Why do I get more than one designated router (DR) in an OSPF area.

[mad_mavula]

I created a test OSPF network and the LSDB of the area 0 is as following.

R1#sh ip ospf data

OSPF Router with ID (192.168.3.1) (Process ID 1)

Router Link States (Area 0)

Link ID                ADV Router    Age    Seq#                Checksum   Link count

192.168.9.1    192.168.9.1    1655    0x80000011    0x00e1ad           2

192.168.5.1    192.168.5.1     402     0x80000016    0x008723           3

192.168.3.1    192.168.3.1     392     0x80000017    0x00b204           3

192.168.7.1    192.168.7.1     392     0x8000000e    0x00c7d2            2




Net Link States (Area 0)

Link ID                   ADV Router       Age          Seq#                Checksum

192.168.4.2         192.168.7.1      1709        0x80000003     0x008191

192.168.1.2         192.168.5.1      1693        0x80000002     0x004aed

192.168.5.2         192.168.9.1      1660        0x80000004     0x0053bc

192.168.3.2         192.168.9.1      1655        0x80000003     0x00e153

192.168.2.2         192.168.7.1        392        0x80000003     0x00e142




Summary Net Link States (Area 0)

Link ID                        ADV Router       Age         Seq#             Checksum

192.168.10.0            192.168.9.1     1754      0x80000005     0x002e43

192.168.6.0              192.168.7.1       674      0x80000003     0x005823

192.168.7.0              192.168.7.1       674      0x80000004     0x00c376

192.168.8.0              192.168.9.1       117      0x80000006     0x00a68e

192.168.9.0              192.168.9.1       107      0x80000007     0x002151

192.168.11.0            192.168.9.1           5      0x80000008     0x00135b

And when I searched OSPF neighbors, I got 3 designated routers which are only other 3 routers in the area.

Can someone please explain me the reason for this.

Thanks in advance.

[Giuseppe Larosa · Accepted solution]

Hello @mad_mavula ,

what you see is normal an OSPF DR is elected for each LAN segment or more correctly for each link of type broadcast of other type that requires a DR to be elected.

As you can see from your show ip ospf neighbors all the listed nerighbors are DR in their associated link subnet, but they are seen out of different local interfaces in different IP subnets.

This is normal

DR is a per broadcast  link concept not per area concept.

Hope to help

Giuseppe

[mad_mavula]

Thanks for your help.

[Joseph W. Doherty]

As Giuseppe correctly notes, DR is per shared (i.e. broadcast) LAN segment.

However, did want to add, you can set priorities for router DR elections.

For example, given:

R1<shared1>R2<shared2>R3

Both shared segments would have a DR and BDR, but R1 and R2 would be the candidate routers for shared1 while R2 and R3 would be the candidate routers for shared2.  I.e. you might have R1 and R3 as DRs, or R2 (for shared1) and R3 as DRs, or R1 and R2 (for shared2) as DRs, or R2 as DR (for both shared1 and shared2).