Implicit & Explicit Null Labels

Question

Hi..
I am a bit confused in Implicit and Explicit null labels. The RFC Says as follows :
IMPLICIT NULL LABEL:This label value is only legal at the bottom of the label stack. It indicates that the label stack must be popped,
and the forwarding of the packet must then be based on the IPv4 header.
EXPLICIT NULL LABEL : This is a label that an LSR may assign and distribute, but which never actually appears in the encapsulation. When an LSR would otherwise replace the label at the top of the stack with a new label, but the new label is "Implicit NULL", the LSR will pop the stack instead of doing the
replacement. Although this value may never appear in the encapsulation, it needs to be specified in the Label Distribution Protocol, so a value is reserved.
My question is, which one is used in PHP operation. Consider I have a scenario like this :
CE1--PE1---P---PE2---CE2
How do I make the P router as the PHP router in this case???I need to have a implicit null label set on P router????
What about explicit null label then???
Regards,
RAJ

Answer 1

by defaulf, the P router will use implicit null label. For you to use the explicit null label, use the command
mpls ldp explicit-null.
The explicit null label is often useful in QoS implementation scenarios.
See the following url
http://www.cisco.com/en/US/products/ps6350/products_command_reference_chapter09186a0080430c63.html#wp1020450