Solved: OSPF DR

rossmillan · ‎06-14-2007

Hi,

I noticed one thing whilst doing the lab in ch4 from cisco BSCI third edition by Diane Teare and Catherine Paquet.

For simplicity I have two Routers connected to each other via fastethernet. P1R1 and P1R3 (just had ospf priority set to 0) I also did a clear ip ospf process to make sure election held again.

My question is why does P1R3 show the neighbor as 10.0.0.11 (which is the correct router id of P1R1) yet the printout shows DR is 10.1.1.1. Is that in that part is always puts the ip address of the interface it is connected to rather than the router id?

P1R3#sh ip ospf neig det

Neighbor 10.0.0.11, interface address 10.1.1.1

In the area 1 via interface FastEthernet0/0

Neighbor priority is 1, State is FULL, 12 state changes

DR is 10.1.1.1 BDR is 0.0.0.0

P1R1#show ip proto

Routing Protocol is "ospf 1"

Outgoing update filter list for all interfaces is not set

Incoming update filter list for all interfaces is not set

Router ID 10.0.0.11

mohammedmahmoud · ‎06-14-2007

Hi,

show ip ospf neighbor displays OSPF-neighbor information on a per-interface basis and thus it uses the IP address of the interface to which this neighbor is directly connected (the interface IP address of the neighbor), please check the following links:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a85.shtml

http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017d02e.html#wp1038133

HTH,

Mohammed Mahmoud.

View solution in original post

smothuku · ‎06-14-2007

Hi ,

I think you set the ospf priority to 0 after DR was elected ...which was 1 on P1R1 router.

Clear the ospf process using clear ip ospf process which electes new DR and BDR.

mohammedmahmoud · ‎06-14-2007

Hi,

show ip ospf neighbor displays OSPF-neighbor information on a per-interface basis and thus it uses the IP address of the interface to which this neighbor is directly connected (the interface IP address of the neighbor), please check the following links:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094a85.shtml

http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_command_reference_chapter09186a008017d02e.html#wp1038133

HTH,

Mohammed Mahmoud.

smothuku · ‎06-14-2007

Hi Mohammed ,

thanks for reply...What is the use of configuring router id on router. Acoording to me in the selection process of DR abd BDR , router comes into the picture..corret..the highet ip address whick is defined using router id command becomes the DR...

Pl correct me if i am wrong...

then in the example mentioned , why it is howing 10.1.1.1 as DR instaed of highet ip address whick is defined using router id .

thanks,

satish

rossmillan · ‎06-14-2007

Hi,

It makes sense to me now I think. Basically the router-id is still 10.0.0.11 which is fine. Its just that the ospf neighbor command shows things relative to the interface. So it knows it goes out interface 10.1.1.1 to get to the DR. The DR address is still 10.0.0.11.

P1R3#sh ip ospf neig det

Neighbor 10.0.0.11, interface address 10.1.1.1

In the area 1 via interface FastEthernet0/0

Neighbor priority is 1, State is FULL, 12 state changes

DR is 10.1.1.1 BDR is 0.0.0.0

mohammedmahmoud · ‎06-14-2007

Hi Satish,

You are very welcomed, and its very nice having this discussion with you, first of all you are absolutely right in the use of the RID in the DR election.

OSPF and its show commands are interface specific utilizing information on a per-interface basis, and thus the ospf neighbor command output shows things relative to the interface, plus please note that the output has told both "Neighbor 10.0.0.11, interface address 10.1.1.1" in order to prevent any confusion while troubleshooting.

In a similar case, for NBMA the neighbor command configures the routers with the IP addresses of the interfaces of its neighbors rather than using their RID, and thus the RID has its roles and needs but it is not always used as the RID is just an ID and not a routing entity or a routable IP (it doesn't have to be reachable in routing essence between routers).

HTH,

Mohammed Mahmoud.