66
Figure 20 DR and BDR in a network
NOTE:
In OSPF, neighbor and adjacency are different concepts.
fter startup, OSPF sends a hello packet
on each OSPF interface. A receiving router checks parameters in the packet. If the parameters
match its own, the receiving router considers the sending router an OSPF neighbor. Two OSPF
neighbors establish an adjacency relationship after they synchronize their LSDBs through exchange
of DD packets and LSAs.
DR and BDR election
DR election is performed on broadcast or NBMA networks but not on P2P and P2MP networks.
Routers in a broadcast or NBMA network elect the DR and BDR by router priority and ID. Routers
with a router priority value higher than 0 are candidates for DR and BDR election.
The election votes are hello packets. Each router sends the DR elected by itself in a hello packet to
all the other routers. If two routers on the network declare themselves as the DR, the router with the
higher router priority wins. If router priorities are the same, the router with the higher router ID wins.
If a router with a higher router priority is added to the network after DR and BDR election, the router
cannot become the DR or BDR immediately because no DR election is performed for it. Therefore,
the DR of a network might not be the router with the highest priority, and the BDR might not be the
router with the second highest priority.
Protocols and standards
• RFC 1765, OSPF Database Overflow
• RFC 2328, OSPF Version 2
• RFC 3101, OSPF Not-So-Stubby Area (NSSA) Option
• RFC 3137, OSPF Stub Router Advertisement
• RFC 4811, OSPF Out-of-Band LSDB Resynchronization
• RFC 4812, OSPF Restart Signaling
• RFC 4813, OSPF Link-Local Signaling
OSPF configuration task list
To run OSPF, you must first enable OSPF on the router. Make a proper configuration plan to avoid
incorrect settings that can result in route blocking and routing loops.
To configure OSPF, perform the following tasks:
DR BDR
DR other DR otherDR other
Physical links Adjacencies
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