[nsp] Duplicate Interface Addresses !!

[Eric Osborne]

On Mon, Feb 17, 2003 at 10:19:08PM +0000, kevin gannon wrote:
> I spent the last three hours troubleshooting an EIGRP duplicate router 
> ID issue.
> The route cause was two interface on the same router with the same address.
> Both interfaces were up/up and before the work started only one was up and
> the duplicate address was then added to an already up/up address. To find
> the duplicate address you need some hidden IOS command in EIGRP otherwise
> I would still be looking !!!.

If they were serial/p2p interfaces, this is expected (see below) - if
not, sounds like a bug.

> 
> Anyone know why IOS only sometimes warns about this issue ?? You know
> the warning you usually get:
> 
> kevin(config)#int loop 100
> kevin(config-if)#ip address 10.10.10.1 255.255.255.0
> 10.10.10.0 overlaps with Ethernet0
> kevin(config-if)#

Does not apply to serial interfaces (historical way to do
load-balancing), but applies to Ethernet and other such inteface types.

> 
> I know its not isolated to one IOS revision as I have managed to
> trick other IOS versions into creating this duplicate situation.
> 
> I have also spent hours troubleshooting MPLS problems due to
> the same issue. Its madness !.

Yeah, it can be a mess.  I'm not sure if there's still a good reason
to allow this behavior, but given the installed base of low-end
routers (which is where most of this will show up) and the unpleasant
things it would do to network design if we just removed the behavior,
extracation is going to be tricky.  Best short-term advice I can give
is "don't do that"...:)



eric

> 
> Regards,
> Kevin
> 
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