F3215-HP Load Balancing Module Appendix Protocol Reference-6PW101

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DR and BDR
On a broadcast or NBMA network, any two routers must establish an adjacency to exchange routing
information with each other. If n routers are present on the network, n(n-1)/2 adjacencies are established.
Any topology change on the network results in an increase in traffic for route synchronization, consuming
many system and bandwidth resources.
The DR and BDR mechanisms can solve this problem.
• 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. When 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 9, soli
d lines ar
e Ethernet physical links, and dashed lines represent OSPF adjacencies. With
the DR and BDR, only seven adjacencies are established.
Figure 9 DR and BDR in a network
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 or 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.
DR BDR
DR other DR otherDR other
Physical links Adjacencies