Designated Routers In Multi-Access Networks; Designated Router Election In Multi-Access Networks - Dell PowerConnect B-FCXs Configuration Manual

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Overview of OSPF
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 963.

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 139
FIGURE 139
Designated and backup router election
Designated Backup Router
priority 10
Router A
Designated Router
priority 5
priority 20
Router C
Router B
If the DR goes off-line, the BDR automatically becomes the DR. The router with the next highest
priority becomes the new BDR. This process is shown in Figure 140.
924 PowerConnect B-Series FCX Configuration Guide
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