3Com 3C13636 Configuration Manual page 617

3Com Router 3000 Ethernet Family
Configuration Guide
DR is elected by all the routers on the segment. Routers with the priorities greater than
0 in the network are eligible "candidates". Among all the routers self-proclaimed to be
the DR, the one with the highest priority will be elected. If two routers have the same
priority, the one with the highest router ID will be elected as the DR. Votes are the hello
packets. Each router writes the elected DR in the packet and sends it to all the other
routers on the segment. If two routers attached to the same segment concurrently
declare themselves to be the DR, choose the one with higher priority. If their priorities
are equal, the one with greater router ID will be preferred. A router, whose priority is 0,
cannot be elected as a DR or BDR.
If a DR becomes invalid due to some fault, it must be reelected from routers on the
network and synchronized with other routers. It takes time and meanwhile the route
calculation is incorrect. In order to speed up this process, the concept of BDR is instilled
in OSPF. In fact, BDR is a backup for DR. DR and BDR are elected in the mean time.
The adjacencies are also established between the BDR and all the routers on the
segment, and routing information is also exchanged between them. When the DR fails,
the BDR will become the DR instantly. Since no re-election is needed and the
adjacencies have already been established, the process is very short. But in this case,
a new BDR should be elected. Although it will also take a quite long period of time, it will
not exert any influence upon the route calculation.
But please note:
When the precedence of an interface is 0, the interface cannot be a DR/BDR
anyway, which may result in a network without DR or BDR.
The DR on the network is not necessarily the router with the highest priority.
Likewise, the BDR is not necessarily the router with the second highest priority. If a
new router is added after DR and BDR election, it is impossible for the router to
become the DR even if it has the highest priority.
A DR is on a certain network segment, in the sense of router interface. Maybe a
router is a DR on one interface, but can be a BDR or DR Other on the other
interface.
Only when an interface is of broadcast or NBMA type, is it necessary to elect DR. A
p2mp or a p2p interface needs not to elect DR. On a broadcast network or NBMA
network, if no one declares that it is DR in the hello packets received, the election
process starts; if multiple routes declare that they are the DR/BDR, the election
process also starts; if a router declares that it is the DR/BDR, new-added routers
will accept the existing DR/BDR without considering about its precedence; when
the DR fails, the BDR will become the DR, a new BDR will be elected.
Perform the following configuration in interface view.
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Chapter 4 OSPF Configuration