300-410 Question #37: Real Exam Question with Answer & Explanation

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.

Explanation

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.

Common mistakes.

• A. 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.
• B. 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.
• C. 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.

Concept tested. BGP next-hop reachability requirement for advertisement

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

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