When you have a redundant switched topology, spanning-tree will block some of the interfaces to create a loop-free topology. Another method to deal with redundant topologies is by using flexlinks. When you configure FlexLinks you’ll have an active and standby interface. Here’s an example:

flex links active standby link

In the topology above spanning-tree would normally block one of the interfaces to create that loop-free topology. This time however we are going to use flexlinks. What happens is that one interface will be the active link and the other one will be a backup link. The backup link is down and only comes up when the active link fails.

I can configure this on SW3:

  • Fa0/14 will be the active interface.
  • Fa0/16 will be the backup interface (this one is blocked!).

When you configure interfaces as FlexLinks they will not send BPDUs. There is no way to detect loops because we don’t run spanning-tree on them. Whenever our active interface fails the backup interface will take over.

Let’s look at the configuration…


I will configure SW3 so that interface Fa0/14 is the active interface and Fa0/16 becomes a backup:

SW3(config)#interface fa0/14
SW3(config-if)#switchport backup interface fa0/16

This is how we make interface fa0/16 a backup of interface fa0/14.

%SPANTREE-6-PORTDEL_ALL_VLANS: FastEthernet0/14 deleted from all Vlans 
%SPANTREE-6-PORTDEL_ALL_VLANS: FastEthernet0/16 deleted from all Vlans

You can see spanning-tree is being disabled for these interfaces.

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Forum Replies

  1. Hi Rene,

    Can you explain what if failed link get backup, will it automatically switch over to active link.?

  2. Hi Zaman,

    The backup link is not used at all…unless the primary link fails. One flexlink has 1 primary and 1 backup link, you can configure multiple flexlink pairs though…the maximum number depends on the platform.


  3. Hei Rene,

    Cisco says something else:

    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.

    But I Strongly agree with you! because i have done it on ME3400 live with customer:

    When it was nothing connected to Gig 0/2 on ME3400:
    GigabitEthernet0/1 GigabitEthernet0/2 <strong>Active Up/Backup Down</strong>

    When he connected the lan cabel to Gig 0/2:
    GigabitEthernet0/1 GigabitEthernet0/2 <strong>Active

    ... Continue reading in our forum

  4. Thanks Rene for sharing this concept. But do you in real network people using flexlinks over STP?


  5. Some customers don’t like the overhead that STP causes on their CPU while computing its BPDUs. They want the entire cycle to be dedicated to the actual Users’ data forwarding. Therefore, they have to find something which consumes little to 0 load on the CPU. Flex links is their option

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