BGP — end of module

What this checks. That you can read a path selection as a policy decision, and describe what BGP does and does not verify. You need 3 of 4 to pass, and every option is explained afterwards — including the ones you did not pick.

1. 1/4What does the AS path length count?

   Tell two ideas apart

   Autonomous systems crossed — not routers, and not distance

   Router hops along the path

   Latency in milliseconds
2. 2/4An operator sets LOCAL_PREF 200 on everything heard from one provider, while a different provider offers a shorter AS path for the same prefix. Which route is selected?

   Predict what happens next

   The LOCAL_PREF 200 route — that step is compared before AS path length here, so the path length is never reached

   The shorter AS path, because it is the primary criterion

   Both are installed and traffic is shared
3. 3/4Someone says 'BGP verifies that an AS is entitled to announce a prefix'. Why is that wrong?

   Say why an answer is wrong

   BGP as specified has no origin validation — an announcement is believed because a configured peer sent it

   It verifies only the first AS in the path

   Verification exists but is optional and usually switched off
4. 4/4A peering's TCP connection to port 179 succeeds, both sides send OPEN, and then one sends a NOTIFICATION. What class of problem is this?

   Diagnose a failure

   A parameter disagreement — the session parameters are checked in the OPEN, before anything else happens

   A reachability problem between the two routers

   A policy problem — one side is filtering the other's routes