Flexlink+ is supported only on Layer 2 trunk ports and port channels, not on VLANs or on Layer 3 ports.
Information about Flexlink+
The following sections provide information about Flexlink+
The Flexlink+ feature enables the user to configure a pair of a Layer 2 interfaces (trunk ports or port channels) where one
interface is configured to act as a backup to the other. The feature provides an alternative solution to the Spanning Tree
Protocol (STP). Users can disable STP and still retain basic link redundancy. Flexlinks are typically configured in service
provider or enterprise networks where customers do not want to run STP on the device. If the device is running STP, Flexlinks
are not necessary because STP already provides link-level redundancy or backup.
On devices, Flexlink+ can be on the same device or on another device in the stack. When one of the links is up and forwarding
traffic, the other link is in standby mode, ready to begin forwarding traffic if the active link shuts down. If the primary
link shuts down, the standby link starts forwarding traffic. When the active link comes back up, it goes into standby mode
and does not forward traffic.
In the following figure, ports 1 and 2 on switch A are connected to uplink switches B and C. Because they are configured with
Flexlink+, only one of the interfaces is forwarding traffic; the other is in standby mode. If port 1 is the active link, it
begins forwarding traffic between port 1 and switch B; the link between port 2 (the backup link) and switch C is not forwarding
traffic. If port 1 goes down, port 2 comes up and starts forwarding traffic to switch C. When port 1 comes back up, it goes
into standby mode and does not forward traffic; port 2 continues forwarding traffic.
If STP is configured on the uplink switch interfaces that connect to the Flexlink+ ports (Switch B and Switch C in this case),
we recommend running the spanning-tree portfast trunk command on such uplink switches, for better convergence values.
Flexlink+ includes an optimization for improved multicast traffic convergence. The optimization uses Layer 2 multicast snooping
mechanisms and requires that the uplink switches connected to the Flexlink+ configured ports have the same Layer 2 multicast
snooping feature enabled.
For IPv4 multicast IGMP snooping is on by default. If IGMP snooping needs to be disabled on the uplink switches it must also
be disabled on the Flexlink+ host switch. Otherwise IGMP reports may be looped around the active and standby Flexlink+ ports
leading to excessively high CPU utilization.
How to configure Flexlink+
The following sections provide information on how to configure Flexlink+.
Configuring the active port for Flexlink+
Command or Action
Enables privileged EXEC mode.
Enter your password if prompted.
Device# configure terminal
Enters global configuration mode.
Device# interface Port-channel2
Specifies the interface, and enters interface configuration mode.
The following table provides release information about the feature or features described in this module. This table lists
only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise,
subsequent releases of that software release train also support that feature.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco
Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.