[eng/rtg] changing loopbacks

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1

It's worth noting that C's don't need actual IP address space assigned to the router-id for OSPF. It's just an arbitrary value; it's probably better karma to set it to whatever you want (maybe something that doesn't look like an IP address).

RFC 2328:

        Router ID
            A 32-bit number assigned to each router running the OSPF
            protocol. This number uniquely identifies the router within
            an Autonomous System.

CCO:
<http://www.cisco.com/en/US/products/sw/iosswrel/ps1839/products_command_reference_chapter09186a008010a39c.html#wp1049279>

Usage Guidelines

You can configure an arbitrary value in the IP address format for each router. However, each router ID must be unique.

If this command is used on an OSPF router process which is already active (has neighbors), the new router-ID is used at the next reload or at a manual OSPF process restart. To manually restart the OSPF process, use the clear ip ospf command.

2

eek! There are a couple of downsides to having the
router-ID divorced from a physical address:

1) you get an additional number which you have to have
to track to ensure uniqueness.

2) you lose the benefit of being able to double check
reachability (ping/ssh to router ID)

3) RFC 1403 says that the BGP router identifier must
be the same as the OSPF router ID, and do you really
want your BGP to reflect an unreachable ID?

I've had a customer who used unreachable router IDs,
and it made their NOC work quite a bit harder than
they otherwise would have had to...

-David

3

eek! There are a couple of downsides to having the
router-ID divorced from a physical address:

1) you get an additional number which you have to have
to track to ensure uniqueness.

2) you lose the benefit of being able to double check
reachability (ping/ssh to router ID)

No doubt, but the OP was trying to fend off OSPF adjacency teardowns when renumbering loopbacks.

3) RFC 1403 says that the BGP router identifier must
be the same as the OSPF router ID, and do you really
want your BGP to reflect an unreachable ID?

Wait a second...
<RFC 1403>

3. BGP Identifier and OSPF router ID

   The BGP identifier MUST be the same as the OSPF router id at all
   times that the router is up.

   This characteristic is required for two reasons.

     i Synchronisation between OSPF and BGP

          Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3,
          belong to the same autonomous system.