Quick Reference Guide

660 PowerConnect B-Series TI24X Configuration Guide
53-1002269-02
Overview of OSPF
23
NOTE
Priority is a configurable option at the interface level. You can use this parameter to help bias one
router as the DR.
FIGURE 107 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 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, refer to
“Changing the router ID” on page 584.
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.
When only one router on the network claims the DR role despite neighboring routers with higher
priorities or router IDs, this router remains the DR. This is also true for BDRs.
The DR and BDR election process is performed when one of the following events occurs:
• An interface is in a waiting state and the wait time expires
• An interface is in a waiting state and a hello packet is received that addresses the BDR
• A change in the neighbor state occurs, such as:
• A neighbor state transitions from 2 or higher
• Communication to a neighbor is lost
• A neighbor declares itself to be the DR or BDR for the first time
OSPF RFC 1583 and 2178 compliance
Routers are configured, by default, to be compliant with the RFC 1583 OSPF V2 specification.
Routers can also be configured to operate with the latest OSPF standard, RFC 2178.
X
Designated Backup Router
Router C
priority 5
Router B
priority 20
Designated Router
priority 10
Router A