Design for OSPF in Multiple-Access network to manage LSA flooding.
Designated router represents all OSPF routers in its area. Designated router is port-based, a router may be the DR for one area but may not be the DR of another area.
To better manage LSA floods, DR is elected.
DRothers will form adjacency with DR and BDR only. DRother is a router that’s neither a DR nor a BDR.
DRother routers still send hello packet to ALLSPFROUTERS using 224.0.0.5 to check the existence of neighbouring OSPF routers. However update will only be multicast using 224.0.0.6, only DR and BDR will listen to this MC.
DR will send update to ALLSPFROUTERS using 224.0.0.5.
DR and BDR also forms adjacency to each other.
Backup Designated Router (BDR) is elected to replace DR should DR fail. Hence DRother routers also form adjacency with BDR.
When OSPF process started, the process will look for existing DR and BDR, if neither exists DR election starts.
If there’s no DR but there’s a BDR, BDR will be promoted to DR.
During the election process the highest priority router wins the DR, the next highest priority wins BDR. If all routers’ priority is a tie, then the highest router ID will be the tie-breaker.
There’s a catch, OSPF router that’s configured first will be the DR regardless of priority and the router id.
R2#sh ip ospf neighb
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DR 00:00:33 172.30.0.1 FastEthernet0/0
3.3.3.3 1 FULL/BDR 00:00:33 172.30.0.3 FastEthernet0/0
1.1.1.1 has a lower router id however it is the DR just because it was configured first. Default priority value for all OSPF router is 1, 0 means not eligible for DR/BDR election.
The command clear ip ospf 1 process is used to reset the OSPF process.
After the OSPF process restarted for all routers:
R2#sh ip ospf neighb
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DROTHER 00:00:33 172.30.0.1 FastEthernet0/0
3.3.3.3 1 FULL/DR 00:00:34 172.30.0.3 FastEthernet0/0
R3#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 1 FULL/DROTHER 00:00:37 172.30.0.1 FastEthernet0/0
2.2.2.2 1 FULL/BDR 00:00:36 172.30.0.2 FastEthernet0/0
10.20.3.1 0 FULL/ – 00:00:34 172.30.20.4 Serial0/0
I have got a strange request:
Ensure R1 becomes the DR on the Ethernet segment in Area 0. R2 and R3 should not become a DR or BDR for the
Ethernet segment in Area 0.
R1(config-if)#ip ospf priority 2
Increase the OSPF priority which was default 1. As mentioned earlier, DR and BDR are properties of a router’s port and not the entire router. Interface fa0/0 of R1 is solely in Area 0, to make R1 to be the DR, I increase the ospf priority.
R3(config-if)#ip ospf priority 0
R2(config-if)#ip ospf priority 0
Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 0 2WAY/DROTHER 00:00:31 172.30.0.3 FastEthernet0/0
Hmm.. surprised… no need to clear ip ospf process
R3#sh ip ospf neighb
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 2WAY/DROTHER 00:00:34 172.30.0.2 FastEthernet0/0
10.20.3.1 0 FULL/ – 00:00:32 172.30.20.4 Serial0/0
R1#sh ip ospf neighb
R1#
Nothing?! No adjacency formed?! wow….
R1#sh ip ospf database
OSPF Router with ID (192.168.1.100) (Process ID 2)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 2272 0x80000002 0x004E40 1
2.2.2.2 2.2.2.2 2281 0x80000003 0x00087E 1
3.3.3.3 3.3.3.3 2282 0x80000003 0x00CCAF 1
192.168.1.100 192.168.1.100 491 0x80000002 0x00851F 0
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.30.0.3 3.3.3.3 552 0x80000004 0x00BBBA
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
172.30.20.0 3.3.3.3 2367 0x80000001 0x00FB8D
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.0.0 1.1.1.1 2283 0x80000001 0x0053D7 0
172.16.1.0 1.1.1.1 2286 0x80000001 0x0048E1 0
172.16.2.0 1.1.1.1 2286 0x80000001 0x003DEB 0
172.16.3.0 1.1.1.1 2286 0x80000001 0x0032F5 0
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 5 0x80000002 0x005CFF 1
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
172.16.0.0 1.1.1.1 59 0x80000001 0x0053D7 0
172.16.1.0 1.1.1.1 59 0x80000001 0x0048E1 0
172.16.2.0 1.1.1.1 59 0x80000001 0x003DEB 0
172.16.3.0 1.1.1.1 59 0x80000001 0x0032F5 0
Hmm.. process 2? I think I have created ip ospf 2 area 0 accidentally just now on interface fa0/0 of R1.
R1(config-if)#ip ospf 1 area 0
R1(config-if)#end
R1#sh ip ospf neighb
Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 0 2WAY/DROTHER 00:00:36 172.30.0.2 FastEthernet0/0
3.3.3.3 0 2WAY/DROTHER 00:00:36 172.30.0.3 FastEthernet0/0
R1#
R3#sh ip ospf neighb
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 2 FULL/DR 00:00:31 172.30.0.1 FastEthernet0/0
2.2.2.2 0 2WAY/DROTHER 00:00:31 172.30.0.2 FastEthernet0/0
10.20.3.1 0 FULL/ – 00:00:30 172.30.20.4 Serial0/0
R2#sh ip ospf neighb
Neighbor ID Pri State Dead Time Address Interface
1.1.1.1 2 FULL/DR 00:00:39 172.30.0.1 FastEthernet0/0
3.3.3.3 0 2WAY/DROTHER 00:00:38 172.30.0.3 FastEthernet0/0
R2#sh ip route ospf
172.16.0.0/24 is subnetted, 4 subnets
O E2 172.16.0.0 [110/200] via 172.30.0.1, 00:03:59, FastEthernet0/0
O E2 172.16.1.0 [110/200] via 172.30.0.1, 00:03:59, FastEthernet0/0
O E2 172.16.2.0 [110/200] via 172.30.0.1, 00:03:59, FastEthernet0/0
O E2 172.16.3.0 [110/200] via 172.30.0.1, 00:03:59, FastEthernet0/0
172.30.0.0/24 is subnetted, 2 subnets
O IA 172.30.20.0 [110/201] via 172.30.0.3, 00:03:59, FastEthernet0/0
R3#sh ip route ospf 1
172.16.0.0/24 is subnetted, 4 subnets
O E2 172.16.0.0 [110/200] via 172.30.0.1, 00:04:25, FastEthernet0/0
O E2 172.16.1.0 [110/200] via 172.30.0.1, 00:04:25, FastEthernet0/0
O E2 172.16.2.0 [110/200] via 172.30.0.1, 00:04:25, FastEthernet0/0
O E2 172.16.3.0 [110/200] via 172.30.0.1, 00:04:25, FastEthernet0/0
I was looking for the priority command and mistook ip ospf 2 area 0 to be the priority command….-_-a
cyruslab 
very nice demonistation, thank you so much. but still i have one confiusion regarding the meaning of 2way/full what is the diffrance
“Designated router is port-based, a router may be the DR for one area but may not be the DR of another area.”
This is a tad misleading and incorrect. DR is segment based – the same router may be the DR on multiple segments in the same area, as well as in different areas. In fact the same router may be DR on one subnet, BDR on another and DROther on a third segment, all within the same area.
Best explanation ever. Thanks