H3C Technologies H3C S5560 Series Switches - Manual (Page 85)

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Using 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
without BDR is time-consuming and is prone to route calculation errors.
• BDR—Elected along with the DR to establish adjacencies with all other routers. If 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 DR Others. 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 DR
Other on another interface.
As shown in
Figure 20 , solid lines are Ethernet physical links, and dashed lines represent OSPF
adjacencies. With the DR and BDR, only seven adjacencies are established.
Figure 20 DR and BDR in a network
NOTE:
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 and 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.
If a router with a higher router priority is added to the network after DR and BDR election, the router
cannot become the DR or BDR immediately because no DR election is performed for it. Therefore, the DR
of a network might not be the router with the highest priority, and the BDR might not be the router with
the second highest priority.
DR BDR
DR other DR other DR other
Physical links Adjacencies