[eng/rtg] changing loopbacks

Thu Sep 29 19:53:19 UTC 2005

On Sep 29, 2005, at 12:05 PM, Randy Bush wrote:
> so i have junipers, ciscos, and a few <gasp> zebras in an ospf
> and ibgp mesh.  they're peering via loopbacks, of course.
> unfortunately, i need to recover the space from which the
> loopbacks are taken.  of course, i would like to do so with
> minimal disruption.  i am thinking of something like the
> following:
>
>   o add second loopbacks to all routers with new address in new
>     block
>   o set up ibgp peerings to new addresses from existing
>     peerings
>   o change the source of routing updates to new addresses
>   o remove old peerings
>   o remove old loopbacks
>
> what [else] am i missing?

I vaguely remember going through this exercise in a previous life...

As people have already mentioned, some things that are generally tied  
to loopback address (router-id for one) require a reboot on some  
vendors equipment. Because of that, we followed a procedure something  
like this:

1) for all routers:
     Add IBGP sessions to new loopback IPs. Don't add new loopback IP  
to the router, or change IBGP source address. At this point, all  
added sessions will be down.

2) for each router:
     Change "startup config" to the new loopback IP and reload the  
router on the new config. Router still has IBGP sessions for both old  
and new loopback IPs of each IBGP peer.

3) for all routers:
     Remove all IBGP sessions to old loopback IPs. At this point all  
sessions to old IPs will be down and all sessions to new IPs will be up.

Each IBGP sessions ends up transitioning through these stages:

old <-> old
reboot one of the routers on new loopback
new <-> old
reboot other router on new loopback
new <-> new

--Stacy