Nortel 5100 Series Release 2.3.3, Руководство пользователя и справочник команд, Страница: 98 / 430

Nortel Switched Firewall 2.3.3 User’s Guide and Command Reference
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„
Open Shortest Path First
213455-L, October 2005
„ Autonomous System Boundary Router (ASBR)—a router that acts as a gateway between
the OSPF domain and non-OSPF domains, such as RIP, BGP, and static routes (see
Figure
54 ).
Figure 54 OSPF domain and an autonomous system
Neighbors and adjacencies
In areas with two or more routing devices, neighbors and adjacencies are formed.
Neighbors are routing devices that maintain information about each others’ health. To
establish neighbor relationships, routing devices periodically send hello packets on each of
their interfaces. All routing devices that share a common network segment, appear in the same
area, and have the same health parameters (
hello
and
dead
intervals) and authentication
parameters respond to each other’s hello packets and become neighbors. Neighbors continue to
send periodic hello packets to advertise their health to neighbors. In turn, they listen to hello
packets to determine the health of their neighbors and to establish contact with new neighbors.
Adjacencies are neighbors that exchange OSPF database information. In order to limit the
number of database exchanges, not all neighbors in an area (IP network) become adjacent to
each other. Instead, the hello process is used for electing one of the neighbors as the area’s
Designated Router (DR) and one as the area’s Backup Designated Router (BDR).
The DR is adjacent to all other neighbors and acts as the central contact for database
exchanges. Each neighbor sends its database information to the DR, which relays the
information to the other neighbors.
Backbone
Area 0
Area 3
Area 2
Area 1
Inter-Area Routes
(Summary Routes)
ABR
ABR
ABR
ASBR
Internal
Router
ASBR
External
Routes
BGP
RIP
OSPF Autonomous System