Operation Manual – IPv4 Routing
H3C S5500-EI Series Ethernet Switches
Chapter 3 OSPF Configuration
3-11
the new DR in a very short period by avoiding adjacency establishment and DR
reelection. Meanwhile, other routers elect another BDR, which requires a relatively long
period but has no influence on routing calculation.
Other routers, also known as DRothers, establish no adjacency and exchange no
routing information with each other, thus reducing the number of adjacencies on
broadcast and NBMA networks.
In the following figure, real lines are Ethernet physical links, and dashed lines represent
adjacencies. With the DR and BDR in the network, only seven adjacencies are enough.
DR
BDR
DRother
DRother
DRother
Figure 3-7
DR and BDR in a network
II. 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 that:
z
The DR election is available on broadcast, NBMA interfaces rather than P2P, or
P2MP interfaces.
z
A DR is an interface of a router and belongs to a single network segment. The
router’s other interfaces may be a BDR or DRother.
z
After DR/BDR election and then a new router joins, it cannot become the DR
immediately even if it has the highest priority on the network.
z
The DR may not be the router with the highest priority in a network, and the BDR
may not be the router with the second highest priority.
