3Com Switch 7754, Configuration Manual

Page: 305 / 940

OSPF Overview 305
In fact, 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.
Neither neighboring relationship is established nor routing information is
exchanged between DR Others (routers other than DR and BDR). This reduces the
number of adjacencies among routers on the broadcast or NBMA network.
As shown in Figure 72, 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 72 DR and BDR
DR/BDR election
Instead of being manually configured, DR and BDR are elected by all the routers on
the current network segment. The priority of a router interface determines the
qualification of the interface in DR/BDR election. All the routers with DR priorities
greater than 0 in the current network segment are eligible “candidates”.
Hello packets serve as the “votes” in the election. Each router writes the DR it
selects to the Hello packet and sends the packet to each router running OSPF in
the network segment. If two routers on the same network segment declare
themselves to be the DR, the one with the highest DR priority will be preferred. If
their priorities are the same, the one with greater router ID will be preferred. A
router whose DR priority is 0 can neither be elected as the DR nor be elected as
the BDR.
Note the following points:
■
DR election is required for broadcast or NBMA interfaces but is not required for
P2P or P2MP interfaces.
■
DR is based on the router interfaces in a certain segment. A router may be a DR
on an interface and a BDR or DR Other on another interface.
■
If a new router is added after DR and BDR election, the router does not become
the DR immediately even if it has the highest DR priority.
DR BDR
DRother DRother DRother