From: Andrew Bratchell (a.bratchell@xxxxxxxxx)
Date: Mon Feb 11 2002 - 18:37:57 GMT-3
Elsayed,
This is a topic that drives many people crazy.
The best explanation I have seen is in Cisco Press's CCIE Practical Studies.
It explains it very clearly.
This book also has an excellent explanation of the 3920 Token Ring Switch as
well that I found useful.
This is taken from my study notes that you may find useful.
Integrated Routing and Bridging allows bridged and routed traffic of the
same protocol to be interchanged between different interfaces on the same
routers - hence the name IRB. IRB uses the concept of a Bridge-Group Virtual
Interface (BVI). The BVI is a virtual interface that acts like a normal
routed interface. A BVI does not support bridging, but represents the
corresponding bridge group to routed interfaces within the router. The
interface number is the link between the BVI and the bridge-group.
Therefore, the BVI number must be the same as the bridge-group number. All
layer 3 information such as IP addressing, filters etc is applied to the BVI
not the physical interface. For example if you have IP and IPX addresses
configured on Ethernet0/0, and you want to just route IP but route/bridge
IPX. Then you would take the IPX addressing off the Ethernet0/0 interface
and apply it to the BVI
The default route/bridge behaviour on a router is to route all packets first
and then bridge them. However, when IRB is enabled the default is to bridge
packets first and to route them second. Consequently all interfaces in the
the bridge group you configure, or all interfaces that have transparent
bridging enabled in them, will now bridge all protocols first. This will
cause major problems for other protocols using those link. Therefore all
layer 3 protocols must be told whether to route ,bridge or do both. This is
an important concept to understand. Note - Bridging attributes cannot be
configured on the BVI Interface itself
Thanks
Andy
-----Original Message-----
From: Elsayed Mohamed [mailto:elsayedm@hotmail.com]
Sent: 11 February 2002 20:50
To: ccielab@groupstudy.com
Subject: BVI and IRB
Guys,
I am having trouble understanding the IRB and BVI topic. All books and white
paper repeat the same thing but it seems that I am memorizing it more than I
understand it. If you UNDERSTAND this topic, can you please email me with
your source of reading or a lab I should do or anything else.......
Signature
Elsayed Mohamed
frustrated ccie candidate
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