[c-nsp] GigE sub-int won't come up

[Justin Shore]

I'm trying to add a simple sub-int to a built-in GigE interface on a 
7206VXR G2.  I already have several sub-ints on the same interface and 
they're working great (typing this email across one of them).  However 
when I added a new sub-int it refused to come up and I can't figure out 
why.

Here's the physical interface config:

interface GigabitEthernet0/2
  description TO 4948-1.amherst Gi1/45
  no ip address
  duplex auto
  speed 1000
  media-type sfp
  negotiation auto

Very simple.  Here's the sub-int config:

interface GigabitEthernet0/2.999
  description Acme LAN PoC
  encapsulation dot1Q 999
  ip vrf forwarding acme-elan
  ip address 100.100.100.13 255.255.255.252

Also very simple.  The other sub-ints aren't in VRFs but I doubt if that 
would be a problem here (could be but I doubt it).

GigabitEthernet0/2.999     100.100.100.13  YES manual down 
     down

I was using VLAN 1001 for the lab test but switched to 999 after 
thinking that perhaps 1001 was used internally on the 7200 (I don't 
think it is but just in case); still no go.  The VRF is up.  I have 
MLPPP bundle in it and they're working on the same 7206.  I'm running 
12.4(15)T7.  It appears to stay up/up until I define the 1Q VLAN ID on 
the sub-int.  Then it goes down/down.  Oddly enough the sub-int doesn't 
go back to up/up (when the 1Q VLAN is removed) after going down/down 
(when the 1Q VLAN is assigned).

What in the world would keep a sub-int on an Ethernet interface from 
coming up?  I'm not even sure what debug options are available for 
something like this.  Connected to Gi0/2 is a 4948 and that VLAN is 
permitted across the trunk.

Thoughts?  I'm sure someone will noticed my error right off but I sure 
can't find it.  Thanks
  Justin