Passing Vlan traffic through 2 routers (IRB?)

Unanswered Question
Aug 8th, 2007

Hi,


I'm trying to configure two routers to pass vlan traffic via a t1. I've found how to do it on a single router, but not how to do it with multiple routers.


I'm assuming that I need to bridge a couple of subinterfaces, then assign a bvi to the bridge. And that I need to do this on both ends.


I also need to not disturb the current routed traffic passing through the t1.


Thanks!


-Jeff

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gmarogi Tue, 08/14/2007 - 09:17

Integrated Routing and Bridging, Transparent Bridging, and PVST+ Between VLANs with IEEE 802.1Q Encapsulation feature. It includes information on the benefits of the new feature, supported platforms, supported standards, and the commands necessary to configure the Integrated Routing and Bridging, Transparent Bridging, and PVST+ Between VLANs with IEEE 802.1Q Encapsulation feature

http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a008008019c.html

Edison Ortiz Tue, 08/14/2007 - 15:03

Let's say you want to bridge Vlan 20


On each router, you type the following:


bridge crb

bridge 20 protocol ieee


On the LAN/WAN interface


bridge-group 20


With that said, I don't recommend doing this since you are spanning your L2 over slow links. The Spanning-Tree Root for that Vlan will be located in the other side of the link and any spanning-tree decision will be send over the WAN. Not an ideal design.


What prompted you to do this ?

jcw009 Thu, 08/16/2007 - 12:44

So it's concurrent routing and bridging, not integrated, eh?


Client has a vendor (PBX) that wants to be able to communicate via TCP/IP between the PBX switches. However, the PBX switches in question have to be on their own vlan, otherwise the applicaton barfs, according to the vendor.


Thanks!

Paolo Bevilacqua Thu, 08/16/2007 - 12:58

Sometime PBX maintainers have a confused idea about IP. Try a regular routed configuration with each PBX in a subnet and proper QoS settings.

If that doesn't work, there are more tricks to play on the cisco to make the PBX believe they are in the same subnet.

jcw009 Thu, 08/16/2007 - 13:22

I had proposed that originally to the client, but they decided to go with the vlan configuration. I've already made the vlans work at two other locations. I don't think the client would be amenable to other changes.


Thank you for the input/confirmation. I was surprised when the PBX vendor said that they had to be on the same vlan.

Paolo Bevilacqua Thu, 08/16/2007 - 14:04

The thing is you will end having a very ugly configuration. First of all you will need to configure the T1 for frame-relay. This will allow you to separate vlans over the WAN. Then, depending on the overall desing, you will have some DLCI carrying bridged IP and some routed IP. Kludgey and confusing even once you get to a working configuration.


Believe me, when they say "have to be the same VLAN" they really mean "I don't know enough about routing and that worked for me in the past". It is worth an effort for a sane setup then.


Edison Ortiz Thu, 08/16/2007 - 16:10

Depends on your design, if you want these routers to be the gateway between the PBX Vlan and the rest of the network, go with IRB. If you want to have another device route between these Vlans, go with CRB.


I will second Paolo on the suggestion. This is a cool thing to do in a practice Lab but building this for a customer, well - let me put it this way, be sure to charge by the hour ...

wilson_1234_2 Thu, 04/01/2010 - 06:54

Edison,


I have a question relating to the Bridging across the WAN link vs using Metro E.


You mentioned below that the spanning tree infor would be sent across a slow link, which is not ideal.


Would the reason be that any STP decisong being made on the remote end would be much too slow to be useful, or possibly cause even less stability?


Also, in regard to Bridging across the WAN vs a Metro E, I have seen numerous posts where bridging across the WAN is considered a no no.


Aside from the reason you mentioned above, the only thing I have seen where it is a problem is because of increasing the broadcast domain, which is wasting bandwidth across an expensive link, correct?


But, if that is the case, wouldn't you be doign the exact same thing in trunking vlans across a Metro E?

Edison Ortiz Thu, 04/01/2010 - 09:42

Would the reason be that any STP decisong being made on the remote end would be much too slow to be useful, or possibly cause even less stability?


Spanning a L2 domain over an unreliable/slow WAN link is something to avoid at all cost. If all the links in your STP domain are inter-switch links with speed ranging from 10Mbps to 1000Mbps and you add a T1 link into the STP topology, this link will cause instability for the entire L2 domain.


Also, in regard to Bridging across the WAN vs a Metro E, I have seen numerous posts where bridging across the WAN is considered a no no.

Aside from the reason you mentioned above, the only thing I have seen where it is a problem is because of increasing the broadcast domain, which is wasting bandwidth across an expensive link, correct?


Metro-E is not considered a WAN connection but a MAN connection. You could extend your L2 domain over a MAN connection as the speed and reliability is similar to a local inter-switch connection.


Regards


Edison

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