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300-410 · Question #37

Refer to the exhibit. R2 is a route reflector, and R1 and R3 are route reflector clients. The route reflector learns the route to 172.16.25.0/24 from R1, but it does not advertise to R3. What is the…

The correct answer is D. R2 does not have a route to the next hop, so R2 does not advertise the prefix to the clients. A BGP router will not advertise a route if it does not have a valid and reachable next-hop to that route in its IP routing table, a fundamental rule for ensuring forwarding capability.

Layer 3 Technologies

Question

Refer to the exhibit. R2 is a route reflector, and R1 and R3 are route reflector clients. The route reflector learns the route to 172.16.25.0/24 from R1, but it does not advertise to R3. What is the reason the route is not advertised?

Options

  • AR1 is an external BGP peer to R2.
  • BR2 is configured to not advertise a route back to an internal BGP peer in the same cluster if it learned the route from an external BGP peer.
  • CR2 is configured to not advertise a route learned from one client to another client.
  • DR2 does not have a route to the next hop, so R2 does not advertise the prefix to the clients.

How the community answered

(29 responses)
  • A
    3% (1)
  • B
    10% (3)
  • C
    3% (1)
  • D
    83% (24)

Why each option

A BGP router will not advertise a route if it does not have a valid and reachable next-hop to that route in its IP routing table, a fundamental rule for ensuring forwarding capability.

AR1 is an external BGP peer to R2.

The question states R1 and R3 are route reflector clients, implying they are iBGP peers, not external BGP peers. Even for eBGP, the next-hop reachability rule still applies.

BR2 is configured to not advertise a route back to an internal BGP peer in the same cluster if it learned the route from an external BGP peer.

Route reflectors are designed to advertise routes learned from one client to other clients, which is their primary function for overcoming the iBGP full-mesh requirement.

CR2 is configured to not advertise a route learned from one client to another client.

This statement contradicts the fundamental purpose of a route reflector, which is to reflect routes between clients to ensure full reachability within the AS without a full iBGP mesh.

DR2 does not have a route to the next hop, so R2 does not advertise the prefix to the clients.Correct

A core principle of BGP is that a router will only advertise a prefix if its next-hop attribute is resolvable and reachable via the IP routing table. If R2, the route reflector, does not have an IGP route (e.g., via OSPF or EIGRP) to reach the next-hop IP address of the 172.16.25.0/24 route learned from R1, it will mark the BGP route as invalid and will not advertise it to R3 or any other peers.

Concept tested: BGP next-hop reachability requirement for advertisement

Source: https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13761-4.html

Topics

#BGP Route Reflectors#BGP Next-Hop Reachability#iBGP#BGP Route Advertisement

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