-metric
Example:

RP/0/RSP0/CPU0:router(config-if-dagr-peer)# route metric normal 84 priority 80

(Optional) Configures the route metric for the DAGR group.

Step 7

timers query query-time standby standby-time
Example:

RP/0/RP1/CPU0:router(config-if-dagr-peer)# timers query 2 standby 19

(Optional) Configures the time in seconds between successive ARP requests being sent out for the virtual IP address.

Step 8

priority-timeout time
Example:

RP/0/RSP0/CPU0:router(config-if-dagr-peer)# priority-timeout 25

(Optional) Configures a timer for the length of time in seconds to wait before reverting to normal priority from a high-priority DAGR route.
Step 9

end
or

commit
Example:

RP/0/RSP0/CPU0:router(config-if-dagr)# end
or

RP/0/RSP0/CPU0:router(config-if-dagr)# commit
Saves configuration changes.

•When you issue the end command, the system prompts you to commit changes:
Uncommitted changes found, commit them before 
exiting(yes/no/cancel)? 
[cancel]:
–Entering yes saves configuration changes to the running configuration file, exits the configuration session, and returns the router to EXEC mode.

–Entering no exits the configuration session and returns the router to EXEC mode without committing the configuration changes.

–Entering cancel leaves the router in the current configuration session without exiting or committing the configuration changes.

Use the commit command to save the configuration changes to the running configuration file and remain within the configuration session.
Step 10

show arp dagr [interface [IP-address]]
Example:

RP/0/RSP0/CPU0:router# show arp dagr

(Optional) Displays the operational state of all DAGR groups. Using the optional interface and IP-address arguments restricts the output to a specific interface or virtual IP address.
Configuration Examples for ARP Configuration on Cisco IOS XR Software

This section provides the following ARP configuration examples:

•Creating a Static ARP Cache Entry: Example

•Enabling Proxy ARP: Example

•Displaying the ARP Table: Example

•Enabling DAGR and Configuring a DAGR Group: Example

•Displaying the Operational State of DAGR Groups: Example

Creating a Static ARP Cache Entry: Example

The following is an example of a static ARP entry for a typical Ethernet host:
configure
arp 192.168.7.19 0800.0900.1834 arpa
The following is an example of a static ARP entry for a typical Ethernet host where the software responds to ARP requests as if it were the owner of both the specified IP address and hardware address, whether proxy ARP is enabled or not:
configure
arp 192.168.7.19 0800.0900.1834 arpa alias
The following is an example of configuring a static ARP entry on an SRP device:
configure
arp 192.168.8.20 0800.0900.1723 srp
Enabling Proxy ARP: Example

The following is an example of enabling proxy ARP:
configure
interface MgmtEth 0/RSP0/CPU0/0
proxy-arp
Displaying the ARP Table: Example

The following example shows how to display the ARP table:
show arp
------------------------------------------------------------------------------
0/4/CPU0
------------------------------------------------------------------------------
Address         Age        Hardware Addr   State      Type  Interface
10.0.0.7        -          0014.a912.b7a0  Interface  ARPA  GigabitEthernet0/2/0/0
10.0.0.200      -          0001.0002.0003  Static     ARPA  GigabitEthernet0/2/0/0
10.0.0.201      -          0004.0005.0006  Alias      ARPA  GigabitEthernet0/2/0/0
10.0.0.252      00:28:20   0015.171f.9ef5  Dynamic    ARP!  GigabitEthernet0/2/0/0
Enabling DAGR and Configuring a DAGR Group: Example

The following is an example of enabling DAGR and configuring a DAGR group peer:
configure
 interface gigabitethernet 0/1/0/0.1
  arp dagr
   peer ipv4 192.168.7.19
    priority-timeout 25
    route distance normal 48 priority 5
    route metric normal 48 priority 5
    timers query 2 standby 40
    commit
Displaying the Operational State of DAGR Groups: Example

The following example shows how to display the current operational state of the DAGR groups:

RP/0/RSP0/CPU0:router# show arp dagr
--------------------------------------------------------------------------------
0/1/CPU0
--------------------------------------------------------------------------------
Interface                 Virtual IP      State     Query-pd Dist Metr
GigabitEthernet0/1/0/2     10.168.7.19    Active    None     150  100
GigabitEthernet0/1/0/2     10.24.0.45     Query     1        None None
GigabitEtherget0/1/0/3     10.66.0.45     Init      None     None None
Additional References

The following sections provide references related to ARP.

Related Documents

Related Topic

Document Title

Cisco ASR 9000 Series Router ARP commands

ARP Commands on Cisco ASR 9000 Series Routers module in Cisco ASR 9000 Series Aggregation Services Router IP Addresses and Services Command Reference

Cisco ASR 9000 Series Router getting started material

Cisco ASR 9000 Series Aggregation Services Router Getting Started Guide

Standards

Standards

Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—

MIBs

MIBs

MIBs Link

—

To locate and download MIBs using Cisco IOS XR software, use the Cisco MIB Locator found at the following URL and choose a platform under the Cisco Access Products menu: http://cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml

RFCs

RFCs

Title

RFC 826

Ethernet Address Resolution Protocol: Or converting network protocol addresses to 48.bit Ethernet address for transmission on Ethernet hardware

RFC 1027

Using ARP to implement transparent subnet gateways

Technical Assistance

Description

Link

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http://www.cisco.com/techsupport