Explicit Null on P routers as well

[ankittagra]

As I understand explicit null have to be configured on PE to advertise label 0.

What will happen if explicit null is configured on all P routers as well. Will we have only label 0 flowing all over the topology ?

For both the cases while using LDP and SR.

[Harold Ritter]

Hi @ankittagra ,

The explicit null is advertised in place of the implicit null advertised by default. Both implicit and explicit null are advertised by the egress PE and can't be advertised by upstream LSRs.

Regards,

[MHM Cisco World]

I think from Yesterday, PE can use explicit null since the prefix is direct connect BUT P use explicit null!!
I dont think so, I will run lab try that with static MPLS label, but not sure I will success or not. 

[Harold Ritter]

Hi @MHM Cisco World ,

The implicit/explicit null label can only be advertised to the penultimate hop router (PHR). The PHR will advertise a non null label to its peers.

Regards,

[MHM Cisco World]

this what I meaning, 
FIB and LFIB in PE so it can send explicit null to P, 
but P dont have FIB it have only LFIB so how that will work? I think you can not even if we use static MPLS label. 
thanks 
MHM

[Harold Ritter]

Hi @MHM Cisco World ,

It is not a really question of FIB vs LFIB, but rather the fact that only the egress router (router directly connected to the destination) should advertise the implicit/explicit null. If a router other than the egress PE was to advertise an implicit/explicit null, it would break the LSP.

By the way, the P routers have a FIB too and can be used as P or PE as needed.

Regards,

[MHM Cisco World]

Yes he ask about P and explicit Null, so I answer him even if you can with static MPLS (I not try before) the traffic will drop. 
and for point for P have FIB sure it have 
what I mention here that prefix connect to PE is NOT found in FIB of P, and if P send explicit null to neighbor then it must lookup in FIB to forward prefix traffic and here the traffic drop. 
So He can not use explicit Null in P. 
thanks 
MHM