352-001 Question #182: Real Exam Question with Answer & Explanation

The correct answer is B: IP addresses used to peer are also being sent via EBGP.. If the IP addresses used for EBGP peering are also redistributed into EBGP, the router creates a recursive dependency where the BGP session is needed to resolve the route to the BGP peer itself, causing continuous session flapping.

Question

A planned EBGP network will use OSPF to reach the EBGP peer addresses. Which of these conditions should be avoided in the design that could otherwise cause the peers to flap continuously?

Options

AAn ACL blocks TCP port 179 in one direction.
BIP addresses used to peer are also being sent via EBGP.
CThe OSPF area used for peering is nonbackbone (not area 0).
DThe routers are peered by using a default route sent by OSPF.

Explanation

If the IP addresses used for EBGP peering are also redistributed into EBGP, the router creates a recursive dependency where the BGP session is needed to resolve the route to the BGP peer itself, causing continuous session flapping.

Common mistakes.

A. Blocking TCP 179 in one direction prevents the BGP session from ever forming at all, not causing a flap cycle once established.
C. Using a non-backbone OSPF area for peer reachability is a design consideration but does not inherently create a recursive routing dependency or continuous flapping.
D. Reaching an EBGP peer via an OSPF-advertised default route is stable as long as the default route persists; it does not create a recursive dependency between BGP session state and route reachability.

Concept tested. BGP recursive routing and peer address redistribution loop

Reference. https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/13753-25.html

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