[c-nsp] "ip unnumbered" with "ip address negotiated" on the same router

[Reuben Farrelly]

Greetings,

I've recently come across a case whereby an 877 router running 12.4(4)T7 I was 
looking at had:

interface Dialer0
  ip address negotiated

interface Vlan1
  ip address 203.123.155.233 255.255.255.248

However the Dialer was being assigned the address 203.123.155.233 by the 
remote LNS.

The routing table looks like this:

      203.123.155.0/24 is variably subnetted, 2 subnets, 2 masks
C       203.123.155.232/29 is directly connected, Vlan1
C       203.123.155.233/32 is directly connected, Dialer3

[IPs changed slightly to obscure the innocent]

router#show int di3
Dialer3 is up, line protocol is up (spoofing)
   Hardware is Unknown
   Description: Internet Network
   Internet address is 203.123.155.233/32

router#show int vl1
Vlan1 is up, line protocol is up
   Hardware is EtherSVI, address is 001b.8fd3.3c05 (bia 001b.8fd3.3b15)
   Description: LAN
   Internet address is 203.123.155.233/29


This seems to be a little odd, and I'm unsure if this is in fact a valid 
configuration.  My gut reaction to this is that that's basically 'ip 
unnumbered' but not configured explicitly, and that having a /32 assigned to 
the Dialer/WAN which overlapped the Vlan interface may not be supported.

Has anyone else seen this done before, or is it known to have any ill effects? 
   There seems to be no problems apparent but then that doesn't imply that it 
is right and will survive IOS upgrades in the future ;-)

If in fact this is all OK, is there any point in the 'ip unnumbered' command 
at all?

Thanks,
Reuben