When designing a robust network, the requirement that should make the top of the list is redundancy.  Most of the time, this is pretty easy.  When linking switches together, connect multiple links.  When connecting servers to the network, using multiple network adapters with teaming is the norm.  Connecting more than one interface on a router for redundancy is usually a little different.  Most of the time, you would take two interfaces on a router and assign an IP network to each and use a routing protocol for interacting with other connected routers for link selection.  However, there are some devices that are router types that don’t perform routing functions. 

IOS devices functioning as some type of gateway comes to mind as a type of device that won’t route traffic – however network redundancy to the device is still desired.  In this example, a Cisco VG224 is used in a network to provide dial tone to analog devices over an IP network.  The VG224 gateway has multiple interfaces and does have the ability to run a routing protocol for reachability, but would be considered a bloated solution just to provide redundant network connections to the device.

In this example, we will accomplish network link redundancy by using a feature called Integrated routing and bridging or IRB for short.  In an IRB configuration, multiple physical interfaces are assigned to a common bridge group.  The two interfaces then form a bridge what communicates with a bridge virtual interface.  The device IP address then gets assigned to the virtual interface.

As with any bridge, loop detection is necessary to prevent problems in the network.  Spanning tree is configured on the specific bridge which will communicate with the connected access switches.  In this particular scenario, we would never want this bridge to become the root for the connected network.  Therefore it is important to specify a priority, even though it is optional, to influence the root bridge selection process.

To get started, specify a bridge number and spanning tree protocol used. The ieee option is the only real valid choice here, which is the 802.1D standard spanning tree.

GW-VG224-01(config)#bridge 1 protocol ?
  dec          DEC protocol
  ibm          IBM protocol
  ieee         IEEE 802.1 protocol
  vlan-bridge  vlan-bridge protocol

GW-VG224-01(config)#bridge 1 protocol ieee

Every bridge that participates in a spanning-tree domain goes through the root bridge election process. Specify the highest bridge priority for influencing the election process to not select this device as a root bridge:

GW-VG224-01(config)#bridge 1 priority ?
  <0-65535>  Priority (low priority more likely to be root)

GW-VG224-01(config)#bridge 1 priority 65535

This device in this example has two Fast Ethernet interfaces. Assign both of these physical interfaces to the bridge group number previously assigned:

GW-VG224-01(config)#int fa0/0
GW-VG224-01(config-if)#bridge-group 1
GW-VG224-01(config-if)#int fa0/1
GW-VG224-01(config-if)#bridge-group 1

Specify the use of irb:

GW-VG224-01(config)#bridge ?
  <1-255>            Bridge Group number for Bridging.
  crb                Concurrent routing and bridging
  irb                Integrated routing and bridging
  mac-address-table  MAC-address table configuration commands

GW-VG224-01(config)#bridge irb

Enter interface configuration mode for the virtual interface that is created. This interface is where the layer3 configuration goes:

GW-VG224-01(config)#int bvI ?
  <1-255>  BVI interface number

GW-VG224-01(config)#int bvI 1
GW-VG224-01(config-if)#ip address 10.32.16.20 255.255.255.0

Traffic will not be forwarded until the bridge is configured to route the IP traffic through this virtual bridge.

GW-VG224-01(config)#bridge 1 route ?
  ip  IP
GW-VG224-01(config)#bridge 1 route ip

Lastly, take the physical interfaces out of shutdown mode:

GW-VG224-01(config-if)#int fa0/0
GW-VG224-01(config-if)#no shut
GW-VG224-01(config-if)#int fa0/1
GW-VG224-01(config-if)#no shut