Question on OSPF router ID?

[aeroplane]

Hi Folks

I am sorry but a very basic question. I have site to site route based IPSEC VPN with five spokes and I created separate st0 (tunnel interface) interfaces for each spoke. I am running OSPF over these tunnels.

If I use the OSPF router ID as one of the st0 interface IP (set routing-options router-id ) and that tunnel is down and ultimately st0 interface is also down then there is any impact on OSPF routing on other tunnel interfaces?

Thanks

[mhariry]

Hi,

I don't think so because you are not in NBMA environment which means that no DR & BDR election just point-to-point so changing router-id shouldn't affect your OSPF performance

Regards,

Mohamed

[aeroplane]

My friend Router ID is just not using in NBMA envir for DR/BDR election... It is used for representing the node in the SPF calcualtion.

Any one can take this?

[ronf]

As long as the router-id is unique, it should not matter if one is down. I typically use a loopback interface though, as it can be useful to have an "always-reachable" address show up in the output of "show ospf neighbor" rather than one that might be down. Ron

[aeroplane]

OK. Let me repharase my question. If I configure router-id using belwo command.

set routing-options router-id 192.168.1.1

This IP 192.168.1.1 is assigned to one tunnel interface. If tunnel interface is down then router-id would be change? I believe I specified the router-id using command and there is no relation with tunnel interface. Am I correct in  understanding?

Thanks

[ronf · Accepted solution]

The router-id has nothing to do with the interface IP. If the interface is down, never existed, was deleted, etc. it will have no impact on the router-id. The only requirement is that the router-id be unique within the OSPF area. I tend to use the loopback IP of the device though, as I like to see something useful in there, but that is just me. Ron

[spuluka]

As you note, the router id is explictly configured in your scenario, so it will not change.  The impact of the interface being down will be that the router will not be reachable as an ospf neighbor. 

But in your design, if the tunnel is the only way the neighbor relationship can establish, then this might not matter.

That is why Ron is saying it is safer to use an ip address that is always up as the router id.  Since the loopback can never go down, you can always respond to those neighbor requests regardless of which interface receives the traffic.  Even if the primary path interface is down.

[aeroplane]

Thanks to both