Cisco – Flex links

Cisco Flex links give the ability to have a layer2 redundant connection, or pair of connections configured as an Etherchannel for a primary link. This is an active/passive setup where if the primary connection’s link status goes down, the Flex link will become active, and if the primary comes back it will go into a standby mode and not take back primary functionality unless told to do so with preemption commands. STP is disabled automatically on Flex Links so no need to bother with Portfast.

Flex links started in code a long while back, and not sure how I missed them. I have needed this functionality before if I was connecting to a backup Firewall, or some device that Spanning-tree would have issues with. This options gives a great way to have a backup link configured if you just need it to become active if something happens to the primary link. In the below scenario I have a Fortigate firewall and Cisco 3560 switch. Port FA 0/1 is my primary and goes to Port 1 of the FGT, and Port 12 is the backup port for this link.



Switch config:

interface FastEthernet0/1
description “Connected to Fortigate”
switchport trunk encapsulation dot1q
switchport mode trunk
switchport backup interface Fa0/12
spanning-tree portfast

interface FastEthernet0/12
description “Connected to Fortigate – Backup”
switchport trunk encapsulation dot1q
switchport mode trunk

Below shows that status of the Flex link


Notice that the primary state is active and up. Now, I will cause a physical port state change by unplugging the interface and see how many pings/time it takes to failover.

After unplugging the primary connection, the link light of port 12 instantly came on and went green, I didn’t even lose a ping to my switch. All mac/uplinks moved over the backup port but no loss. Below shows the status after. Notice that the backup state is up, not the primary. The primary port after plugging it back up is amber like a blocked port, and both interfaces port status show up, even though the primary port at this point is not forwarding traffic.





One response to “Cisco – Flex links

  1. dustinreeves February 13, 2021 at 9:39 am

    Wow, thanks for posting this, this just solved a problem ive been dealing with for a few weeks now!

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