CISCO FLEX Configuration (L2 Failover)

Default Configuration

The Flex Links are not configured, and there are no backup interfaces defined.

The preemption mode is OFF.

The preemption delay is 35 seconds.

The MAC address-table move update feature is not configured on the switch.

Restrictions for Flex Links 

• 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 any STP features (for example, PortFast, BPDU Guard, and so forth) on Flex Links ports or ports to which the links connect. 

• Local administrative shut down or a link that starts forwarding again due to preemption is not considered a link failure. In those cases, the feature flushes the dynamic hosts and and does not move them. 

• Static MAC addresses that are configured on the primary link are not moved to the standby link. 

• Static MAC addresses configured on a flex links port are restored when it starts forwarding again.

Information About Flex Links 

Flex Links are a pair of 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. Flex Links provide link-level redundancy that is an alternative to Spanning Tree Protocol (STP). STP is automatically disabled on Flex Links interfaces. 

Release 15.0SY supports a maximum of 16 Flex Links. Flex Links are supported only on Layer 2 ports and port channels, not on VLANs or on Layer 3 ports. 

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.When a primary link fails, the feature takes these actions: 

• Detects the failure. 

• Moves any dynamic unicast MAC addresses that are learned on the primary link to the standby link. 

• Moves the standby link to a forwarding state. 

• Transmits dummy multicast packets over the new active interface. The dummy multicast packet format is: 

– Destination: 01:00:0c:cd:cd:cd 

– Source: MAC address of the hosts or ports on the newly active Flex Link port. 

You can configure preemption in the following three modes:

Forced—The active interface always preempts the backup.

Bandwidth—The interface with the higher bandwidth always acts as the active interface.

Off—There is no preemption; the first interface that is up is put in forwarding mode.

In Figure 1-2, ports 1 and 2 on switch A are connected to switches B and D through a Flex Link pair. Port 1 is forwarding traffic, and port 2 is in the blocking state. Traffic from the PC to the server is forwarded from port 1 to port 3. The MAC address of the PC has been learned on port 3 of switch C. Traffic from the server to the PC is forwarded from port 3 to port 1. If port 1 shuts down, port 2 starts forwarding traffic. If there is no traffic from the PC to the server after failover to port 2, switch C does not learn the MAC address of the PC on port 4, and because of that, switch C keeps forwarding traffic from the server to the PC out of port 3. There is traffic loss from the server to the PC because port 1 is down. To alleviate this problem, the feature sends out a dummy multicast packet with the source MAC address of the PC over port 2. Switch C learns the PC MAC address on port 4 and start forwarding traffic from the server to the PC out of port 4. One dummy multicast packet is sent out for every MAC address. 

A Flex Links port can be a physical Ethernet port or a port channel.

You cannot configure Flex Links port on the following types of interface:

Fabric Extender (FEX) fabric port and FEX host port

Virtual Fibre Channel interface

Virtual network tag (VNTag)

Interface with port security enabled

Layer 3 interface

Switched Port Analyzer (SPAN) destination

Port channel member

Interface configured with private VLAN

Endnode mode

Fabric path core interface (Layer 2 multipath)

Flex Configuration

Switch# configure terminal 

Switch(conf)# interface gigabitethernet0/21

Switch(conf-if)# switchport backup interface gigabitethernet0/22

Switch(conf-if)# end

Switch# show interface switchport backup

Switch# configure terminal

Switch(conf)# interface gigabitethernet0/1

Switch(conf-if)#switchport backup interface gigabitethernet0/2 preemption mode forced

Switch(conf-if)#switchport backup interface gigabitethernet0/2 preemption delay 30

Switch(conf-if)# end

Switch# show interface switchport backup detail

Switch# show interface switchport backup detail 

Switch# show running-config backup 

Switch# show startup-config backup

Switch# show running-config flexlink 

Switch# show startup-config flexlink 

Reference Links:

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst6500/ios/15-0SY/configuration/guide/15_0_sy_swcg/flexlinks.pdf

https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3550/software/release/12-2_25_see/configuration/guide/swflink.html

https://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5000/sw/layer2/503_n2_1/503_n2_1nw/Cisco_n5k_layer2_config_gd_rel_503_N2_1_chapter12.html#task_EBBB3C692C394A1EB384B9AF830CE203