What is summarization?

[MLB메니아]

OSPF and Route Summarization

Summarizing is the consolidation of multiple routes into one single advertisement. This is normally done at the boundaries of Area Border Routers (ABRs). Although summarization could be configured between any two areas, it is better to summarize in the direction of the backbone. This way the backbone receives all the aggregate addresses and in turn will injects them, already summarized, into other areas. There are two types of summarization:

• Inter-area route summarization

• External route summarization

Inter-Area Route Summarization

Inter-area route summarization is done on ABRs and it applies to routes from within the AS. It does not apply to external routes injected into OSPF via redistribution. In order to take advantage of summarization, network numbers in areas should be assigned in a contiguous way to be able to lump these addresses into one range. To specify an address range, perform the following task in router configuration mode:

 area area-id range address mask

Where the "area-id" is the area containing networks to be summarized. The "address" and "mask" will specify the range of addresses to be summarized in one range. The following is an example of summarization:

In the above diagram, RTB is summarizing the range of subnets from 128.213.64.0 to 128.213.95.0 into one range: 128.213.64.0 255.255.224.0. This is achieved by masking the first three left most bits of 64 using a mask of 255.255.224.0. In the same way, RTC is generating the summary address 128.213.96.0 255.255.224.0 into the backbone. Note that this summarization was successful because we have two distinct ranges of subnets, 64-95 and 96-127.

It would be hard to summarize if the subnets between area 1 and area 2 were overlapping. The backbone area would receive summary ranges that overlap and routers in the middle would not know where to send the traffic based on the summary address.

The following is the relative configuration of RTB:

 RTB#
 router ospf 100
 area 1 range 128.213.64.0 255.255.224.0

Prior to Cisco IOS^?? Software Release 12.1(6), it was recommended to manually configure, on the ABR, a discard static route for the summary address in order to prevent possible routing loops. For the summary route shown above, you can use this command:

ip route 128.213.64.0 255.255.224.0 null0

In IOS 12.1(6) and higher, the discard route is automatically generated by default. If for any reason you don't want to use this discard route, you can configure the following commands under router ospf:

[케빈 o_o/]

앗,,, 영어닷!! ㅋㅋ... 솔직히 전 영어가 더 익숙하네여..ㅋㅋ..농담임다. (ㅠ_ㅠ;)

[MLB메니아]

케빈님은 그러실만 하네용 ㅋㅋㅋ