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

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QuestionQ175

Unicast Routing

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Refer to the exhibit. The four routers in this network have established adjacencies. Router R2 is an IS-IS level 1-2 router that connects to other areas, and the attach bit is set in R2's level 1 LSP advertisements to R1. By design, R2 should send level 2 routes from its routing table to R1's routing table. However, R1 has installed a default route in its routing table. Which action resolves this issue?

  • A Use the is-type level-1-2 backdoor command to unset the attach bit on R1 and use a prefix list to filter prefixes from R2.
  • B Use the is-type level-1-2 backdoor command to unset the attach bit on R2 and configure a distribute-list to redistribute prefixes to R1.
  • C Set the attach bit on R1 and use a prefix list to filter prefixes from R2.
  • D Configure R2 as a level 1 router and use a prefix list to filter prefixes to R1
Explanation

An IS-IS Level 1-2 router sets the attach bit in its Level 1 LSPs to indicate reachability to other areas, causing Level 1 routers to install a default route toward it. Suppressing the attach bit on R2 with is-type level-1-2 backdoor prevents that default-route behavior. A distribute-list can then control the Level 2-to-Level 1 route leaking so that the required Level 2 prefixes are advertised to R1.

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