65
•
DR
—Elected to advertise routing information among other routers. If the DR fails, routers on the
network must elect another DR and synchronize information with the new DR. Using this mechanism
alone is time-consuming and prone to route calculation errors.
•
BDR
—Elected along with the DR to establish adjacencies with all other routers. If the DR fails, the
BDR immediately becomes the new DR, and other routers elect a new BDR.
Routers other than the DR and BDR are called "DROthers." They do not establish adjacencies with one
another, so the number of adjacencies is reduced.
The role of a router is subnet (or interface) specific. It might be a DR on one interface and a BDR or
DROther on another interface.
In
Figure 20
, solid lines are Ethernet physical links, and dashed lines represent OSPF adjacencies. With
the DR and BDR, only seven adjacencies are established.
Figure 20
DR and BDR in a network
NOTE:
In OSPF, "neighbor" and "adjacency" are different concepts. After 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 becomes active after DR and BDR election, the router cannot
replace the DR or BDR until a new election is performed. 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
DR
BDR
DR other
DR other
DR other
Physical links
Adjacencies
