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How to configure IRB

Core Issue

A Cisco router can be configured for routing specific protocols and bridging other protocols at the same time. The Concurrent Routing and Bridging (CRB) feature allows a specific protocol to be routed on certain interfaces and bridged on other interfaces on the same device at the same time. However, this feature does not allow the routed and bridged domains to communicate.  This limitation is overcome by the Integrated Routing and Bridging (IRB) feature while providing the benefit of CRB at the same time. 

IRB can be used for interconnecting routed and bridged domains using the same router. This is useful while migrating from a bridged network to a routed network. IRB is also useful for connecting to a remote site that does not have routing capabilities. IRB is only supported for Transparent Bridging (TB) and cannot operate with CRB at the same time on the same router.


To resolve this issue, perform these steps:

  1. Before configuring IRB, enable bridging and routing for the desired protocols. To create a bridge group, issue the bridge <bridge-group> protocol {dec | ieee} command from global configuration mode. To include interfaces under the defined bridge group, issue the bridge-group command under the interfaces.  


    Enable routing for the desired protocols. Cisco routers by default route IP and bridge all other protocols. If you have disabled IP routing and want to enable it, issue the ip routing command from global configuration mode. Issue the equivalent command for enabling routing for other protocols that you require, such as Internetwork Packet Exchange (IPX) or AppleTalk.


  2. IRB provides the ability to route between a bridged domain and a routed domain using Bridge Group Virtual Interface (BVI). The BVI is a virtual interface within the router that acts like a normal routed interface that does not support bridging, but represents the corresponding bridge group to routed interfaces within the router. The interface number of the BVI is the number of the bridge group that the virtual interface represents. The number is the link between the BVI and the bridge group.  

        When you configure and enable routing on the BVI, packets that come in on a routed interface, which are destined for a host on a segment in a bridge group, are routed to the BVI. From the BVI, the packet is forwarded to the bridging engine, which forwards it through a bridged interface. This is forwarded based on the destination MAC address. Similarly, packets that come in on a bridged interface, but are destined for a host on a routed network, first go to the BVI. Next, the BVI forwards the packets to the routing engine before sending them out of the routed interface.  


    Since the BVI represents a bridge group as a routed interface, it should be configured only with Layer 3 (L3) characteristics, such as network layer addresses. Similarly, the interfaces configured for bridging a protocol should not be configured with any L3 characteristics.


    Enable IRB by issuing the bridge irb command from global configuration mode. Configure the BVI by assigning the corresponding number of the bridge group to the BVI. This is done by issuing the interface bvi command from global configuration mode. Assign a network layer address for the protocol being routed. If you are using IRB for IP, issue the ip address command. This assigns an IP address to the BVI interface. This should belong to the same network as the network on the hosts that are connected to the bridging domain.


  3. After IRB is enabled, the default behavior in a bridge group is to bridge all protocols. Make sure you explicitly configure routing on the bridge group for the protocols you want routed. This is done by issuing the bridge route        command from global configuration mode. Configuring routing enables both bridging and protocol routing in the interfaces that belong to the bridge group.  


    If you want to only route certain protocols over the interfaces that belong to the bridge group and not bridge them, disable bridging for such protocols by issuing the no bridge bridge        command from global configuration mode.


For more information on configuring IRB with examples, refer to the Configuring Integrated Routing and Bridging section of Configuring Transparent Bridging.

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‎06-22-2009 05:11 PM
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