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Multihomed - BGP - 2 Cisco Routers with 3 circuits.

Posted on 2011-02-26
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Last Modified: 2012-05-11
Good Day Experts.

We have a routing challenge and were hoping that you could guide us in the right direction.
Here are the details:

On Router 1 (cisco 1841):
ISP A – T1 Circuit1
ISP B – T1 Circuit2

On Router 2 (cisco 1941):
ISP A – T1 Circuit3

ISP A is the same for both routers, but the circuits are different.

We are using BGP and getting default routes.

We have failover working well. We can shutdown any interface and the other pick up right away.

The problem that we are facing is how to load share all three circuits. We are currently load sharing Circuit1 and Circuit2 but seem to be unable to include Circuit3 into the equation.

Any ideas?
Thanks in advance for your help.
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Question by:QuestionManA
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17 Comments
 
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Expert Comment

by:rfc1180
ID: 34988392
Are you able to post your configs and possibly a network diagram depicting the connections.

Thanks
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Author Comment

by:QuestionManA
ID: 34988610
Thanks for the reply.

I am able to post some information. What commands would you like me to post?

As for the network diagram.
Our 3 circuits come in via a smartjack and go to the various routers. From there we have a managed switch and then a cisco ASA/ The ASA then connects to the servers.
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Expert Comment

by:rfc1180
ID: 34988615
>I am able to post some information. What commands would you like me to post?
show run

remove any information that you do not want to make public. There are several ways to load balance but this is based solely on your current design and how the routers are connected in your network. Just draw up a diagram on how your network is logically connected.

Billy
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Author Comment

by:QuestionManA
ID: 34988762
Both Router configs are attached.

1841-config-a.txt
1941-config-a.txt
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LVL 79

Expert Comment

by:lrmoore
ID: 34988819
The ISPA might not have load-sharing enabled on your 2 separate circuits. Even though you have the same weight on your end, they might not on their end.

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Author Comment

by:QuestionManA
ID: 34988946
Ok. I will ask them. Do you see anything else that it could be or that we should change?
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Expert Comment

by:rfc1180
ID: 34989088
On the LAN side of the 1841 and 1941, you are using GLBP, so you are load balancing between both routers, then you are load sharing utilizing 2 static routes on the 1841 which is accomplished using IP CEF and not BGP. At this point, BGP is not being used for load sharing. What is your expectation of load sharing? Are you expecting a specific percentage of load sharing via all 3 circuits? What is your expectation on outbound and inbound?

Billy
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Author Comment

by:QuestionManA
ID: 34989217
Ideally we would like to see a 33% spilt (or close) on all outbound traffic.
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Expert Comment

by:rfc1180
ID: 34989404
>Ideally we would like to see a 33% spilt (or close) on all outbound traffic.
not going to happen with your configuration

Load balancing on the two links is not possible because BGP chooses the single best path among the networks that is learned from eBGP and internal BGP (iBGP); however, if you were receiveing full internet tables, then you could load share among the multiple paths to two different or three different AS numbers. You have the best configuration that you can acheive; if all 3 circuits were on the same router, then you could load share acrossed on 3 circuits, but your best best is what you have, load share acrossed 2 and using GLBP two load balance the 2 different edge routers.


Billy
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Author Comment

by:QuestionManA
ID: 34989698
If all three were on one router - it would be possible? Could you elaborate?
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Author Comment

by:QuestionManA
ID: 34989704
And also, if they were all on one router, would we need full routes?
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Expert Comment

by:rfc1180
ID: 34990288
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Author Comment

by:QuestionManA
ID: 34991930
rfc1180 - Thanks for the post.

We are doing that on Router 1 (1841) between ispa and isapb.

What we are having difficulty with is router2 and including that circuit.

The ideas currently on the lsit are (please feel free to add any) are:
1. Combine all 3 circuits on one router.
2. Put the two ISPA circuits on one router and ISP B on the other.

Will either work?

Any idea's or comments are appreciated.
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LVL 24

Expert Comment

by:rfc1180
ID: 34992703
>We are doing that on Router 1 (1841) between ispa and isapb.
Whether you do it via static routes or via BGP, CEF is load sharing, not BGP.When using the maximum-paths statement in BGP, the statement allows BGP to add the routes into the local RIB if the routes are equal.

If you want to load share all thress circuits, your only option is to combine all 3 circuits on one router.
1. Combine all 3 circuits on one router.

Why?
As already stated; load sharing on the two links is not possible because BGP chooses the single best path among the networks that is learned from eBGP and iBGP.

Billy
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Author Comment

by:QuestionManA
ID: 34992895
Thank you.

What we have done today is to place both ISPA circuits on the same router with ISPB on the other.
ISPA has the most amount of traffic and we can load share between them.
We will leave ISPB as is (it it has roughly half the load of ISPA).

Of course this is just a short term solution since we plan on adding ISPC (ethernet over copper) and using the T1's as failovers.

If we combined all three onto one circuit, could you elaborate on how we could acheive load sharing.

Thanks.
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Accepted Solution

by:
rfc1180 earned 500 total points
ID: 34992940
router bgp x
maximum-paths 3

Billy
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Author Closing Comment

by:QuestionManA
ID: 34993069
Thanks! We're try it!
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