Foundry Networks Switch and Router User Manual

Configuring OSPF
December 2000 17 - 3
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
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.
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.
Link
Activity
FDX
Link/Act
FDX
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Power
Console
Router A
priority 10
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FDX
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FDX
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Power
Console
Router C
priority 5 priority 20
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FDX
Link/Act
FDX
Link/Act
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Power
Console
Router B
Designated Router
Designated Backup Router
Link
Activity
FDX
Link/Act
FDX
Link/Act
Link
Activity
Power
Console
Router A
priority 10
Link
Activity
FDX
Link/Act
FDX
Link/Act
Link
Activity
Power
Console
Router C
priority 5 priority 20
Link
Activity
FDX
Link/Act
FDX
Link/Act
Link
Activity
Power
Console
Router B
Designated Router
Designated Backup Router
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Foundry Networks Switch and Router Specifications

General

Brand	Foundry Networks
Model	Switch and Router
Category	Switch
Language	English