102 CHAPTER 15: ROUTING CONFIGURATION GUIDE
Precautions ■Before configuring OSPF basic functions, configure a router ID for each OSPF
process to ensure OSPF runs normally. You are recommended to use the ospf
command to configure router IDs for the processes, especially on a device
running multiple processes.
■To prevent route leaking and enhance network security, use the
silent-interface command on the interfaces not running OSPF to disable them
from sending OSPF packets.
Configuring OSPF DR Election
On broadcast or NBMA networks, any two routers need to exchange routing
information with each other. If n routers are present on a network, n × (n-1)/2
adjacencies are required. Any route change on a router in such a network
generates traffic for routing information synchronization, consuming network
resources. The Designated Router (DR) is defined to solve the problem. All the
other routers on the network send routing information to the DR, which is
responsible for advertising link state information.
On a network, a BDR is elected along with a DR and establishes adjacencies with
all the other routers for routing information exchange. When the DR fails, the BDR
will become the new DR in a very short period by avoiding adjacency
establishment and DR re-election. Meanwhile, other routers elect another BDR,
which requires a relatively long period but has no influence on routing calculation.
A router that is neither a DR nor a BDR is a DRother. It forms adjacencies with the
DR and BDR, but it neither establishes adjacencies nor exchange routing
information with each other, thus reducing the number of adjacencies on
broadcast and NBMA networks.
The DR and BDR in a network are elected by all the routers attached to the
network. 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.
Network Diagram Figure29 Network diagram for DR/BDR election
Device Interface IP address Router ID Interface priority
Switch A Vlan-int1 196.1.1.1/24 1.1.1.1 100
Switch B Vlan-int1 196.1.1.2/24 2.2.2.2 0
Switch C Vlan-int1 196.1.1.3/24 3.3.3.3 2
Switch D Vlan-int1 196.1.1.4/24 4.4.4.4 1
DR
BDR
Switch A Switch D
Switch CSwitch B
Vlan-int1
Vlan-int1
Vlan-int1
Vlan-int1