Autonomous System And Areas - Avaya 8800 Configuration Manual

OSPF and RIP fundamentals
7. LSAs are flooded throughout the area to ensure that all routers in an area have in
8. From this database each router calculates a shortest-path tree, with itself as root.

Autonomous system and areas

The autonomous system (AS) can be subdivided into areas that group contiguous networks,
routers connected to these networks, and attached hosts. Each area has a topological
database, which is invisible from outside the area. Routers within an area know nothing of the
detailed topology of other areas. Subdividing the AS into areas significantly reduces the
amount of routing protocol traffic compared to treating the entire AS as a single link-state
domain.
You can attach a router to more than one area. When you do, you can maintain a separate
topological database for each connected area. Two routers within the same area maintain an
identical topological database for that area. Each area is assigned a unique area ID and the
area ID 0.0.0.0 is reserved for the backbone area.
Packets are routed in the AS based on their source and destination addresses. If the source
and destination of a packet reside in the same area intra-area routing is used. If the source
and destination of a packet reside in different areas, inter-area routing is used. Intra-area
routing protects the area from bad routing information because no routing information obtained
from outside the area can be used. Inter-area routing must pass through the backbone area,
which is described in
In large networks with many routers and networks, the link-state database (LSDB) and routing
table can become excessively large. Large route tables and LSDBs consume memory. The
processing of link-state advertisements results in additional SF/CPU cycles to make forwarding
decisions. To reduce these undesired effects, an OSPF network can be divided into
subdomains called areas.
An area comprises a number of OSPF routers that have the same area identification (ID).
By dividing a network into multiple areas, a separate LSDB, consisting of router link-state
advertisements (LSA) and network LSAs, are maintained for each area. Each router within an
area maintains an LSDB only for the area to which it belongs. This means that area router
LSAs and network LSAs do not flood beyond the area borders.
Therefore, the impact of a topology change is localized to the area in which it occurs. The only
exception is for the area border routers, which must maintain an LSDB for each area to which
they belong. Changes in topology are advertised to the remainder of the network by the area
border routers by advertising summary LSAs.
Note:
ERS 8800/8600 could take a minimum of about 10 seconds to populate an OSPF learned
route into the Route Table Manager as it does not rebuild its router LSAs immediately when
the neighbor router changes to FULL state.
36 Configuration — OSPF and RIP
identical topological database.
This shortest-path tree in turn yields a routing table for the protocol.
Backbone area on page 37.
June 2011