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BGP Full mesh

Hi ,

Want to know bit in depth about BGP full mesh.

In a BGP AS (this AS is running some IGP for internal reachbility purpose). There can be two scenarios possible:

1. All the routers in AS are running IBGP with each other.

2. All edge routers are running IBGP with each other. Say R1, R3 and R5 in attached diagram are running IBGP. these are the only routers on which BGP is enabled.

As we all know 1st scenario is BGP full mesh as all routers are participating in BGP and forming neighbourship with all other routers in AS. But when only three routers are running BGP and forming neighbourship with remaining two, will that also be called Full mesh?

Just to simplify my question, in case only limited routers are running BGP and they are forming neighbourship with remaining BGP enabled routers, will it be called Full mesh or partial mesh.

2 ACCEPTED SOLUTIONS

Accepted Solutions

BGP Full mesh

Manoj,

A full mesh is when all iBGP speakers have a peering to every other speaker. In your example, if R3 has a peering with R1 and R5, but R1 and R5 does NOT have a peering between each other then that's a partial mesh. If R3 has a peering with R1 and R5 and R5 and R1 peer with each other and R3 then that's a full mesh.

You can figure out how many peerings you'll need based on the amount of nodes that you have by using the formula:

n(n-1)/2

Say you have 5 routers. You would need 5(5-1)/2 = 10 peerings for a full mesh. If you had 10 routers, you would need 10(10-1)/2 = 45 peerings. It exponentially grows which is why most people look into route reflectors.

John

HTH, John *** Please rate all useful posts ***
Hall of Fame Super Silver

BGP Full mesh

Manoj

I agree with John. As you said "But when only three routers are running BGP and forming neighbourship with remaining two, will that also be called Full mesh?". So yes if R1 forms neighbor relationship with R3 and R5, R3 forms neighbor relationship with R1 and R5, and R5 forms neighbor relationship with R1 and R3 then it is still called full mesh. It does not matter that R2 and R4 are inside the AS. If they are not running BGP then they do not affect the full mesh.

HTH

Rick

3 REPLIES

BGP Full mesh

Manoj,

A full mesh is when all iBGP speakers have a peering to every other speaker. In your example, if R3 has a peering with R1 and R5, but R1 and R5 does NOT have a peering between each other then that's a partial mesh. If R3 has a peering with R1 and R5 and R5 and R1 peer with each other and R3 then that's a full mesh.

You can figure out how many peerings you'll need based on the amount of nodes that you have by using the formula:

n(n-1)/2

Say you have 5 routers. You would need 5(5-1)/2 = 10 peerings for a full mesh. If you had 10 routers, you would need 10(10-1)/2 = 45 peerings. It exponentially grows which is why most people look into route reflectors.

John

HTH, John *** Please rate all useful posts ***
Hall of Fame Super Silver

BGP Full mesh

Manoj

I agree with John. As you said "But when only three routers are running BGP and forming neighbourship with remaining two, will that also be called Full mesh?". So yes if R1 forms neighbor relationship with R3 and R5, R3 forms neighbor relationship with R1 and R5, and R5 forms neighbor relationship with R1 and R3 then it is still called full mesh. It does not matter that R2 and R4 are inside the AS. If they are not running BGP then they do not affect the full mesh.

HTH

Rick

Silver

BGP Full mesh

That would be full-mesh edge.

If you can tunnel edge routers to each other and core routers need not have any knowledge of external networks then you can go for such design.

HTH.

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