Exchange of routing information, Designated and backup designated routers – Lucent Technologies 6000 User Manual
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Configuring OSPF Routing
OSPF overview
MAX 6000/3000 Network Configuration Guide
8-3
Masks (VLSM), or Classless Inter-Domain Routing (CIDR). The MAX routes a packet to the
best (longest, or most specific) match. The MAX considers host routes to be subnets whose
masks are all ones (0xFFFFFFFF).
Note:
Although OSPF is very useful for networks that use VLSM, Lucent recommends that
you attempt to assign subnets as contiguously as possible, to prevent excessive link-state
calculations by all OSPF routers on the network.
Exchange of routing information
OSPF uses a topological database of the network and propagates only changes to the database.
Part of the SPF algorithm involves acquiring neighbors and forming an adjacency with one
neighbor, as shown in Figure 8-1.
Figure 8-1. Adjacency between neighboring routers
An OSPF router dynamically detects its neighboring routers by sending Hello packets to the
multicast address All SPFRouters. It then attempts to form adjacencies with some of its
newly acquired neighbors.
Adjacency is a relationship formed between selected neighboring routers for the purpose of
exchanging routing information. Not every pair of neighboring routers becomes adjacent.
Adjacencies are established during network initialization in pairs, between two neighbors. As
the adjacency is established, the neighbors exchange databases and build a consistent,
synchronized database between them.
When an OSPF router detects a change on one of its interfaces, it modifies its topological
database and multicasts the change to its adjacent neighbor, which in turn propagates the
change to its adjacent neighbor until all routers within an area have synchronized topological
databases. The result is quick convergence among routers.
Designated and Backup Designated Routers
In OSPF terminology, a broadcast network is any network that has more than two OSPF
routers attached and that supports the capability to address a single physical message to all of
the attached routers.
Router-1
Router-2
Router-3
Adjacency