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

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Mar 3rd, 2009
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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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Giuseppe Larosa Tue, 03/03/2009 - 07:25
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Hello,


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.1 255.255.255.0



bridge 1 protocol ieee

bridge 1 route ip


do it on both ends/routers


Hope to help

Giuseppe


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


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

Giuseppe


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


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Edison.



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


HTH,


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Edison.

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


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


Jon

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


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.


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


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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