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300-515 · Question #153

300-515 Question #153: Real Exam Question with Answer & Explanation

The correct answer is B. Change the mask on the P3 loopback 0 interface to /32.. MP-BGP VPNv4 exchange requires an LSP to each PE loopback. If P3 advertises its loopback with a non-/32 mask, the IGP/LDP FEC can be ambiguous and break the LSP toward PE2 after reboot. Advertising the loopback as a host route (/32) restores correct LDP/IGP reachability to the PE

Question

Refer to the exhibit. A large regional company has two primary offices in different cities, connected via a service provider that provides Layer 3 connectivity between the two sites. Although the two company offices are physically separated, traffic from site B to the Internet uses the PE1 internet connection. The service provider is running OSPF in its core, with the loopback 0 IP address used as the OSPF and BGP router IDs. After the most recent service provider maintenance window, during which P3 was rebooted, the connection between CE1 and CE2 failed to come up. Which action must network engineers take to resolve the issue?

Options

  • AConfigure BGP to use router IDs with a/24 mask different from the loopback 0 IP address.
  • BChange the mask on the P3 loopback 0 interface to /32.
  • CImplement BGP as the PE-CE routing protocol at both sites.
  • DDisable LDP on the interface from P3 to PE2.

Explanation

MP-BGP VPNv4 exchange requires an LSP to each PE loopback. If P3 advertises its loopback with a non-/32 mask, the IGP/LDP FEC can be ambiguous and break the LSP toward PE2 after reboot. Advertising the loopback as a host route (/32) restores correct LDP/IGP reachability to the PE loopbacks and brings the CE1-CE2 connectivity back.

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