3Com 5500-EI PWR Install Manual page 297

solve this problem, DR is defined in OSPF so that all routers send information to the DR only and the
DR broadcasts the network link states in the network.
If the DR fails, a new DR must be elected and synchronized with the other routers on the network. The
process takes quite a long time; in the process, route calculation is incorrect. To shorten the process,
BDR is introduced in OSPF.
A BDR provides backup for a DR. DR and BDR are elected at the same time. Adjacencies are also
established between the BDR and all the other routers on the segment, and routing information is also
exchanged between them. Once the DR becomes invalid, the BDR becomes a DR. Since no re-election
is needed and the adjacencies already exist, the switchover process is very short. Now, a new BDR
should be elected. Although this election process will also take quite a long time, route calculation will
not be affected.
On an OSPF network, a router which is neither DR nor BDR is called DR Other. It establishes
adjacencies with the DR and BDR, but not with other DR Others. This reduces not only the number of
adjacencies among routers, but also the network traffic and occupied bandwidth resources on the
broadcast or NBMA network.
In 0, the solid lines represent physical Ethernet connections and the dotted lines represent adjacencies
established. The figure shows that, with the DR/BDR mechanism adopted, seven adjacencies suffice
among the five routers.
Figure 4-7 DR/BDR
DR
DRother
DR/BDR election
The DR and BDR in a network are elected by all routers rather than configured manually. The DR
priority of an interface determines its qualification for DR/BDR election. Interfaces attached to the
network and having priorities higher than '0" are election candidates.
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
DR priority wins. If DR priorities are the same, the router with the higher Router ID wins. In addition, a
router with the priority 0 cannot become the DR/BDR.
Note the following points:
DR election is required for broadcast or NBMA interfaces but is not required for P2P or P2MP
interfaces.
BDR
DRother DRother
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