Bridging / IRB

This is just a summary about the bridging and IRB functions in Cisco routers and switches. The configuration on routers and switches is identical, but only the interfaces are different (real interfaces on routers, vlan interfaces on switches).
If you have a network and a router splits this network, you’ll need the bridging functionality.

This means:
On a router interface fa0/1 is a host with IP 192.168.1.5/24
On the same router but interface fa1/0 is another host with IP 192.168.1.10/24
These two hosts can’t communicate by default. To let these host communicate with eachother, a bridge configuration will be needed.
The configuration in this scenario looks like this:
router(conf)#bridge 50 protocol ieee
router(conf)#interface fa0/1
router(conf-if)#bridge-group 50
router(conf)#interface fa1/0
router(conf-if)#bridge-group 50
The default gateway for this network can be configured on a BVI interface. This BVI interface number has to be the same as the bridge-group ID
router(conf)#bridge irb
router(conf)#interface bvi 50
router(conf-if)#ip address 192.168.1.254
When configuring irb, the router will route all the IP traffic by default for the particual bridge group.
For bridging between the interfaces in a irb, the following configuartion is needed:
router(conf)#bridge 50 bridge ip
for routing IP between the BVI and other interfaces:
router(conf)#bridge 50 route ip
With this configuration: clns and decnet are routed by default. If you don’t need these protocols, disabled these with the ‘no’ command:
router(conf)#no bridge 5o route clns
router(conf)#no bridge 5o route decnet