OSPF - DR/BDR

When mulitple routers are on the same segment, running OSPF, they form adjacencies with a designated router, and a backup designated router, this prevents routers from having to form large numbers of adjacencies with all routers.

Designated router is the router with highest interface IP address, or preferably loopback interface, BDR is router with second highest. A new router with highest IP address can not preempt the current DR. To force a DR re-election use command "clear ip ospf process *" on DR router. Use the router-id command to force the ip address used for ospf process router-id.

The DR will receive updates and send updates to segment routers. Segment routers advertise changes to the DR/BDR using multicast address 224.0.0.6.

*Mar 1 00:30:44.531: Processing link 0, id 172.16.0.1, link data 255.255.255.255, type 3
*Mar 1 00:30:44.531: Add better path to LSA ID 172.16.0.1, gateway 172.16.0.1, dist 2
*Mar 1 00:30:44.531: Add path: next-hop 192.168.0.1, interface FastEthernet0/0
*Mar 1 00:30:44.531: Processing link 1, id 192.168.0.2, link data 192.168.0.1, type 2
*Mar 1 00:30:44.535: Ignore newdist 2 olddist 1
*Mar 1 00:30:44.535: OSPF: Adding Stub nets
*Mar 1 00:30:44.535: OSPF: Add Network Route to 172.16.0.1 Mask /32. Metric: 2, Next Hop: 192.168.0.1
*Mar 1 00:30:44.535: OSPF: insert route list LS ID 172.16.0.1, type 0, adv rtr 192.168.0.1

Router#sh run sec router osp
router ospf 72
router-id 172.16.0.1
log-adjacency-changes
network 172.16.0.0 0.0.0.255 area 51
network 192.168.0.0 0.0.0.255 area 51