How to bridge two ports on a Cisco Router

It’s no very often that the opportunity is there to use a Bridge Group on a Cisco Router. A Bridge Group will allow two routed interfaces on a router to act like two switch ports. In some cases Bridge Groups are a must have due to the network the router is connected to.

A few years ago I had a client that was buying a circuit from the cable company. The cable company had it’s fiber lines going past the business on the far side of the campus. Due to a costly build to have them run the fiber to the correct side, it was decided to terminate the service across campus. The internal network was then used to transport that data to the rest of the network.


The fiber was terminated on a router. The router was then connected to two different switches that were used to complete their fiber ring. There was an East switch and a West switch, all layer 2. Instead of adding another VLAN all of the way around the ring, they simply used the bridging feature on the router.

Below is a configuration example of how to configure a bridge group on a Cisco Router.

interface GigabitEthernet0/0
no ip address
duplex auto
speed auto
!
interface GigabitEthernet0/0.21 (21 is the VLAN number)
encapsulation dot1Q 21
bridge-group 1 (1 needs to match your BVI Interface)
!
interface GigabitEthernet0/1
no ip address
duplex auto
speed auto
!
interface GigabitEthernet0/1.21 (21 is the VLAN number)
encapsulation dot1Q 21
bridge-group 1
!
interface BVI1 (The number after BVI needs to match the Bridge-group number)
description Layer 3 Interface on vlan 21
ip address -ip address- -subnet-mask-
end

What are some good reason’s why you have used Bridge Groups?
Have you ever found Bridge Groups used for the wrong reason?

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About Scape

Over 10 Years in the networking field. Have worked in the Service provider and Enterprise environments. I have worked with Cisco, Foundry/Brocade, F5, Riverbed, Scientific Atlanta, Routers, Switches, Firewalls, Load Balancers, WAN Accelerators, DWDM, SONET, Multicast etc...

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