Click the exhibit. If router A originates an update for prefix 192.168.0.1/27, what will the update contain when it reaches router B?

Question

Options

• AAS Path of 65200, local-preference of 100.
• BAS Path of 65200, local-preference of None.
• CNull AS Path, local-preference of 100.
• DNull AS Path, local-preference of None.

• A
  6% (2)
• B
  15% (5)
• C
  70% (23)
• D
  9% (3)

Explanation

Option C is correct because Router A and Router B are iBGP peers (same AS). When a router originates a prefix and advertises it via iBGP, it does not prepend its own AS number to the AS Path - that only happens on eBGP (inter-AS) advertisements - so the AS Path arrives at Router B as empty/null. Local Preference defaults to 100 and is carried in iBGP updates (it's an iBGP-only attribute), so Router B receives it.

• A & B are wrong because AS 65200 would only appear in the AS Path if this update crossed an eBGP boundary; iBGP peers never modify the AS Path.
• D is wrong because Local Preference is included in iBGP updates (at the default value of 100) - it's only stripped when sending to eBGP neighbors.

Memory tip: Think "iBGP = Internal = Local Pref stays, AS Path untouched." iBGP shares local policy (Local Preference) but has no reason to stamp an AS number, since the route hasn't left the AS yet.

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