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as the title says, I am working with a rather simple topology:

R1<--eBGP-->R2<--iBGP-->R3

R1 has an eBGP neighborship with R2. I recently added an iBGP session between R2 and R3 (both are XR routers) and I was surprised to see that the routes R2 learns from R1 were advertised to R3.

Does a router that learns networks from an eBGP session automatically propagates them to its iBGP peers?

2 Answers 2

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Does a router that learns networks from an eBGP session automatically propagates them to its iBGP peers?

Yes this is one of the propagation rules of BGP. The full set of rules which elaborates on what Ron covered above is.

  1. When multiple feasible routes to a destination exist, BGP advertises only the best route to its peers.
  2. BGP advertises only routes that it uses.
  3. A BGP speaker advertises routes learned from an EBGP peer to all BGP peers, including both EBGP and IBGP peers.
  4. A BGP speaker advertises routes learned from an IBGP peer to EBGP peers.
  5. A BGP speaker does not advertise routes learned from one IBGP peer to other IBGP peers by default.

Rule 5 can be overcome with the use of route-reflectors/confederations as per the first response from Ron. However by default it's not allowed due iBGP bypassing the as-path loop prevention semantics of BGP.

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It is BGP. The distinction is eBGP is to a peer in a different AS, while iBGP is to a peer in the same AS, but it is the same BGP process on R2.

There is a rule for iBGP that any iBGP-learned routes cannot be advertised to an iBGP peer, which is why a mitigation, such as route reflectors or confederations are used for that.

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  • Added an additional post that elaborates on this to help the OP. The distinctions matter but a clear explanation of the announcement rules helps here I believe.
    – moogzy
    Jan 24 at 17:22

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