Neighbors And Adjacencies; The Link-State Database - Lenovo RackSwitch G8264CS Application Manual

Neighbors and Adjacencies

The Link-State Database

470
G8264CS Application Guide for ENOS 8.4
Figure 49. OSPF Domain and an Autonomous System
BGP
External
ASBR
Routes
RIP
ASBR
In areas with two or more routing devices, neighbors and adjacencies are formed.
Neighbors are routing devices that maintain information about each others' state. 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.
The hello process is used for electing one of the neighbors as the network segment's
Designated Router (DR) and one as the network segment's Backup Designated
Router (BDR). The DR is adjacent to all other neighbors on that specific network
segment 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.
The BDR is adjacent to all other neighbors (including the DR). Each neighbor sends
its database information to the BDR just as with the DR, but the BDR merely stores
this data and does not distribute it. If the DR fails, the BDR will take over the task
of distributing database information to the other neighbors.
OSPF is a link‐state routing protocol. A link represents an interface (or routable
path) from the routing device. By establishing an adjacency with the DR, each
routing device in an OSPF area maintains an identical Link‐State Database (LSDB)
describing the network topology for its area.
Each routing device transmits a Link‐State Advertisement (LSA) on each of its
active interfaces. LSAs are entered into the LSDB of each routing device. OSPF uses
flooding to distribute LSAs between routing devices. Interfaces may also be passive.
Passive interfaces send LSAs to active interfaces, but do not receive LSAs, hello
OSPF Autonomous System
Backbone
Area 0
Inter-Area Routes
(Summary Routes)
ABR
Area 1
Area 3
ABR
ABR
Internal
Router
Area 2