Configuration Guide User guide

1228 FastIron Configuration Guide
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OSPF overview
OSPF point-to-point links
One important OSPF process is Adjacency. Adjacency occurs when a relationship is formed
between neighboring routers for the purpose of exchanging routing information. Adjacent OSPF
neighbor routers go beyond the simple Hello packet exchange; they exchange database
information. In order to minimize the amount of information exchanged on a particular segment,
one of the first steps in creating adjacency is to assign a Designated Router (DR) and a Backup
Designated Router (BDR). The Designated Router ensures that there is a central point of contact,
thereby improving convergence time within a multi-access segment.
In an OSPF point-to-point network, where a direct Layer 3 connection exists between a single pair of
OSPF routers, there is no need for Designated and Backup Designated Routers, as is the case in
OSPF multi-access networks. Without the need for Designated and Backup Designated routers, a
point-to-point network establishes adjacency and converges faster. The neighboring routers
become adjacent whenever they can communicate directly. In contrast, in broadcast and
non-broadcast multi-access (NBMA) networks, the Designated Router and Backup Designated
Router become adjacent to all other routers attached to the network.
To configure an OSPF point-to-point link, refer to “Configuring an OSPF point-to-point link” on
page 1269.
Designated routers in multi-access networks
In a network that has multiple routers attached, OSPF elects one router to serve as the designated
router (DR) and another router on the segment to act as the backup designated router (BDR). This
arrangement minimizes the amount of repetitive information that is forwarded on the network by
forwarding all messages to the designated router and backup designated routers responsible for
forwarding the updates throughout the network.
Designated router election in multi-access networks
In a network with no designated router and no backup designated router, the neighboring router
with the highest priority is elected as the DR, and the router with the next largest priority is elected
as the BDR, as shown in Figure 159