3Com S7906E - Switch, Configuration Manual

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1-10
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.
OSPF Packet Formats
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).
Figure 1-8 OSPF packet format
OSPF packet header
OSPF packets are classified into five types that have the same packet header, as shown below.
Figure 1-9 OSPF packet header
z
Version: OSPF version number, which is 2 for OSPFv2.
z
Type: OSPF packet type from 1 to 5, corresponding with hello, DD, LSR, LSU and LSAck
respectively.
z
Packet length: Total length of the OSPF packet in bytes, including the header.