350-001 · Question #170
Two BGP peers connected through a routed firewall are unable to establish a peering relationship. What could be the most likely cause?
The correct answer is B. EBGP multihop is not configured. BGP uses TCP port 179 (not UDP), so option D is factually wrong on the protocol. Option A is false - BGP does not require Layer 2 adjacency and can peer across routed hops. Option C is wrong - IP protocol 89 is OSPF, not BGP. The key issue here is that EBGP, by default, sets…
Question
Two BGP peers connected through a routed firewall are unable to establish a peering relationship. What could be the most likely cause?
Options
- ABGP peers must be Layer 2-adjacent.
- BEBGP multihop is not configured.
- CThe firewall is not configured to allow IP protocol 89.
- DThe firewall is not configured to allow UDP 179.
How the community answered
(30 responses)- A3% (1)
- B70% (21)
- C7% (2)
- D20% (6)
Explanation
BGP uses TCP port 179 (not UDP), so option D is factually wrong on the protocol. Option A is false - BGP does not require Layer 2 adjacency and can peer across routed hops. Option C is wrong - IP protocol 89 is OSPF, not BGP. The key issue here is that EBGP, by default, sets the IP TTL to 1, meaning it only expects directly connected peers. When a routed firewall sits between the BGP peers, it adds an additional hop, causing the TTL to expire before the packet reaches the peer. EBGP multihop (configured with 'neighbor x.x.x.x ebgp-multihop') increases the TTL to allow peering across multiple hops. Without it, the BGP TCP session will never establish through the firewall.
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