From: Sam.MicroGate@xxxxxxxxxxxxxx
Date: Fri Feb 15 2002 - 10:02:40 GMT-3
I have been studying the confusing subject of IRB. I also read most of the
emails in this site. And here are my notes. I hope it helps. To configure
IRB, you need to answer a few questions.
1- What is the number of the BVI interface?
This one is the easiest one. The number would be equal to the
bridge-group number.
2- What network layer 3 addresses the BVI interface will be
assigned?
To answer this question, you have to look at each protocol
separately and the interfaces of the router. If a protocol needs to be
switched between a bridged interface to a routed interface or vice versa, a
network layer 3 address must be assigned to the BVI interface. For example,
suppose that you have a router with 5 interfaces. Three of those interfaces
are in one bridge group and no ip addresses is configured in them. The other
two interfaces have an ip addresses. Now if the IP traffic to be routed from
a bridged interface to a routed interface or vice versa, you must assign an
IP address to the BVI interface. This IP address will be allocated from the
same subnet as IP hosts located in the bridge-group. At the hosts in the
bridge-group, configure the default gateway to the IP address of the BVI
interface. Now what happens if all interfaces in the router have IP
addresses? The answer is there is no IP address required at the BVI
interface. IP will be routed normally between the 5 interfaces of the. You
have to go through this process for each protocol (IPX and AT if needed)
Remember, bridged interfaces may or may not have IP address but still have
IP hosts attached to it.
3- you are not finished configuring IRB yet. With the above
configurations IP traffic will be bridged within the bridge group and will
not make it to the routed interfaces. The reason for that is when you enable
IRB in a router, the bridge-group will bridge all packets. If you need to
enable routing over the defined bridge-group, use the following command:
Router (config)#bridge bridge-group number route ip
Also you have to configure this command for each protocol if needed
Finally there are two commands to check your configuration:
1- Router# show interface bvi1
2- Router# show interface Ethernet0 irb
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