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400-007 · Question #103

Refer to the exhibit. This network is running OSPF and EIGRP as the routing protocols. Mutual redistribution of the routing protocols has been configured on the appropriate ASBRs. The OSPF network…

The correct answer is B. route summarization on the appropriate ASBRS. To prevent EIGRP route flaps from triggering OSPF SPF recalculations, route summarization must be applied on the ASBRs performing redistribution so that individual flapping routes are hidden behind a stable summary LSA.

Designing Network Infrastructure

Question

Refer to the exhibit. This network is running OSPF and EIGRP as the routing protocols. Mutual redistribution of the routing protocols has been configured on the appropriate ASBRs. The OSPF network must be designed so that flapping routes in EIGRP domains do not affect the SPF runs within OSPF. The design solution must not affect the way EIGRP routes are propagated into the EIGRP domains. Which technique accomplishes the requirement?

Exhibit

400-007 question #103 exhibit

Options

  • Aroute summarization the ASBR interfaces facing the OSPF domain
  • Broute summarization on the appropriate ASBRS.
  • Croute summarization on the appropriate ABRS.
  • Droute summarization on EIDRP routers connecting toward the ASBR

How the community answered

(26 responses)
  • A
    12% (3)
  • B
    58% (15)
  • C
    27% (7)
  • D
    4% (1)

Why each option

To prevent EIGRP route flaps from triggering OSPF SPF recalculations, route summarization must be applied on the ASBRs performing redistribution so that individual flapping routes are hidden behind a stable summary LSA.

Aroute summarization the ASBR interfaces facing the OSPF domain

Specifying summarization on 'interfaces facing the OSPF domain' is an imprecise and technically incorrect target - external route summarization for redistributed routes is configured at the ASBR redistribution process level, not directionally on a specific interface.

Broute summarization on the appropriate ASBRS.Correct

When route summarization is configured on the ASBRs redistributing EIGRP routes into OSPF, individual EIGRP route instability is absorbed at the redistribution boundary and does not produce new or changed LSAs inside the OSPF domain, thereby eliminating unnecessary SPF runs while leaving EIGRP propagation within the EIGRP domains entirely unaffected.

Croute summarization on the appropriate ABRS.

ABRs connect OSPF areas to each other and have no authority over Type 5 (external) LSAs originated by ASBRs; only the originating ASBR can summarize redistributed external routes, making ABR-based summarization ineffective here.

Droute summarization on EIDRP routers connecting toward the ASBR

Summarizing on EIGRP routers toward the ASBR reduces the number of prefixes the ASBR receives from EIGRP, but each summarized prefix can still flap and generate updated Type 5 LSAs inside OSPF, which does not prevent SPF recalculations.

Concept tested: OSPF ASBR external route summarization to prevent SPF instability

Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13682-10.html

Topics

#route redistribution#OSPF#EIGRP#route summarization

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