OSPF Neighbor State and show ip route ospf output

[Irakli G.]

Hi,

I have OSPFv2 lab  using IOSv Software (VIOS-ADVENTERPRISEK9-M), Version 15.6(2)T, RELEASE SOFTWARE (fc2) devices, where I am using only one area (backbone area, 0). I use 5 routers, 4 of them have interfaces connected to the same broadcast domain. On topology (that I have attached) you can see those routers: R2,R3,R4 and R5. On that shared segment of my network (192.168.0.16/29) R5 and R4 are DR,BDR respectively. R2 and R3 have 2WAY/DROTHER neighboring state with each other. Both R2 and R3 have directly connected subnets: 172.16.2.0/24 and 172.16.3.0/24 respectively. I am confused about output of ip route 172.16.2.0 and  ip route 172.16.3.0 commands.

R2's router-id is 2.2.2.2

R3's router-id is 3.3.3.3

R2's interface that is connected to that broadcast domain owns ip address 192.168.0.17

R3's interface that is connected to that broadcast domain owns ip address 192.168.0.2.

R3>sho ip route 172.16.2.0
Routing entry for 172.16.2.0/24
Known via "ospf 1", distance 110, metric 2, type intra area
Last update from 192.168.0.17 on GigabitEthernet0/2, 01:14:42 ago
Routing Descriptor Blocks:
* 192.168.0.17, from 2.2.2.2, 01:14:42 ago, via GigabitEthernet0/2
Route metric is 2, traffic share count is 1

R2>sho ip route 172.16.3.0
Routing entry for 172.16.3.0/24
Known via "ospf 1", distance 110, metric 2, type intra area
Last update from 192.168.0.22 on GigabitEthernet0/2, 01:15:45 ago
Routing Descriptor Blocks:
* 192.168.0.22, from 3.3.3.3, 01:15:45 ago, via GigabitEthernet0/2
Route metric is 2, traffic share count is 1

R2>show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
3.3.3.3 1 2WAY/DROTHER 00:00:34 192.168.0.22 GigabitEthernet0/2
4.4.4.4 1 FULL/BDR 00:00:39 192.168.0.20 GigabitEthernet0/2
5.5.5.5 1 FULL/DR 00:00:37 192.168.0.18 GigabitEthernet0/2
1.1.1.1 1 FULL/BDR 00:00:37 192.168.0.1 GigabitEthernet0/0

R3>sho ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
2.2.2.2 1 2WAY/DROTHER 00:00:32 192.168.0.17 GigabitEthernet0/2
4.4.4.4 1 FULL/BDR 00:00:36 192.168.0.20 GigabitEthernet0/2
5.5.5.5 1 FULL/DR 00:00:33 192.168.0.18 GigabitEthernet0/2
4.4.4.4 1 FULL/DR 00:00:32 192.168.0.38 GigabitEthernet0/1
1.1.1.1 1 FULL/BDR 00:00:39 192.168.0.9 GigabitEthernet0/0

Question is why R2's show ip route command at third line shows ip address of R3 as source of update, when they are in 2WAY/DROTHER state neighbors.? Why R3's show ip route command at third line shows ip address of R2 as source of update, when they are in 2WAY/DROTHER state neighbors.? 

[MHM Cisco World]

2-Way

This state designates that bi-directional communication has been established between two routers. Bi-directional means that each router sees the hello packet from the other router. This state is attained when the router receiving the hello packet sees its own Router ID within the received hello packet neighbor field. At this state, a router decides whether to become adjacent with this neighbor. On broadcast media and non-broadcast multi-access networks, a router becomes full only with the designated router (DR) and the backup designated router (BDR); it stays in the 2-way state with all other neighbors. On Point-to-point and Point-to-multipoint networks, a router becomes full with all connected routers.

it learn the LSA from DR but it point to router that originate the LSA

[gdy1039]

Hello Irakli

First, this output is normal and no problem at your configuration.

R2, R3, R4 and R5 around the switch will work in same subnet for OSPF.  R5 as DR and R4 as BDR, R2 and R3 send LSA to R5, R5 group LSA update to all members in this subnet 192.168.0.16/29. R2 and R3 know each other by stay in 2-way status, all DR-Others stay in this way. This design reduce router resource and speed up elect as DR once current DR dead. Because of DR-Others don't need to take time find out who in subnet and will trying to be a DR.

Cause this is normal status, R1 will know it can direct reach R2 instead of go R1 or R4. Assume each interface cost is same, then the prefer route would be R1 G0/2 to R2 G0/2, because of it's lowest cost path.

Please let me know if still have question.

Thank you.

Best Regards.

Scott Gao

[David Ruess]

Hello,

This is normal configuration output. Remember OSPF is a link state protocol so all routing information is known within the area. So when all routers send their information to the DR on a broadcast link all the DR is doing is relaying that info to the rest of the routers. It doesn’t change the info. Even though it comes from the DR it will still have the originating routers information. The pint of the DR is to reduce many talkers on the same broadcast domain but the LSUs contain the info needed to build the SPF tree from each routers perspective. 
Think of it like this. If the DR changed the Info of the Link state update to its own information then all traffic would have to flow through it for packets to get to their destination. That’s not very efficient. So that’s why it’s done the other way.

Hope that helps

-David