eBGP square or triangle?

Unanswered Question
Apr 17th, 2008


Hopefully a simple eBGP question:

If I have 2 edge routers in an AS, and 2 edge routers in another AS is it best practices to have them in a square, or triangles?

By this I mean AS1 has router 1 and router 2 (running iBGP between them), and AS 2 has the same.

Is it best practice to have an eBGP peer between AS1 router 1 <-> AS2 router 1, and AS1 router 2 <-> AS2 router 2, or:

AS1 router 1 peering with AS2 router 1 and 2, and AS1 router 2 peering with AS2 router 1 and 2.

Either way, can somebody confirm the best practice and why?

Any help would be greatly appreciated.

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cisco_lad2004 Fri, 04/18/2008 - 10:50


if layer ONE allows for redundancy, meaning R1-AS1 has 2 different links to R1&R2-AS2 then I would fully mesh.

if not, there are still some benefits.

I would set up 3 session on each router.

in this case traffic outboud, would always exit via external AS. so R1-AS1 >> R1 or R2 -AS2 (Unless u have manipulated IBGP attributes).

why would I fully mesh ?


2-less hops, by meshing. u are allowing BGP to use less hops which sometimes customers like to see (unless MPLS is used)



andy.taylor Fri, 04/18/2008 - 13:38


Many thanks. I deployed with multiple eEBP peers i.e. each router has 2 x eBGP and 1 x iBGP for redundancy and found faster convergence.

Many thanks for your reply, it's good to get validation :-)

All the very best,



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