3Com Switch 4800G 24-Port Configuration Manual page 282

282 C 28: OSPF C
HAPTER
n
OSPF Packet Formats
ONFIGURATION
become 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.
Figure 79 DR and BDR in a network
DR
DRother
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.
The DR election is available on broadcast, NBMA interfaces rather than P2P, or
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P2MP interfaces.
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.
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.
The DR may not be the router with the highest priority in a network, and the
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BDR may not be the router with the second highest priority.
OSPF packets are directly encapsulated into IP packets. OSPF has the IP protocol
number 89. The OSPF packet format is shown below (taking a LSU packet as an
example).
BDR
DRother DRother