FlexLinks

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:

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…

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.

SW3#
%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.

SW3#show interfaces switchport backup 

Switch Backup Interface Pairs:

Active Interface Backup Interface State
------------------------------------------------------------------------
FastEthernet0/14        FastEthernet0/16        Active Up/Backup Standby

Verify our configuration with the show interfaces switchport backup command. That’s all there is to it. It’s an interesting solution because we don’t need spanning-tree anymore. After all only one interface is active at any moment. Let’s see what happens when I disable the active interface:

SW3(config)#interface f0/14
SW3(config-if)#shutdown

Take a look below:

SW3#show interfaces switchport backup 

Switch Backup Interface Pairs:

Active Interface Backup Interface State
------------------------------------------------------------------------
FastEthernet0/14        FastEthernet0/16        Active Down/Backup Up

You can see that Fa0/16 has taken over. That’s all there is to it!

hostname SW3
!
interface FastEthernet0/14
 switchport backup interface Fa0/16
!
interface FastEthernet0/16
!
end