300-510 SPRI: Implementing Cisco Service Provider Advanced Routing SolutionsDemo

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QuestionQ238

MPLS and Segment Routing

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Refer to the exhibit. ISP_1 uses OSPF and LDP within its backbone. Client_A and Client_B both reside in the same VRF, VRF_Shared. An iBGP session using the VPNv4 address family has been established between R6 and R14. Following a recent replacement of router R13, Client_A has lost connectivity to Client_B. A network engineer needs to troubleshoot the connection from R1 to R23 through ISP_1.

Which action must the engineer take to resolve the issue?

  • A Set the network 13.13.13.1 0.0.0.0 area 0 command under the OSPF configuration on R13.
  • B Set the ip cef command under the global configuration on R6.
  • C Set the next-hop-self command under the BGP configuration on R6.
  • D Set the mpls label mode vrf VRF_Shared command under the global configuration on R13.
Explanation

The traceroute mpls output shows a valid label being received at the first hop but then "No Label" (code 'B', unlabeled output interface) from R13 onward toward R14's loopback (14.14.14.1/32), even though a standard IP ping to that same address succeeds — indicating the IGP-based IP route is fine but the LDP/MPLS control plane is broken specifically at the newly replaced R13. Cisco IOS LDP sessions are built using each router's LDP router-id (by default the highest loopback address) as the TCP transport address, so if R13's loopback address (13.13.13.1) was not re-added into the OSPF process/area 0 after the router swap, R14 cannot route to that loopback and the LDP neighbor relationship (and therefore label exchange) with R13 cannot be correctly (re)established. This breaks the label-switched path across R13 even though plain IP reachability to R14 still functions, which explains the loss of the MPLS L3VPN connectivity between Client_A and Client_B. Adding the network statement for R13's loopback under OSPF area 0 restores IGP reachability to R13's LDP identifier, allowing the LDP session with R14 to form and labels to be advertised again, repairing the LSP.

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