Autonomous Systems (As); Confederations - Alcatel-Lucent 7450 Configuration Manual

Configuring IP Router Parameters

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.

Confederations

Configuring confederations is optional and should only be implemented to reduce the IBGP
mesh inside an AS. An AS can be logically divided into smaller groupings called sub-
confederations and then assigned a confederation ID (similar to an autonomous system
number). Each sub-confederation has fully meshed IBGP and connections to other ASs
outside of the confederation.
The sub-confederations have EBGP-type peers to other sub-confederations within the
confederation. They exchange routing information as if they were using IBGP. Parameter
values such as next hop, metric, and local preference settings are preserved. The
confederation appears and behaves like a single AS.
Confederations have the following characteristics.
28
• A large AS can be sub-divided into sub-confederations.
• Routing within each sub-confederation is accomplished via IBGP.
• EBGP is used to communicate between sub-confederations.
• BGP speakers within a sub-confederation must be fully meshed.
Router Configuration Guide