Cisco IRB clarification

[bbiandov]

I think this is a great article both in terms of conceptual explanation and practical hands-on examples:

http://www.cisco.com/en/US/tech/tk389/tk815/technologies_tech_note09186a0080094663.shtml

What I am struggling to find in the content is a good explanation of why is it so important that the 2 joined VLANs appear as ONE VLAN? Often the author refers to the misconception that even though the 2 VLANs may be numbered identically, such as vlan1 on both ends, it is NOT truly the same VLAN.

Then the articale goes to explain how using IRB the 2 VLANs do become one.

I would like to find a paragraph explaining why is it important that the two truly appear as one VLAN?

What's the problem of removing the header at each end of the respective VLAN and then re-inserting the new header as long as both are bridged and arp goes from one to another; etc regular properties of bridging? Where is the benefit of IRB that can not be fulfilled using conventional bridging?

Thanks

[Giuseppe Larosa]

Hello Bbiandov,

in the setup described in the article VLAN1 and VLAN2 are joined via the two ethernet interfaces on the router configured for IRB.

One could say that this setup allows one IP subnet to span over two different VLANs but actually as you noted IRB joins the two broadcast domains so the previous sentence is not correct.

There are some aspects of IRB that are not covered in the article but that are interesting to know

bridge 1 protocol ieee

actually IRB starts an IEEE compliant Spanning Tree on the router itself that interacts

with PVST running on the switch.

In other words if the two ports of the switch connecting to the router E0 and E1 were in the same VLAN, depending on the resulting root bridge, one side will be in STP blocking state.

IRB so may provide a way to have link redundancy without load sharing if those two ports are in the same VLAN on the switch.

>> Where is the benefit of IRB that can not be fulfilled using conventional bridging?

it provides a way to bridge and route at the same time when necessary, so for in the example it provides bridging between the two VLANS but also to send and receive traffic from other IP networks with the BVI interface acting as the default gateway for hosts connected in the single broadcast domain.

Some use IRB for providing link redundancy as a way to overcome the limitation that two different LAN interfaces cannot have overlapping IP addresses.

Other uses of IRB are possible.

IRB was handy at first to terminate DSL services using RFC 1483 bridged mode (before RBE was introduced)

IRB is not limited to IPv4: it can be used to route and bridge other protocols like CLNS.

Hope to help

Giuseppe