basically, IRB allows you to bridge and route the same protocol on the same router. With CRB (Concurrent Routing & Bridging), you can either route or bridge a given protocol on the same router, but not both. With IRB, as an example, your Ethernet interface can be in a bridge group, and traffic coming in on that interface can still be routed through your serial interface. The way to do it is to create a Bridge Virtual Interface (BVI), which is basically a logical Layer 3 interface that allows the bridged traffic to be routed out through other Layer 3 interfaces. The BVI has an IP address just like other layer 3 interfaces, the only difference is that you have to specify which protocols need to be routed.
Here is a sample configuration. In this example, the interface Ethernet 0 is member of bridge group 1. Without IRB, there would be no way to route traffic coming in on that interface to other locations, but with IRB, the BVI makes that possible:
interface bvi 1
ip address 220.127.116.11 255.0.0.0
ip address 18.104.22.168 255.255.255.252
bridge 1 protocol ieee
bridge 1 route ip
So IRB is mostly used in environments where traffic needs to be, for one reason or another, bridged and routed at the same time.
Check this document, it gives you a fairly straightforward explanation of how IRB works:
Thanks for concise reply to my question .please tell me one more thing if i have an ATM interface and one serial interface in my router,using IRB in that case is a right solution in order to make the data travel.
When you have ATM intefaces some ISP's force you to use bridging; you are correct while thinking about using IRB, since it can bridge but also can brige.
Now here is the trick, you need to split this router in to two, so the portion that goes to the ISP should be bridged but the part to the internakl network should be ROUTED.
Therefore, the BVI interface should be the one that has the ip address provided by your ISP; be aware of NOT USE the bridge X command in the Ethernet (could be FastEthernet) to make it work. I've done this in the past and this should work. By the way DO NOT FORGET that this router will still routing and it will need a DEFAULT ROUTE pointing to the ISP.
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