[c-nsp] MPLS VPN aggregate label / route lookup

[Peter Rathlev]

Hi,

We're about to change the netmask of an interface which is part of an
MPLS L3 VPN. During preparation we've been wondering about how this
would affect connectivity to the net. It's 6500/Sup720/12.2SXF, and the
configuration looks something like this:

ip vrf A
 rd 64512:1
 route-target both 64512:1
!
interface Vlan123
 ip vrf forwarding A
 ip address 10.0.0.1 255.255.255.128
!
router bgp 64512
 address-family ipv4 vrf A
  network 10.0.0.0 mask 255.255.255.128
 !
!

We want to change Vlan123 from 10.0.0.1/25 to 10.0.0.1/24, and we'd like
to do it without any connectivity loss. The plan would be something
like:

- Add "ip route vrf A 10.0.0.0 255.255.255.0 Null0 250"
- Add "network 10.0.0.0 mask 255.255.255.0" to MP-BGP
- Wait for the new prefix to be visible all over the network
- Change Vlan123 to 10.0.0.1/24
- Clean up

The question is: When the routers build a Null adjacency for the /24
network, will they just throw the traffic away upon receiving the tagged
traffic from the core? Or will they perform a lookup in the local
RIB/FIB and see that the connected /24 net has a better admin distance
than the Null route, even though it was that latter that the label was
built for?

If it makes any difference, the VRF only takes up a single aggregate
label in the TIB:

R1#show mpls forwarding-table 
Local  Outgoing    Prefix           Bytes tag  Outgoing   Next Hop    
tag    tag or VC   or Tunnel Id     switched   interface              
<cut>
36     Aggregate   vrf:A             155825439                          
<cut>
R1#

If this means the PFC will look at the FIB after VPN label popping then
all should be fine.

On another note: We have other PEs that build "full" mpls
forwarding-tables, including all local prefixes, but the one in question
only has aggregate labels for the VRFs. It's neighbor at the POP has
~3800 label assigned, this one only has ~50, and they hold the same VRFs
and the same prefixes. They should be configured the same way. Can
anybody point at how I can find out what makes this one use aggregates?

Thanks,
Peter