Which of the following regarding the TTL of a BGP message is TRUE?

Question

Options

• ATTL is a well-known mandatory BGP attribute.
• BThe default TTL value of an eBGP message is 1, and 64 for an iBGP message.
• CThe default TTL value of an iBGP message can be modified using the multihop command.
• DThe default TTL value of an eBGP message can be modified using the "ttl-security" command.

• A
  3% (1)
• B
  90% (27)
• C
  3% (1)
• D
  3% (1)

Explanation

Option B is correct because BGP uses different default TTL values depending on whether the session is external (eBGP) or internal (iBGP). eBGP peers are assumed to be directly connected (one hop away), so the IP TTL defaults to 1 - packets die if they traverse more than one router. iBGP peers are within the same AS and typically use the OS default TTL of 64 (or 255 on some platforms), since they may not be directly connected.

Why the distractors are wrong:

• A is false - TTL is an IP header field, not a BGP path attribute at all. BGP well-known mandatory attributes are ORIGIN, AS_PATH, and NEXT_HOP.
• C is false - the multihop (or ebgp-multihop) command is used for eBGP, not iBGP, to raise the eBGP TTL above 1 when peers aren't directly connected. iBGP already has a high TTL by default.
• D is false - ttl-security (GTSM) works in the opposite direction: instead of raising the TTL, it sets the TTL to 255 and drops packets that arrive with a TTL below a threshold, hardening against spoofed BGP attacks. It secures eBGP sessions rather than simply modifying the default TTL in the traditional sense.

Memory tip: Think "eBGP = 1 hop, 1 TTL" - eBGP neighbors must be directly connected by default, so TTL=1 enforces that. Anything that breaks that rule (multi-hop eBGP) needs ebgp-multihop to fix it.

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