Cisco 7600 Series Router Software Configuration Guide, Cisco IOS Release 15S
Configuring Flex Links
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Table of Contents

Configuring Flex Links

Understanding Flex Links

Configuring Flex Links

Flex Links Default Configuration

Flex Links Configuration Guidelines and Restrictions

Configuring Flex Links

Monitoring Flex Links

Configuring Flex Links

This chapter describes how to configure Flex Links on the Cisco 7600 series router.


Note For complete syntax and usage information for the commands used in this chapter, refer to the Cisco 7600 Series Routers Command References at this URL:

http://www.cisco.com/en/US/products/hw/routers/ps368/prod_command_reference_list.html


 

The chapter consists of these sections:

Understanding Flex Links

Flex Links are a pair of a Layer 2 interfaces (switchports or port channels), where one interface is configured to act as a backup to the other. Flex Links are typically configured in service-provider or enterprise networks where customers do not want to run STP. Flex Links provide link-level redundancy that is an alternative to Spanning Tree Protocol (STP). STP is automatically disabled on Flex Links interfaces.

To configure the Flex Links feature, you configure one Layer 2 interface as the standby link for the link that you want to be primary. With Flex Links configured for a pair of interfaces, only one of the interfaces is in the linkup state and is forwarding traffic. If the primary link shuts down, the standby link starts forwarding traffic. When the inactive link comes back up, it goes into standby mode.

In Figure 11-1, ports 1 and 2 on router A are connected to uplink routers B and C. Because they are configured as Flex Links, only one of the interfaces is forwarding traffic and the other one is in standby mode. If port 1 is the active link, it begins forwarding traffic between port 1 and router B; the link between port 2 (the backup link) and router C is not forwarding traffic. If port 1 goes down, port 2 comes up and starts forwarding traffic to router C. When port 1 comes back up, it goes into standby mode and does not forward traffic; port 2 continues to forward traffic.

Figure 11-1 Flex Links Configuration Example

 

If a primary (forwarding) link goes down, a trap notifies the network management stations. If the standby link goes down, a trap notifies the users.

Flex Links are supported only on Layer 2 ports and port channels, not on VLANs or on Layer 3 ports.

Configuring Flex Links

These sections contain this configuration information:

Flex Links Default Configuration

There is no default Flex Links configuration.

Flex Links Configuration Guidelines and Restrictions

When configuring Flex Links, follow these guidelines and restrictions:

  • You can configure only one Flex Links backup link for any active link, and it must be a different interface from the active interface.
  • An interface can belong to only one Flex Links pair. An interface can be a backup link for only one active link. An active link cannot belong to another Flex Links pair.
  • Neither of the links can be a port that belongs to an EtherChannel. However, you can configure two port channels (EtherChannel logical interfaces) as Flex Links, and you can configure a port channel and a physical interface as Flex Links, with either the port channel or the physical interface as the active link.
  • A backup link does not have to be the same type as the active link (Fast Ethernet, Gigabit Ethernet, or port channel). However, you should configure both Flex Links with similar characteristics so that there are no loops or changes in operation if the standby link becomes active.
  • STP is disabled on Flex Links ports. If STP is disabled on the router, be sure that there are no Layer 2 loops in the network topology.
  • Do not configure any STP features (for example, PortFast, BPDU Guard, and so forth) on Flex Links ports or the ports to which the links connect.

Configuring Flex Links

To configure Flex Links, perform this task:

 

Command
Purpose

Step 1

Router# configure terminal

Enters global configuration mode.

Step 2

Router(conf)# interface {{ type 1 slot/port } | { port-channel number }}

Specifies a Layer 2 interface.

Step 3

Router(conf-if)# switchport backup interface {{ type 1 slot/port } | { port-channel number }}

Configures the interface as part of a Flex Links pair.

Step 4

Router(conf-if)# exit

Exits configuration mode.

Step 5

Router# show interface [{ type 1 slot/port } | { port-channel number }] switchport backup

Verifies the configuration.

Step 6

Router# copy running-config startup config

(Optional) Saves your entries in the router startup configuration file.

1.type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet


Note If you configure the switchport interface using the switchport backup interface command, and then pass traffic to the backup interface, then the ingress counters will increment on the backup interface although the actual traffic does not pass through it.


This example shows how to configure an interface with a backup interface and how to verify the configuration:

Router# configure terminal
Router(conf)# interface fastethernet1/1
Router(conf-if)# switchport backup interface fastethernet1/2
Router(conf-if)# exit
Router# show interface switchport backup
Router Backup Interface Pairs:
 
Active Interface Backup Interface State
------------------------------------------------------------------------------------------
FastEthernet1/1 FastEthernet1/2 Active Up/Backup Standby
FastEthernet1/3 FastEthernet2/4 Active Up/Backup Standby
Port-channel1 GigabitEthernet7/1 Active Up/Backup Standby

Monitoring Flex Links

Table 11-1 shows the privileged EXEC command for monitoring the Flex Links configuration.

 

Table 11-1 Flex Links Monitoring Command

Command
Purpose

show interface [{ type 2 slot/port } | { port-channel number }] switchport backup

Displays the Flex Links backup interface configured for an interface, or displays all Flex Links configured on the router and the state of each active and backup interface (up or standby mode).

2.type = ethernet, fastethernet, gigabitethernet, or tengigabitethernet