HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 659

Route summarization offers two distinct advantages:
Saving bandwidth and router memory—Routers can transmit more infor-
mation at once. Routing tables are simplified.
Cordoning off problem networks—OSPF routers generate a network
topology according to the messages they receive about link states; when-
ever a link goes down or up, the network topology changes. This feature
makes local routing highly accurate, but a flapping link can flood a
network with LSAs. Route summarization isolates this problem.
For example, in Figure 13-9, Router A simply needs to know how to route
traffic toward subnets 192.168.1.0 through 192.168.31.0 in area 1. Back-
bone Router B does not need to inform Router A when, for example, the
link to 192.168.15.0 /24 goes down.
192.168.40.0 /24
Router A
Routing table
192.168.1.0 /19 B
Area 2
Figure 13-9. OSPF Route Summarization
To each directly connected area, an ABR should distribute summaries for
every other area in the WAN. The ABR is responsible for summarizing net-
works in the directly connected areas. It will receive LSAs for other areas from
ABRs connecting to those areas, and it can then advertise these summaries to
routers in the areas that it supports.
For example, in Figure 13-10 ABR A connects to stub area 1 as well as to the
network backbone. ABR A summarizes the range of networks in area 1. ABR
B connects to stub areas 2 and 3. It summarizes the range of networks in these
areas. ABR A receives the summaries for areas 2 and 3 from ABR B and
advertises them to Router D at remote site 1. Similarly, ABR B advertises
summaries for stub area 1 to Routers E and F. (Router C is part of area 0, so
it receives LSAs for all areas.)
IP Routing—Configuring RIP, OSPF, BGP, and PBR
Router B
Routing table
192.168.15.0 /24 C
192.168.17.0 /24 D
Configuring OSPF
Router C
192.168.15.0 /24
Router D
192.168.17.0 /24
Area 1
13-45