Routing

Hello,

Here's an OSPF routing question. I'm trying to fully understand this question in the JNCIP-SP practice exam, see attached gif. I've simulated the setup in a test lab, and indeed the traffic flows as described in answer C.

I do understand that the loopback from R3 will be know as a summary/Type3 LSA on router R6.

Better yet, there will be two Summary LSAs. One originating from R3 and the other from R5. What I don't understand is, why R6 prefers the summary LSA originated by R5. Especially since the metric advertised by R3 will be zero and the metric for the Summary LSA advertised by R5 will be 30. Why then is the LSA from R3 prefered?

Could somebody perhaps elaborate on that?

Output from my test setup can be found in the other gif attachment.

m20-2-test = R6

10.0.0.121 = R3

Notice the metrics

hi,

I see the exam screenshot comes from JN0-660 Practice Test - it's publicly available at http://www.juniper.net/us/en/training/certification/service_provider_track.html#jncipsp

Just a reminder for everyone - we must respect JNCP exams security policies !

Back to the question.

R6 prefers summary originated by R3. Yes ! Look at metric in 'show route 10.0.0.121 exact'

output at R6. The metric should be 20. Right ? (I don't test it now).

The path from R6 to R3 is via R5. Now, look at 'show route 10.0.0.121 exact' at R5. 

Surprise ? R5 prefers route via R2-R1, metric 30 ? It does not take direct route to R3 (metric 10).

Why ? OSPF intra-area routes are preferred over inter-area routes. Going directly to R1 would mean

following inter-area (summary) route.

Since IP forwarding is hop-by-hop decision, the real path doesn't have to reflect R6 point of view.

No more comments please, left as an exercise for the readers in the future

(unless I'm wrong here - comments welcome).

jtb

I do think you may not be counting the cost of the path from R6 to the summary advertising router in your calculations.  The cost from R6 to R3 is 40, the cost from R6 to R5 is 10.  So this would seem to me to lead to a tie if R3 advertises the summary as a metric of 0 and R5 as a metric of 30...since:

0 + 40 = 40

10 + 30 = 40

So I'm still not sure what the tie breaking mechanism is in this case, or if in fact some equal cost load balancing is done (it doesn't sound like it)

I read through jtb's post and he makes an important point, something I missed.

From the point of view of next-hop for R6, it doesn't really matter if R6 prefers the summary from R3 or R5....becasue the 'next-hop' for either summary is going to be R5...so the route that is chosen is to send traffic to R5 no matter what summary is 'better'.  What happens next is all up to R5, as jtb says, intra area routes are used become inter area, so R5 sends traffic off to R2...then R2 to R1...and finally to R3.