OSPF Route Summarization
All OSPF routers in the same area must have identical LSDBs after flooding is complete. As a
result, all routers in the same OSPF area must have the same summary routes, and must be missing
the same component subnets of each summary. To make that happen, OSPF allows route
summarization only as routes are injected into an area, either by an ABR (inter-area routes) or by
an ASBR (external routes).
OSPF uses two different configuration commands to create the summary routes, depending on
whether the summary is for inter-area or external routes. Table 10-9 lists the two commands. Both
commands are configured under router ospf.
The commands have a couple of important attributes. First, the area range command specifies an
area; this area is the area in which the component subnets reside, with the summary being
advertised into all other areas. Also, the area range command can set the cost for the summary
route, instead of using the lowest cost of all component routes. Also, the not-advertise keyword
can essentially be used to filter the subnets implied by the summary, as covered in Chapter 9,
“OSPF.”
Where used Command
ASBR summary-address {{ip-address mask} | {prefix mask}} [not-advertise] [tag tag]
ABR area area-id range ip-address mask [advertise | not-advertise] [cost cost]
OSPF Route Summarization Commands
The summary-address command summarizes external routes as they are injected into OSPF as
an ASBR. The cost can be assigned, and the routes can be filtered using the not-advertise
keyword.