C
HAPTER
20
| Unicast Routing
Configuring the Open Shortest Path First Protocol (Version 2)
– 519 –
Figure 340: Displaying Information on NSSA and Stub Areas
CONFIGURING AREA
RANGES (ROUTE
SUMMARIZATION FOR
ABRS)
An OSPF area can include a large number of nodes. If the Area Border
Router (ABR) has to advertise route information for each of these nodes,
this wastes a lot of bandwidth and processor time. Instead, you can use
the Routing Protocol > OSPF > Area Range (Add) page to configure an ABR
to advertise a single summary route that covers all the individual networks
within its area. When using route summaries, local changes do not have to
be propagated to other area routers. This allows OSPF to be easily scaled
for larger networks, and provides a more stable network topology.
Figure 341: Route Summarization for ABRs
CLI REFERENCES
â—† "router ospf" on page 1043
â—† "area range" on page 1049
COMMAND USAGE
â—† Use the Area Range configuration page to summarize intra-area routes,
and advertise this information to other areas through Area Border
Routers (ABRs). The summary route for an area is defined by an IP
address and network mask. You therefore need to structure each area
with a contiguous set of addresses so that all routes in the area fall
within an easily specified range. If it is not possible to use one
contiguous set of addresses, then the routes can be summarized for
several area ranges.This router also supports Variable Length Subnet
Masks (VLSMs), so you can summarize an address range on any bit
boundary in a network address.
â—† To summarize the external LSAs imported into your autonomous
system (i.e., local routing domain), use the Summary Address
configuration screen (page 519).
â—† This router supports up five summary routes for area ranges.
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