BGP full mesh

Minary

I have a sample Question that asks

What statement is true about IBGP routers? Select the best response.

A-They must be full meshed
B-They can be in a differnent AS
C-They must be directly connected
D-They do not need to be directly connected.


According to the answer with the parctice exam the correct response is D.
Why is A not correct also ? Is the question just a bad example ?

I did some research and "y

ou need to have all your iBGP routers full-meshed unless you have a route-reflector or are using confederations"

.

AwesomeGarrett

Answer A says they must be full meshed which is simply not true. They can be and its recommended but its not requirement of iBGP. Must is the keyword for that answer.

fredrikjj

iBGP should be fully meshed to prevent potential routing loops if you don't use confed/reflector, but it's not a strict requirement.

EdTheLad

Being fully meshed doesn't prevent routing loops, the rule not allowing iBGP learned routes to be advertised to an iBGP neighbor prevents routing loops. Due to this rule, in order to propagate eBGP routes to all neighbors a full mesh is required otherwise traffic will get blackholed. So the full mesh is a design rule of thumb but not a mandatory config requirement.

tomtom1

And the reason for the rule is because of the BGP split horizon rule that EdTheLad mentions. To mitigate this, you could use a route-reflector which reflects the routing updates, just like a mirror.

networker050184

There are also tools to get around needing an iBGP full mesh so it is definitely not mandatory.

Danielh22185

A.) As already stated there are some reasons why you dont HAVE to have a full mesh. It's more considered a best practice so other routers within the iBGP AS can reach other networks.

B.) This is not correct because iBGP essientally means "local" so if you are specifying different AS numbers you are essentially trying to setup eBGP.

C.) iBGP neighbors do NOT have to be directly connected, nor do eBGP neighbors (however this is the general norm of how they are establish fo eBGP). iBGP by default actually very commonly mesh which creats neighbors that are not all the time directly connected.

D.) This is the correct answer for reasons already stated.

d4nz1g

awesomegarrett wrote: »

answer a says they must be full meshed which is simply not true. They can be and its recommended but its not requirement of ibgp. Must is the keyword for that answer.

/\ this!