Foundry Configuration Guide for the FESX, FSX, and FWSX
20 - 4 © Foundry Networks, Inc. December 2005
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
20.2
Figure 20.2 Designated and backup router election
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 20.3.
NOTE: Priority is a configurable option at the interface level. You can use this parameter to help bias one router
as the DR.
Figure 20.3 Backup designated router becomes designated router
If two neighbors share the same priority, the router with the highest router ID is designated as the DR. The router
with the next highest router ID is designated as the BDR.
Router A
priority 10
Router C
priority 5 priority 20
Router B
Designated Router
Designated Backup Router
Router A
priority 10
Router C
priority 5 priority 20
Router B
Designated Router
Designated Backup Router
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