Exam 350-501 topic 1 question 352 discussion

D is correct

Well confusing question and confusing answer! During a recent outage on the B-C link, the IGP traffic path was switched to the alternate path via routers E and F, but label forwarding did not occur on the new path== there was LDP session failure between E and F and it was not detected. Bidirectional Forwarding Detection (BFD) provides low-overhead, short-duration detection of failures in the path between adjacent forwarding engines. It seems the path requires fault detection for the forwarding plane; hence, the correct answer is C.

Sorry for my previous comment. It seems to me that D is wrong. The IGP traffic path was switched to the alternate path via E and F, but label forwarding didn't occur on the new path. LDP sync only holds the IGP forwarding until LDP is up, but it doesn't ensure the end-to-end path traffic. If there was LDP session protection on rotuers A and D, it would keep sending targeted hellos using the alternate path (A-E-F-D); hence, it would ensure traffic in case of one path failure. Therefore, the correct answer is B.

Answer: D Following information from Cisco SPCOR materials (online)-purchased D: Lack of synchronization between LDP and IGP can cause MPLS traffic loss. Upon link-up, for example, IGP can advertise and use a link before LDP convergence has occurred; LDP IGP synchronization synchronizes LDP and IGP so that IGP advertises links with regular metrics only when MPLS LDP is converged on that link. B: B is also a good answer. LDP Session Protection lets you configure LDP to automatically protect sessions with all or a given set of peers (as specified by the peer ACL). When configured, LDP initiates backup targeted hellos automatically for neighbors for which primary link adjacencies already exist. These backup targeted hellos maintain LDP sessions when primary link adjacencies go down. In brief. LDP targeted hellos maintain the protection path for the non-directly connected neighbor. And it is the best solution for the protection path.

MPLS LDP Session Protection allows to protect an LDP session between 2 directly connected routers (RB/RC) by proposing an alternative link (RB-RA-RE-RF-RD-RC) in case the direct link is broken...RA and RD are not directly connected => B is wrong.

B. wrong because no mention about TE -'' MPLS LDP Session Protection protects an LDP session between directly connected neighbors or an LDP session established for a traffic engineering (TE) tunnel.'' C.wrong BFD - help IGP faster to detect failure D.Correct - https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/mp_ldp/configuration/15-s/mp-ldp-15-s-book/mp-ldp-igp-synch.pdf

B or D , but i think B is more Correct

The IGP convergence is already there. MPLS LDP synch is for when the link comes from down to up so IGP waits until LDP is converged. I am thinking the solution here might be mpls session protection?

I am thinking B too

LDP session protection is between directly connected neighbors, which A and D are not So the D) should be the correct answer

I think B is the correct answer - you need LDP seeion protection

D - Enable MPLS LDP IGP Synchronization on all routers