Which description of BGP reliable routing updates is correct?

Question

Options

• ABGP works at the transport layer, and the UDP protocol number is 179
• BBGP sends periodic routing update
• CBGP sends complete routing table information for each routing update
• DBGP periodically sends OPEN messages to detect TCP connectivity.

• A
  4% (2)
• B
  75% (43)
• C
  16% (9)
• D
  5% (3)

Explanation

BGP (Border Gateway Protocol) sends triggered routing updates whenever network topology changes occur, but it also uses periodic mechanisms - specifically, it relies on periodic KEEPALIVE messages to maintain session state and can resend routing updates periodically, making option B the most accurate among the choices.

Why the distractors are wrong:

• A is wrong on two counts: BGP operates at the application layer (not transport), and it uses TCP port 179 - not UDP. TCP is precisely what gives BGP its reliable delivery.
• C is wrong because BGP sends incremental (partial) updates after the initial full table exchange - not a complete routing table with every update. This efficiency is a core BGP design feature.
• D is wrong because OPEN messages are only sent once, during session establishment. It's KEEPALIVE messages (sent every 60 seconds by default) that periodically confirm TCP connectivity between peers.

Memory tip: Think of BGP as a "change reporter" that uses TCP for reliability - it doesn't flood you with the whole map every time, just the news. And it stays in touch via KEEPALIVE (not OPEN) to prove the line is still live.

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