400-007: Cisco Certified Design ExpertDemo

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QuestionQ163

Security Design

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Refer to the exhibit. A customer network design team is planning a phased migration from a legacy TDM-based L2VPN to an MPLS-based L3VPN:

  • The OSPF backbone link between HUB A and HUB B sites will be migrated to eBGP.
  • Spoke A2 and Spoke B1 will be migrated to the L3VPN.

Which solution design can be considered to avoid routing loops during the backbone-link migration?

  • A Redistribute EIGRP 200 and 300 with low cost into BGP.
  • B OSPF backbone area advertises summarized routes to hub.
  • C Enable route-filtering on OSPF backbone routers for spoke traffic.
  • D Advertise low AD value for transit traffic on hub sites.
Explanation

OSPF route summarization toward the hubs limits the OSPF domain to advertising aggregate reachability rather than the individual spoke prefixes that are also introduced through the eBGP/MPLS L3VPN path. This prevents those more-specific migrated routes from being reintroduced through the parallel backbone path and forming a redistribution loop. Administrative distance is local to a router and is not advertised in routing updates; changing it can itself cause routing loops or black holes.

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