Introduction
How to configure a Flex Link for link-level redundancy in Cat switches that run IOS.
Resolution
Flex Link is a Layer 2 availability feature that can co-exist with spanning tree. This enhancement allows a convergence time of less than 50 milliseconds. In addition, this convergence time remains consistent regardless of the number of VLANs or MAC addresses configured on switch uplink ports.
It is a pair of a Layer 2 interfaces, either switchports or port channels, that are configured to act as a backup to another Layer 2 interface. The feature provides an alternative solution to the spanning tree protocol (STP), and it allows users to turn off STP and still provide basic link redundancy.
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 switch, be sure that there are no Layer 2 loops in the network topology.
•Do not configure the following STP features on Flex Links ports or the ports to which the links connect:
–Bridge Assurance
–UplinkFast
–BackboneFast
–EtherChannel Guard
–Root Guard
–Loop Guard
–PVST Simulation
•Flex Links and port security are not compatible with each other.
•Static unicast MAC addresses that are configured on the primary link are not moved to the standby link.
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 {{type1 slot/port} | {port-channel number}} | Specifies a Layer 2 interface. |
| Step 3 | Router(conf-if)# switchport backup interface {{type1 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 [{type1 slot/port} | {port-channel number}] switchport backup | Verifies the configuration. |
| Step 6 | Router# copy running-config startup config | (Optional) Saves your entries in the switch startup configuration file. |
type = fastethernet, gigabitethernet, or tengigabitethernet
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
Reference
Catalyst 6500 Release 12.2SX Software Configuration Guide - Flex Links
