Explicit-Null And Implicit-Null Labels; How To Implement Mpls Ldp - Cisco ASR 9000 Series Configuration Manual

Explicit-Null and Implicit-Null Labels

negotiate for downstream-on-demand mode. If the ACL does not exist or is empty, downstream-on-demand
mode is not configured for any neighbor.
For it to be enabled, the Downstream on demand feature has to be configured on both peers of the session. If
only one peer in the session has downstream-on-demand feature configured, then the session does not use
downstream-on-demand mode.
If, after, a label request is sent, and no remote label is received from the peer, the router will periodically
resend the label request. After the peer advertises a label after receiving the label request, it will automatically
readvertise the label if any label attribute changes subsequently.
Related Topics
Configuring LDP Downstream on Demand mode, on page 48
Explicit-Null and Implicit-Null Labels
Cisco MPLS LDP uses null label, implicit or explicit, as local label for routes or prefixes that terminate on
the given LSR. These routes include all local, connected, and attached networks. By default, the null label is
implicit-null that allows LDP control plane to implement penultimate hop popping (PHOP) mechanism.
When this is not desirable, you can configure explicit-null that allows LDP control plane to implement ultimate
hop popping (UHOP) mechanism. You can configure this explicit-null feature on the ultimate hop LSR. This
configuration knob includes an access-list to specify the IP prefixes for which PHOP is desired.
This new enhancement allows you to configure implicit-null local label for non-egress (ultimate hop LSR)
prefixes by using the implicit-null-override command. This enforces implicit-null local label for a specific
prefix even if the prefix requires a non-null label to be allocated by default. For example, by default, an LSR
allocates and advertises a non-null label for an IGP route. If you wish to terminate LSP for this route on
penultimate hop of the LSR, you can enforce implicit-null label allocation and advertisement for this prefix
using implicit-null-override feature.
If a given prefix is permitted in both explicit-null and implicit-null-override feature, then
Note
implicit-null-override supercedes and an implicit-null label is allocated and advertised for the prefix.
In order to enable implicit-null-override mode, this configuration must be applied at MPLS LDP label
configuration mode:
mpls ldp
!
This feature works with any prefix including static, IGP, and BGP, when specified in the ACL.

How to Implement MPLS LDP

A typical MPLS LDP deployment requires coordination among several global neighbor routers. Various
configuration tasks are required to implement MPLS LDP :
Cisco ASR 9000 Series Aggregation Services Router MPLS Configuration Guide, Release 4.3.x
20
label
implicit-null-override for <prefix><ACL>
Implementing MPLS Label Distribution Protocol
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