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Three Branch Office Connectivity

kevkline asked
Last Modified: 2008-07-29
I currently have three branch offices connected in a triangle with the following scenario:

Each of the three are connected via serial to point-to-point single T-1's.  Each link currently has it's own network.  Two serial cards in each router.  One going to each directly connected site.  Pretty standard I guess.  Three inside networks (, and

Router A--------- B
Router A--------- C

Router B--------- A
Router B--------- C

Router C--------- A
Router C--------- B

I am going to be getting rid of the T-1's and converting to a 10 Mbps fiber based MPLS (AT&T calls this OPT-E-MAN) which is actually a switched network and not a routed network although I can route it of course.

I am not going to be getting rid of the routers I currently have (Cisco 2811, 2821 and 2851) because they are already doing other things and I would like to keep a segmented routed network.  Router A has two PRI's for voice and an FXO card for paging, Routers B and C each have 1 PRI for voice and one FXO for paging.

Each router has 1 extra available ethernet port.  I could configure the ports on Routers A and B to be on the same network (say and and they should talk fine.  But without another ethernet port, what is the best way to connect Router C so it can talk to Router A and B?

Can I put three routers with IPs on the same network, say, plugged into the same switched network and be able to get to the correct network on the other side of the router?  I am thinking this should work.


Router A------ (with the network behind it)
Router B------- the network behind it)
Router C------- the network behind it)

Will this route OK?

Or is it possible to subinterface an ethernet port so that one port would reside on two networks?

Maybe I am overthinking this, but any thoughts would be greatly appreciated!
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Top Expert 2008
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Thanks guys.  Secondary question.  If I am passing data from eth0 to eth1, is my only limitation the speed of the port, whether 100 or 1000 Mbps?  

Asking because these are the specs I found with regards to T1 performance

2811: 2 T1/E1
2821: 4 T1/E1
2851: 6 T1/E1
Does that mean a 2811 can only process 3 Mbps of data regardless of how the data is obtained or whether or not through ethernet, serial, etc?
Top Expert 2008

T1s all have top speed of 1.544 Mbps. These specs area actually defining how many ports there are for example the 2811 listed above 2 T1/E1 ports (E1 is the European standard and runs at like 2.032 Mbps or something like that)

Here is the data sheet on the 2800 Series routers

2811 series router has 2 10/100 ports and the 2821 and 2851 all have gigabit ethernet ports so the traffic that flows from one port to the other will be much faster then any T1 connection.

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