Configuration manual

Configuring OSPF Areas
22-8 Configuring OSPFv2
0 to 4294967295. A value of 0 means that two consecutive SPF calculations are performed one
immediately after the other. +
Configuring OSPF Areas
OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group,
together with the routers having interfaces to any one of the included networks, is called an area.
Each area has its own link-state database.
The topology of an area is invisible from the outside of the area, and routers internal to a given
area know nothing of the detailed topology external to the area. This isolation of area detail
enables the protocol to reduce routing traffic, as compared to treating the entire Autonomous
System as a single link-state domain.
A router has a separate link-state database for each area it is connected to. Routers connected to
multiple areas are called Area Border Routers (ABR). Two routers belonging to the same area
have, for that area, identical area link-state databases.
An autonomous system can have one or more areas. An AS with multiple areas must define one of
the areas as the backbone with an area ID of 0. All non-backbone areas in a multiple area AS must
either be contiguous to the backbone or connected using a virtual-link. The backbone is
responsible for distributing routing information between non-backbone areas. The backbone must
be contiguous, although it need not be physically contiguous. Backbone connectivity can be
established and maintained with virtual links.
Virtual links can be configured between any two backbone routers that have an interface to a
common non-backbone area. Such virtual links belong to the backbone. The protocol treats two
routers joined by a virtual link as if they were connected by an unnumbered point-to-point
backbone network.
See RFC 2328 OSPF Version 2 for further details on inter-area connectivity.
An Area ID can be any value from 0 - 4294967295, but is converted into the 32-bit dotted-quad
format (area 50 would be displayed as 0.0.0.50; area 3546 would be displayed as 0.0.13.218).
Configuring Area Range
An area range is a form of address summarization that defines a range of addresses to be used by
the backbone ABRs when they communicate routes to other areas. Area range is a critical tool that
pares the route tables and update traffic, as well as reduces network recalculation by the Dijkstra
algorithm. Area range configuration summarizes by aggregating an areas’ internal networks to
advertise a single network. Backbone routers see only one update, representing an entire range of
subnets. Area ranges can be configured for purposes of network advertisement as well as
summarization of subnets that should not be advertised.
Use the area range command in router configuration command mode to configure an area
network summarization.
Example
The following code example configures summarization for the topology shown in Figure 22-3 on
page 22-9.
Area 1
ABR1(su)->router(Config)#router ospf 1
ABR1(su)->router(Config-router)#area 0.0.0.1 range 10.2.0.0 255.255.0.0