Setting Up The Neighbor Relationship - Hirschmann MACH 4000 User Manual

OSPF
7.3 Setting up the Neighbor
Relationship
When a router is started, it uses what are called hello packets to contact its
neighboring routers. With these hello packets, an OSPF router finds out
which OSPF routers are near it and whether they are suitable for setting up
a neighbor relationship (adjacency).
In broadcast networks such as Ethernet, the number of neighbors increases
with the number of routers connected, as does the information exchange for
clarifying and maintaining the neighbor relationships. To reduce these
volumes within an area, OSPF uses the hello protocol to determine a
Designated Router (DR) within the corresponding segment. Thus every
router in an area only sets up the neighbor relationship with its designated
router, instead of with every neighbor. The designated router is responsible
for the distribution of all the link state information to its neighbor routers.
For security reasons, OSPF provides for the selection of a Backup
Designated Router (BDR), which takes over the tasks of the DR if the DR
fails. The OSPF router with the highest router priority is the DR. The router
priority is specified by the administrator. If two routers have the same priority,
the router with the higher router ID is selected. The router ID is the smallest
IP address of a router interface. You configure this router ID manually when
starting up the OSPF router
DR
Figure 29: LSA distribution with designated router and
backup designated router
78

7.3 Setting up the Neighbor Relationship

(see on page
BDR
70 "Router ID").
Routing L3P
Release 4.0 11/07