An OSPF area contains the following networks:

Last Updated on August 1, 2021 by Admin 1

An OSPF area contains the following networks:

How can the route to these networks be summarized?

Summarization is the process of advertising a network with a subnet mask such that it includes all of the subnets. For a simple example if you had two Class C networks, you could advertise them as a Class B network and it would encompass them both. Normally summarization should be implemented such that it summarizes ONLY the networks desired and no others (in the simple example it would possibly include other Class C networks). The process for arriving at the “best” summarization is a follows.

First, write the last octet that all networks share in common (third octet in this case) in binary form for each network:–00001000–00001010–00001100–00001110

The addresses have the first five bits in common; therefore, they can be summarized with the third octet 00001000 and a subnet mask of

Another way of looking at it is that covers the range of through, the same range as all the component subnets.

None of the following possible answers is a valid range, nor do most of them cover the correct range of addresses: is not a valid range. A 20-bit mask can only be on a subnet that is a multiple of 16, such as .16.0, .32.0, .48.0 etc. The subnet .8.0 is not a multiple of 16. is not valid. A 22-bit mask requires a multiple of 4 in the third octet, and 10 is not a multiple of four. Even if it were a valid range, it does not cover the entire range of addresses that need to be summarized. is not valid. The 20-bit mask is only usable on ranges that are multiples of 16 in the third octet, and 14 is not a multiple of 16. Even if the mask were valid, it could not cover the correct addresses.

When addresses are summarized the cost of the summary address will the highest cost of the component subnets. For example, in the partial sample output of the show ip route command below, there are three routes. The output is from a router running OSPFv3, so the addresses are IPv6, but the concept is the same.

OI 2001:0D B 8:0:0:7/64 [110/20]
via FE 80::A8BB:CCFF:FE 00:6F00, FastEthernet0/0
OI 2001:0D B 8:0:0:8/64 [110/100]
via FE 80::A8BB:CCFF:FE 00:6F00, FastEthernet0/0
OI 2001:0D B 8:0:0:9/64 [110/40]
via FE 80::A8BB:CCFF:FE 00:6F00, FastEthernet0/0

The routes have metrics (the second value in brackets, [administrative distance/cost]) of 20, 100, and 40. Therefore, the metric for the summarized route would be 100.

Layer 3 Technologies
Configure and verify manual and autosummarization with any routing protocol

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