Bridging Cisco Router Interfaces for High Availability LAN

The figure above shows a sample High Availability LAN. It eliminates the single point of failure in cascading switches topology in which a switch failure would result in total outage of the LAN.

By using Integrated Routing and Bridging (IRB) technique, a Cisco router can be turned into a L3 switch. IP addresses can be assigned on Bridged-Group Virtual Interfaces (BVIs), similar to VLAN interfaces as in L3 switches. A BVI is a virtual routed interface that has all network layer attributes, eg: a network address can be assigned to it, able to perform filtering, and does not support bridging.

Bridge groups are defined by a unique number and are used for router bridging configuration. Network traffic is bridged between all interfaces that belong to the same bridge group.

Integrated Routing and Bridging configuration on RT1:

RT1(config)#int fa0/0
RT1(config-if)#bridge-group 1
RT1(config-if)#no shut
RT1(config-if)#exit
RT1(config)#
RT1(config-if)#int fa1/0
RT1(config-if)#bridge-group 1
RT1(config-if)#no shut
RT1(config-if)#exit
RT1(config)#
RT1(config)#int bvi1
Integrated Routing and Bridging is not configured!
RT1(config)#bridge irb
RT1(config)#int bvi1
RT1(config-if)#ip add 192.168.1.1 255.255.255.0
RT1(config-if)#exit
RT1(config)#bridge 1 protocol ieee
RT1(config)#bridge 1 route ip
RT1(config)#^Z

Note: The bridge 1 protocol ieee global configuration command removes bridge-group 1 spanning-disabled interface subcommand on Fa0/0 and Fa1/0 and hence enables STP.

RT1#sh spanning-tree brief

Bridge group 1
  Spanning tree enabled protocol ieee
  Root ID    Priority    32768
             Address     cc01.01e0.0000
             This bridge is the root
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768
             Address     cc01.01e0.0000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec
             Aging Time 300

Interface                                   Designated
Name                 Port ID Prio Cost  Sts Cost  Bridge ID            Port ID
-------------------- ------- ---- ----- --- ----- -------------------- -----
FastEthernet0/0      128.2    128    19 FWD     0 32768 cc01.01e0.0000 128.2
FastEthernet1/0      128.3    128    19 FWD     0 32768 cc01.01e0.0000 128.3

RT1#
RT1#sh bridge group

Bridge Group 1 is running the IEEE compatible Spanning Tree protocol

   Port 2 (FastEthernet0/0) of bridge group 1 is forwarding
   Port 3 (FastEthernet1/0) of bridge group 1 is forwarding

RT1#
RT1#sh int irb

FastEthernet0/0

 Routed protocols on FastEthernet0/0:
  ip

 Bridged protocols on FastEthernet0/0:
  appletalk  clns       ip

--- output omitted ---

FastEthernet1/0

 Routed protocols on FastEthernet1/0:
  ip

 Bridged protocols on FastEthernet1/0:
  appletalk  clns       ip

--- output omitted ---

FastEthernet2/0

 Routed protocols on FastEthernet2/0:
  ip

BVI1

 Routed protocols on BVI1:
  ip
RT1#