Flex links are Layer-2 interfaces manually configured in primary/failover pairs. The Spanning Tree Protocol (STP, discussed in Chapter 8) normally provides primary/failover functionality, but it was designed for the sole purpose of preventing loops. Flex links ensure that there are backup links for primary links. Only one of the links in a flex-link pair will be forwarding traffic at any time.
Flex links are designed for, and should be used only on, switches that do not run spanning tree. Should flex links be configured on a switch running spanning tree, the flex links will not participate in STP.
Flex links are configured on the primary interface by specifying the backup interface with the switchport backup interface command:
Neither of the links can be an interface that is a member of an EtherChannel. An EtherChannel can be a flex-link backup for another port channel. A single physical interface can be a backup to an EtherChannel as well. The backup link does not need to be the same type of interface as the primary. For example, a 100 Mbps interface can be a backup for a 1 Gbps interface.
Monitor flex links with the show interface switchport backup command:
Flex Links are a pair of a Layer 2 interfaces (ports 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.