QOS & MPLS

In some cases (for example, a plain non-VPN MPLS network), the PHP action on the final P router can expose a plain IP packet when a packet with only one label is received. When this IP packet is received by the egress LSR (PE), it is not possible to classify the packet based on the MPLS EXP bits because there is no label now. In these situations, you must configure the egress PE router to advertise an explicit-null label. When the PHP action is performed on the P router, a label with a value of zero is sent, and with this special label you can mark the EXP bits as normally labeled packets, allowing the correct classification on the egress PE router.

Default Behavior: Penultimate Hop Pop (PHP)

R8#show mpls for 5.5.5.5 32
 Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
 tag    tag or VC   or Tunnel Id      switched   interface
 16     Untagged    5.5.5.5/32        0          Se0/0/0.57 point2point

Behavior with Ultimate Hop Popping (UHP)

R9(config)#mpls ldp explicit-null !this is how you enable UHP

R9#show mpls for 5.5.5.5 32
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface
16     0          5.5.5.5/32        0          Se0/0/0.57 point2point

UHP: in other words: explicit-null, instead of advertising a pop we send a label (0, for both LDP/TDP)

DiffServ Tunneling Modes for MPLS Networks
RFC3270