Router ID

EdTheLad

A problem i have on my 2600s and loopbacks.
I configure an ip address on a serial interface and enable ospf.
The ip address of the serial interface becomes the new router id.I configure
a loopback interface,shutdown the serial and the router id still keeps the old serial ip address??I try a "clear ip process" but still get the same result!
Has anyone got any ideas why this doesnt switch to the loopback interface which is in an up state???

Noods

I may be wrong, but I think the ID only changes when the router is rebooted, or OSPF is disabled/reinabled.

EdTheLad

Your wrong Noods,the loopback address in an address which is always up.Therefore you are adviced to configure the router id to be a loopback address.In theory since my interface drops the router should automatically
choose the highest loopback ip address and if there are none configured it will choose the highest ip address on a physical interface.
Now this is documented everywhere but in practice on my 2600's its not the
case!!

Noods

I realize this, but here is what Im seeing.

Your OSPF took your interface IP because it was the only option at the time.

OSPF only selects another IP as the ID when
A) OSPF is toggled
The router is rebooted
C) NOT when the interface goes down, as you have done.

Have you tried rebooting the router? I would be interested to see if that worked. Again, you may be right, but I thought IDs were recalculated differently.

EdTheLad

Noods wrote:

I realise this, but here is what Im seeing.

Your OSPF took your interface IP because it was the only option at the time.

OSPF only selects another IP as the ID when
A) OSPF is toggled
The router is rebooted
C) NOT when the interface goes down, as you have done.

Have you tried rebooting the router? I would be interested to see if that worked. Again, you may be right, but I thought IDs were recalculated differently.

I stated in my original post that i reset the ospf process,i also shutown the router id interface with no change on the router id.A router reboot will of course cause a change of the router id but this is of no practical use.
So Noods if you dont think the router id is supposed to change after an
interface drops explain to me why a loopback interface was coded in the first place!! The concept of a loopback interface is to have a stable interface on your system to assign a router id.
When ospf runs it sends out hello packets,these hello packets contain the routers router id.All neighbouring routers keep a record of this in their neighbour table to detect when a router goes offline.
If the router ID on a router changes this means all neighbour table need updating which causes a convergence.
This will not happen when you use a loopback address.This is why it exists in the first place.
Can someone try this on a 2600 and let me know if it works for you?
Maybe its a software bug!I dont know my software off hand maybe 12.2(29)

Yankee

Noods is correct. Once a router id is selected it is kept regardless of interface state changes. A reload will let it select a new one.

Yankee

EdTheLad

Yes i think u guys are right, i reckon the book i read is shite.
Anyway with more research i can see the problem with setting the router id to an interface id is, if there is a router failure i.e. it reboots and comes up with a different router ID any virtual links which were previously configured
will no longer work.
So i have my answer