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224 Chapter 7: OSPF, EIGRP, and IGRP
Specific routes within an area's summarized boundaries that change do not need to be changed
in the backbone's routing tables or in other areas. Routers that received summarized routes have
shorter routing tables and can provide faster routing lookups.
Summarization can occur at area borders, where a single summary LSA represents all the routes
in a single area or at ASBR redistribution points where the summarized routes represent the
external routes that are imported into OSPF. Route summarization procedures condense routing
information. Without summarization, each router in a network must retain a route to every
subnet in the network. With summarization, routers can reduce some sets of routes to a single
advertisement, reducing both the load on the router and the perceived complexity of the
network. The importance of route summarization increases with network size.
Summarization
There are two types of summarization:
·
Interarea route summarization--Addresses can be summarized on an ABR into area 0
or from area 0.
·
External route summarization--Addresses can be summarized into OSPF from
external routing protocols on an ASBR.
Figure 7-4 shows OSPF interarea route summarization between Area 0 and Area 1.
Figure 7-4
OSPF Interarea Route Summarization
Figure 7-5 shows OSPF summarization of an external route imported from RIP.
Area 1
RTA
128.213.64-95
255.255.255.0
Area 0
RTB
RTC
RTD
Area 2
128.213.96Р127
255.255.255.0
128.213.96.0 255.255.224.0
128.213.64.0 255.255.224.0
87200333.book Page 224 Wednesday, August 22, 2001 2:37 PM