OSPF DR / BDR

DR – designated router in multi access network topology act as collector and distributor for LSAs. A BDR – backup designated router is elected in case the designated router fails. All other router become DROthers. Instead flooding LSA to all routers in multi access network, DROthers only send their LSAs to the DR and BDR using multicast address 224.0.0.6. The DR use multi access address 224.0.0.5. And the result is that only DR router flood all the LSAs in multi access network.

DR Election:
1- Choose the highest OSPF priority (default is 1, note that if you configure priority of 0 that router wont enter the election process – used on FR networks.) router (config-if)#ip ospf priority {0 to 255}
2- Choose the highest router-id.
3- Choose the highest loopback interface.
4- Choose the highest configured physical interface (must be up/up).

What is the maximum and minimum number of DR and BDR in an Area:
You can only have 1 DR and 1 BDR in an area. When the DR goes down, the BDR becomes DR and then an election is taken place to see who is new BDR.

Network Multiaccess network
I think of multiaccess is that any network segment on which a DR needs to be elected.

DR and BDR is elected on per interface basis.Dr/Bdr is elected on multiaccess network type, so in an area the number of DR/BDRs depends on the number of multiacess interfaces you have in that particular area.

drdr1232
R2#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.10.10.30 1 FULL/DR 00:00:32 1.1.1.2 GigabitEthernet0/0
192.168.10.2 1 FULL/DR 00:00:36 192.168.10.2 GigabitEthernet0/1

Router-id can’t be the same as the other segment.

So if your router has 2 segments in different/same multiacess networks it can be a Dr/BDR on 1 segment at the same time it can be a DRother router on another multiacess network.Per segment an OSPF process can have a single DR with no BDR or a single DR and a single BDR.

Multi access networks create challenge for OSPF because:
a) create multiple adjacencies (one adjacencies for every pair of router)
b) extensive flooding of LSA – link state advertisement
for n -routers it is n(n-1)/2 adjacencies.

Command to verify DR/BDR:
1. show ip ospf interface
2. show ip ospf neighbor

Note: point-to-point, point-to-multipoint, point-to-multipoint non-broadcast networks will not.

Reference:
https://learningnetwork.cisco.com/thread/15136
http://cdesigner.eu/content/6-ospf-dr-and-bdr-slection-multiaccess-network-segment-examination-lab

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