CCIE - Interesting OSPFism

Author

Interesting OSPFism

doctorcisco

2002-05-14, 3:44 pm

Guess what I'm studying today!

Recent IOS code no longer changes the OSPF Router ID when the interface with that IP goes down. That's documented in the CCNP routing book, and I'm sure elsewhere.

I noticed today that it's more simple than that: once the OSPF process picks its router ID, that router ID will NOT change until the OSPF process restarts. If you remove the IP address from the router (delete or readdress the interface), it keeps its router ID. Even cle ip ospf proc won't get it out of there. You have to reboot, or remove and reload the "router ospf process-id" config, or do something I didn't think of. You could, of course, manually configure the router-id. But who does that?

So you can end up with the OSPF router ID being x.x.x.x, even though this address is nowhere to be found on the router. Also, if you configure a new interface on the router with a higher IP address, the router ID won't change either.

Now, what happens if

1) Router ID is x.x.x.x.

2) You remove address x.x.x.x from one router.

3) You re-use that IP address on a new router, and after a reboot it becomes the new router's ID.

Some router output is below my signature if you want to see it. I'll fiddle with this a bit sometime and let y'all know what I see.

doc

4500-1#cle ip ospf proc
Reset ALL OSPF processes? [no]: y

4500-1#sh ip int brie
Interface IP-Address OK? Method Status Prot
ocol
Ethernet0 192.168.1.45 YES NVRAM administratively down down

Ethernet1 unassigned YES NVRAM administratively down down

Loopback0 200.100.100.100 YES manual up up

Serial0 unassigned YES NVRAM up up

Serial0.251 192.168.210.17 YES manual up up

Serial0.252 192.168.210.49 YES NVRAM up up

Serial0.259 192.168.210.33 YES NVRAM up up

Serial0.261 10.210.210.210 YES NVRAM up up

<snip>

4500-1#sh ip ospf int
Loopback0 is up, line protocol is up
Internet Address 200.100.100.100/32, Area 1
Process ID 100, Router ID 192.168.220.17, Network Type LOOPBACK, Cost: 1

<snip>

4500-1#sh run

<snip>

!
router ospf 100
log-adjacency-changes
network 192.168.210.0 0.0.0.255 area 1
network 200.0.0.0 0.255.255.255 area 1
network 10.0.0.0 0.255.255.255 area 0
!

MadChef

2002-05-15, 5:36 am

This is why I made a point in my lab to do this:

Build all your interface configurations:
encap frame
no frame in
ip addr
frame map whatever
no shut

Once all your interfaces are built and up/up, THEN start worring about your routing process. That way you get a RID that's meaningful and won't change if you have to reload the router.

So doc, what happens when you re-address 4500-1 lo0 and put that address somewhere else as a RID? I'm curious.

MadChef

shanelai

2002-05-16, 9:10 am

I usually manually assign router IDs so there's never confusion.

MadChef

2002-05-16, 10:07 am

quote:
Originally posted by shanelai
I usually manually assign router IDs so there's never confusion.

And that's fine if you're running IOS that supports the router-id command, but that's a relatively new command. You've got to do it the old-fashioned way if you're running routers with 12.0 or earlier.

MadChef

doctorcisco

2002-05-16, 12:15 pm

quote:

So doc, what happens when you re-address 4500-1 lo0 and put that address somewhere else as a RID? I'm curious.

MadChef

Partial answer.

Router A, DR, had the ID first.
Router B, BDR.
Router C, has duplicate ID to Router A.
All share an ethernet segment.

Router A will generate error messages about the duplicate RID, and will NOT establish neighborship with C.

Router B happily lists both A and C, with the same ID and different source IP's, in its neighbor list.

Router C thinks Router B is the DR, and gets to FULL with B. He gets stuck in INIT state with router A, since A refuses to answer his intial hello but he knows about A from B (or else knows about A from A's traffic to 224.0.0.5). "sh ip ospf neigh" on C lists B as the DR and C as DROTHER, with no BDR listed. No error messages except some noise about "Trying to delete a max-age LSA." He also gets no routes, for obvious reasons.

I'll fiddle some more another time and check in.

doc

Sponsored Links