Designated Router Election - Foundry Networks Switch and Router Installation And Configuration Manual

Designated Router Election

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
17.2
priority 5
Power
Router C
Figure 17.2 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 17.3.
NOTE: Priority is a configurable option at the interface level. You can use this parameter to help bias one router
as the DR.
Designated Backup Router
priority 5
Power
Router C
Figure 17.3 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.
NOTE: By default, the Foundry router ID is the IP address configured on the lowest numbered loopback
interface. If the Layer 3 Switch does not have a loopback interface, the default router ID is the lowest numbered
IP address configured on the device. For more information or to change the router ID, see "Changing the Router
ID" on page 15-26.
When multiple routers on the same network are declaring themselves as DRs, then both priority and router ID are
used to select the designated router and backup designated routers.
December 2000
Designated Backup Router
priority 10
Link Link
Activity Activity
Console
Power
Router A
Link Link
Activity Activity
FDX
Console
Link/Act
FDX
Link/Act
Designated Router
priority 10
Link Link
Activity Activity
Console
Power
Router A
Link Link
Activity Activity
FDX
Console
Link/Act
FDX
Link/Act
FDX
Link/Act
FDX
Link/Act
Designated Router
priority 20
Link Link
Activity Activity
FDX
Console
Link/Act
FDX
Power Link/Act
Router B
FDX
Link/Act
FDX
Link/Act
X
priority 20
Link Link
Activity Activity
FDX
Console
Link/Act
FDX
Power Link/Act
Router B
Configuring OSPF
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