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IRB? BCP? Ethernet over T1? Best way to trunk Vlans over T1 links?

Hello all,

We have a site that we need some design suggestions for, here is the currect map:

Bridge question.jpg

As you can see we have 3 buildings.  Currently, Building 1 connects to a Microwave Hut that gives access to the Internet and MPLS, over fiber and switches.  We have fiber runs going down to building 2 and 3 from the Hut, but because of building costs and permits, we can not make the fiber hot, yet.  What is currectly supporting building 2 and 3 is a existing microwave link that supports 24 T1 circuits.  (VERY Ugly design, lets just say each building supports 4 routers at the moment)

What we are thinking of doing is robbing two routers between the microwave hut and Building 2, and creating a switch trunk over them.  Trying to create the Blue line, using the Red lines.  We must use bridging (I think) as we have both vlans that support internal connections (10.10.X.X) and need to support direct external connections to outside LAN Blocks (12.x.x.x).  We tried looking at the option of converting the Microwave to Ethernet handoff instead of these T1 circuits, but the microwave supports other voice/data circuits that we can’t move away from just yet.  If I didn’t make is clear, Yes, the fiber is there to replace the Microwave someday.  So this solution is needed only for the next year. 

We really just want to use the routers to transport Ethernet over T1 lines and back.  As T1 is the only layer 1 and 2 that Microwave supports.  If we can, getting VTP to work would be great, but if not, no sweat.  We are talking about 9 vlans in total for both buildings.  So how would you do it?  Or what should we look at?  The switches are 2960’s and 3560’s.  Routers are 1900 and 2900 series.  Another option we are looking at is just getting Ethernet over T1 converters.  But we would rather use the Cisco routers in place.




Does anyone have any thoughts on this?  I'm really just looking for a direction i should be going.  Since it seems there are many options.  Any guidance would be greatly appreciated.

Thanks for the time and help,



Try IRB first. its been around for a while. what ever apps need this localization will not run well.


I started with IRB, and it worked.  VTP, CDP other layer 2 apps didn't but, forwarding dot1q frames did.  So i have to manage vlans on both sides of these routers.  I guess no big deal.  Then i found L2TPv3 and that worked even better, and just as easy to configure:

interface Loopback0

ip address

pseudowire-class Hut2B2

encapsulation l2tpv3

ip local interface Loopback0

interface FastEthernet0/0

xconnect 10 pw-class Hut2B2

Now cdp, vtp, and everything works great.


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