4A0-102 · Question #103
An autonomous system has 7 routers configured with BGP. How many iBGP sessions are required for a fully meshed configuration?
The correct answer is C. C. 21. Option C (21) is correct because a full iBGP mesh requires every router to peer with every other router, calculated using the formula n(n−1)/2, where n is the number of routers. With 7 routers: 7 × 6 / 2 = 21 sessions. A (7) is wrong - this would mean each router only has one…
Question
Options
- AA. 7
- BB. 13
- CC. 21
- DD. 42
How the community answered
(27 responses)- A4% (1)
- B7% (2)
- C85% (23)
- D4% (1)
Explanation
Option C (21) is correct because a full iBGP mesh requires every router to peer with every other router, calculated using the formula n(n−1)/2, where n is the number of routers. With 7 routers: 7 × 6 / 2 = 21 sessions.
- A (7) is wrong - this would mean each router only has one peer, which is far from fully meshed.
- B (13) is wrong - it doesn't correspond to any meaningful formula for this topology.
- D (42) is a classic trap - it's the result of 7 × 6 without dividing by 2, which double-counts each session (a session between router A and B is the same as B and A).
Memory tip: Use the "handshake problem" - if 7 people each shake hands with everyone else, how many handshakes occur? Same formula: n(n−1)/2. This works for iBGP full mesh for the same reason: each session is shared between two peers.
Topics
Community Discussion
No community discussion yet for this question.