Tuesday, March 17, 2009

Flex Links - In Switching Environment

Flex links are layer-2 interfaces manually configuredin primary/failover
pairs. The Spanning Tree Protocol normally provides primary/failover 
functionality, but it was designed for the sole purpose of preventing 
loops.Flex links are used to 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 switches where you do 
not wish to run spanning tree, and should be usedonly 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

interface GigabitEthernet1/0/20
switchport access vlan 10
switchport backup interface Gi1/0/21
interface GigabitEthernet1/0/21
switchport access vlan 10
No configuration is necessary on the backup interface.

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 Ether
Channel 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.

Monitoring flex links is done with the show interface switchport backup 

3750# sh int switchport backup
Switch Backup Interface Pairs:
Active Interface Backup Interface State
GigabitEthernet1/0/20 GigabitEthernet1/0/21 Active Down/Backup Down

Best Regards,
Deepak Arora

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