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Lab
Activity |
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In this lab,
you will learn how to configure OSPF to summarize
addresses within an area. |
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Summarization is off by default. To
configure route summarization on the ABR, perform the following
steps:
- Configure OSPF.
- Instruct the ABR to summarize
routes for a specific area before injecting them into a different
area, as follows:
router(config-router)#area
area-id range address mask
To configure route summarization on
an ASBR to summarize external routes, perform the following steps:
- Configure OSPF.
- Instruct the ASBR to summarize
external routes before injecting them into the OSPF domain, as
follows:
router(config-router) #summary-address
address mask

Figure
shows that route summarization can occur in both directions.
In Router 2's configuration shown in
Figure , the following commands are used:
- area 0 range 172.16.96.0
255.255.224.0
---
Identifies
area 0 as the area containing the range of networks to be
summarized into area 1. The ABR R1 is summarizing the range of
subnets from 172.16.96.0 to 172.16.127.0 into one range:
172.16.96.0 255.255.224.0. This summarization is achieved by
masking the first three left-most bits of subnet 96 using the mask
255.255.224.0.
This summarization was successful
because you are summarizing two distinct subnet ranges into the
backbone: 32 to 63 and 64 to 95.
- area 1 range 172.16.32.0
255.255.224.0
---
Identifies
area 1 as the area containing the range of networks to be summarized
into area 0. The ABR R1 is summarizing the range of subnets from
172.16.32.0 to 172.16.63.0 into one range: 172.16.32.0
255.255.224.0.
The configuration on the right works
exactly the same way. Note that, depending on your network topology,
you may not want to summarize area 0 networks. If you have more than
one ABR between an area and the backbone area, for example, sending
a summary LSA with the explicit network information will ensure that
the shortest path is selected. If you summarize the addresses, a
suboptimal path selection may occur.
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