Bridging Across serial interfaces...same subnet on both ends

Unanswered Question
Mar 3rd, 2009

I have two routers connected via 64K serial interfaces....The ip address on the ethernet of both routers is in the same subnet....My questions is..>Can I bridge like subnets across those serial interfaces...I need to maintain IP integrity at both ends for management purposes...perhaps a tunnel...I just don't know..can't remember...

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Overall Rating: 5 (2 ratings)
Giuseppe Larosa Tue, 03/03/2009 - 07:25


you need to use IRB and bridging over the serial interfaces

int eth0

no ip address

bridge-group 1

int ser0/0

ip addr

bridge-group 1

int bv1

! here the shared subnet

ip address x.x.x.1

bridge 1 protocol ieee

bridge 1 route ip

do it on both ends/routers

Hope to help


apostollic Tue, 03/03/2009 - 08:18

Do the serial interfaces need a 30 bit IP range to connect them///or no ip address on the serial

Giuseppe Larosa Tue, 03/03/2009 - 08:33


you should be fine with no ip address on the serial interfaces.

I've noticed that you say you have only 64kbps link so a bridging solution is probably troublesome:

broadcast traffic is enough to saturate the link if this is a real production network and not a lab.

Hope to help


apostollic Tue, 03/03/2009 - 09:14

The bandwidth is small but, the application is tiny..just polling a RTU...once evrey 5 minutes...the whole network is for that...thanks again

Edison Ortiz Tue, 03/03/2009 - 09:20

Once you configure bridging, the WAN won't be used just for that small application transfer but every single broadcast in either side of the network will traverse that link.

In a LAN segment, broadcast traffic is barely a problem since you usually have a LAN with speeds of 10Mbps or better.

With this design, your LAN speed will be as low as 64kbps depending on the switch/router root placement.

I strongly suggest to revisit this design.



Edison Ortiz Tue, 03/03/2009 - 07:45

Be aware that extending your Layer2 topology over slow unreliable WAN links will create atrocious performance issues in the network at both ends of the link.

I suggest re-examining your network and provide a Layer3 design that fulfills this need without going the bridging path.




Jon Marshall Tue, 03/03/2009 - 07:47


Off the top of your head do you know if L2TPv3 is any better in terms of performance across a WAN link ?


Edison Ortiz Tue, 03/03/2009 - 07:52


They both accomplish the same thing, i.e. extending a L2 topology.

With that said, L2TPv3 is mostly used to accommodate customers on a point-to-point virtual connection.

For instance, a service provider would use 2 routers with L3 inter-connectivity and assign a L2 port for each CE. The CEs, in turn, will be able to create L3 point-to-point connection, pretty much like your typical point-to-point serial link. In this case, that's a proper use of L2TPv3 implementation.



Edison Ortiz Tue, 03/03/2009 - 08:52


Another thing just came to my head :)

When you enable bridging between 2 routers, say a service provider doing what I mentioned before, you are automatically enabling spanning-tree and a root election will take place. This add some overhead over WAN links.

While, when you enable L2TPv3, you are creating a pseudowire for devices that are attached 'xconnect' interfaces. No spanning-tree is running on the service provider devices - hence it is much cleaner solution.




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