Ospf Packet Formats - HP 4800G Series Configuration Manual

If the DR fails to work, routers on the network have to elect another DR and synchronize information
with the new DR. It is time-consuming and prone to routing calculation errors. The Backup Designated
Router (BDR) is introduced to reduce the synchronization period.
The BDR is elected along with the DR and establishes adjacencies for routing information exchange
with all other routers. When the DR fails, the BDR will 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 1-7 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.
Note that:
The DR election is available on broadcast, NBMA interfaces rather than P2P, or 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 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).
BDR
DRother DRother
1-9