Operation Manual – Routing Protocol
H3C S3600 Series Ethernet Switches-Release 1510 Chapter 4
OSPF Configuration
4-6
In Figure 4-2, 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.
DR
DR Other DR Other
BDR
DR Other
Figure 4-2 DR and BDR
IV. 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:
z DR election is required for broadcast or NBMA interfaces but is not required for
P2P or P2MP interfaces.
z 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.
z 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.
z The DR on a network segment is not necessarily the router with the highest
priority. Likewise, the BDR is not necessarily the router with the second-highest
priority.
4.1.5 OSPF Packets
OSPF uses five types of packets:
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