Cisco
352-001 · Question #43
352-001 Question #43: Real Exam Question with Answer & Explanation
The correct answer is D: Use route maps on the ASBRs to filter routes with tags so they are not redistributed.. Mutual OSPF redistribution at multiple points requires tagging routes when they are redistributed and then filtering those tags at other ASBRs to prevent routes from being re-redistributed back into their origin domain.
Question
Your design plan includes mutual redistribution of two OSPF networks at multiple locations, with connectivity to all locations in both networks. How is this accomplished without creating routing loops?
Options
- AUse route maps on the ASBRs to allow only internal routes to be redistributed.
- BUse route maps on the ASBRs to allow internal and external routes to be redistributed.
- CUse route maps on the ASBRs to set tags for redistributed routes.
- DUse route maps on the ASBRs to filter routes with tags so they are not redistributed.
Explanation
Mutual OSPF redistribution at multiple points requires tagging routes when they are redistributed and then filtering those tags at other ASBRs to prevent routes from being re-redistributed back into their origin domain.
Common mistakes.
- A. Restricting redistribution to only internal routes prevents external routes from propagating across the redistribution boundary, breaking connectivity to locations that depend on external route information.
- B. Allowing both internal and external routes to be redistributed without a loop-prevention mechanism causes routing loops, as external routes are repeatedly passed between the two OSPF domains.
- C. Setting tags on redistributed routes is only half of the required solution - without also configuring filters to block routes carrying those tags at the receiving ASBRs, the tagged routes will still be redistributed back into their origin domain.
Concept tested. Mutual OSPF redistribution loop prevention using route tags
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