Alcatel-Lucent 7750 SR OS - Manual (Page 23)

IP Router Configuration
7750 SR OS Router Configuration Guide Page 23
Autonomous Systems (AS)
Networks can be grouped into areas. An area is a collection of network segments within an AS that
have been administratively assigned to the same group. An area’s topology is concealed from the
rest of the AS, which results in a significant reduction in routing traffic.
Routing in the AS takes place on two levels, depending on whether the source and destination of a
packet reside in the same area (intra-area routing) or different areas (inter-area routing). In intra-
area routing, the packet is routed solely on information obtained within the area; no routing
information obtained from outside the area can be used. This protects intra-area routing from the
injection of bad routing information.
Routers that belong to more than one area are called area border routers. All routers in an AS do
not have an identical topological database. An area border router has a separate topological
database for each area it is connected to. Two routers, which are not area border routers, belonging
to the same area, have identical area topological databases.
Autonomous systems share routing information, such as routes to each destination and information
about the route or AS path, with other ASs using BGP. Routing tables contain lists of next hops,
reachable addresses, and associated path cost metrics to each router. BGP uses the information and
path attributes to compile a network topology.