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I have this configuration:

r1 is RR of r2 and R5

r3 is RR of r2

r3 is connected with r4

r4 is connected with r5

My question is: what r2 and r1 know? For example i'm sure that r1 knows r2 and r5 but it knows also r3?

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    Aug 11, 2017 at 16:50

1 Answer 1

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In your case, these are the iBGP routes known:

  1. R1 knows routes from R2 (Direct) and R5 (Direct)
  2. R2 knows routes of R1 (Direct), R3 (Direct), R4 (Reflected by R3) and R5 (Reflected by R1 and R3)
  3. R3 knows routes of R2 (Direct) and R4 (Direct)
  4. R4 knows routes of R3 (Direct) and R5 (Direct)
  5. R5 knows routes of R1 (Direct), R2 (Reflected by R1) and R4 (Direct)

The rules are as simple:

A BGP route will not readvertise iBGP learned routes to other iBGP neighbors, unless they are configured as a Route Reflector for that particular neigbor. So, to get full connectivity between iBGP neighbors, you need either a full mesh between all routers, or a full mesh between the Route Reflectors and all the other iBGP routers.

NB: For the reflection to happen, the RR does not have to be able to route to the destined networks. All calculations and validation of what goes into the routing table, will be done at each router individually.

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