Explicit null is not applicable on a CSC-PE router, accordingly it should have no impact on the PE. This command enables a CSC-CE router to use BGP to distribute MPLS labels with a value of zero for explicit-null instead of implicit-null along with IPv4 routes to a CSC-PE peer router.
You must issue this command only on the local CSC-CE router.
The logic of explicit null labels:
On a CSC-CE with BGP IPv4 label distribution, BGP advertises an implicit null label for directly connected routes. This causes the previous hop (penultimate) router to do penultimate hop popping (PHP). The MPLS?VPN Explicit Null Label Support BGP IPv4 Label Session feature makes the penultimate router swap the incoming label for (or impose) the explicit null label. This action forces the egress router to process the explicit null label by popping it and inspecting the packet that remains.
The explicit null label helps to preserve quality of service (QoS) bits from one service level agreement (SLA) to another until the packets reach their CSC-CE destination.
There are some restrictions that you have to be aware of:
. You must configure your network for MPLS VPN.
. You must configure BGP to distribute labels between the CSC-CE and CSC-provider edge (PE) routers.
. Configure an explicit null label only in a CSC-CE topology.
. Configure an explicit null label only on a per-neighbor basis.
I am sorry, i am in the middle of preparing for CSC project and i got overwhelmed, i think that you are using the explicit null technique to preserve the QoS values in the MPLS header.
Normally, LDP advertises an Implicit Null label for directly connected routes. The Implicit Null label causes the second last (penultimate) label switched router (LSR) to remove the MPLS header from the packet. In this case, the penultimate LSR and the last LSR do not have access to the quality of service (QoS) values that the packet carried before the MPLS header was removed. To preserve the QoS values, you can configure the LSR to advertise an explicit NULL label (a label value of zero). The LSR at the penultimate hop forwards MPLS packets with a NULL label instead of forwarding IP packets.
Note An explicit NULL label is not needed when the penultimate hop receives MPLS packets with a label stack that contains at least two labels and penultimate hop popping is performed. In that case, the inner label can still carry the QoS value needed by the penultimate and edge LSR to implement their QoS policy.
When you issue the mpls ldp explicit-null command, Explicit Null is advertised in place of Implicit Null for directly connected prefixes.
I've never used it but i can't see any impact that can affect the PE.
HTH, i hope that i've been informative, please do rate all helpful replies,
egress LSR needs to do double lookup with explicit null label.
For example, for 7600 box (PFC3B/3BXL) recirculation is necessary in these situations:
•To pop an explicit null top label
Packet recirculation occurs only on a particular packet flow; other packet flows are not affected. The rewrite of the packet occurs on the modules; the packets are then forwarded back to the PFC3BXL or PFC3B for additional processing.
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