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300-410 · Question #46

Refer to the exhibit. An engineer is trying to generate a summary route in OSPF for network 10.0.0.0/8, but the summary route does not show up in the routing table. Why is the summary route missing?…

The correct answer is D. There is no route for a subnet inside 10.0.0.0/8, so the summary route is not generated. An OSPF summary route will not be generated if the router does not have at least one more specific subnet within the defined summary range present in its routing table.

Layer 3 Technologies

Question

Refer to the exhibit. An engineer is trying to generate a summary route in OSPF for network 10.0.0.0/8, but the summary route does not show up in the routing table. Why is the summary route missing? Exhibit details: Router#show ip route output does not list any 10.0.0.0/8 routes. Router#show running-config | section ospf output shows router ospf 1, summary-address 10.0.0.0 255.0.0.0, redistribute static subnets, network 192.168.3.0 0.0.0.255 area 0, and network 192.168.12.0 0.0.0.255 area 0.

Exhibit

300-410 question #46 exhibit

Options

  • AThe summary route is not visible on this router, but it is visible on other OSPF routers in the same area.
  • BThe summary-address command is used only for summary prefixes between areas.
  • CThe summary route is visible only in the OSPF database not in the routing table.
  • DThere is no route for a subnet inside 10.0.0.0/8, so the summary route is not generated.

How the community answered

(41 responses)
  • A
    12% (5)
  • B
    5% (2)
  • C
    2% (1)
  • D
    80% (33)

Why each option

An OSPF summary route will not be generated if the router does not have at least one more specific subnet within the defined summary range present in its routing table.

AThe summary route is not visible on this router, but it is visible on other OSPF routers in the same area.

An ASBR generating a summary route will install a discard route (Null0) for the summary into its own routing table, making it visible on the router itself, not just on other OSPF routers.

BThe summary-address command is used only for summary prefixes between areas.

The `summary-address` command is used by an ASBR to summarize external routes being injected into OSPF, distinguishing it from `area range` used by ABRs for inter-area summarization.

CThe summary route is visible only in the OSPF database not in the routing table.

Once generated, an OSPF summary route is injected into the OSPF database and subsequently into the routing table as an OSPF external route type (E1/E2), or as a Null0 route on the originating ASBR.

DThere is no route for a subnet inside 10.0.0.0/8, so the summary route is not generated.Correct

For an OSPF Autonomous System Boundary Router (ASBR) to generate a summary route using the `summary-address` command, it requires the existence of at least one specific component route that falls within the summarized range in its routing table. If there are no routes for any subnet within 10.0.0.0/8 known to the router, the summary route will not be advertised.

Concept tested: OSPF summary-address generation requirements

Source: https://www.cisco.com/c/en/us/td/docs/ios/iproute_ospf/command/reference/ior_book/ospf_i1.html#wp1010373

Topics

#OSPF summarization#ASBR#Route generation#Routing table

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