Configuring Explicit Null Label; Label Distribution Protocol Auto-Configuration - Cisco NCS 5500 Series Configuration Manual

Configuring Explicit Null Label

Configuring Explicit Null Label
Cisco MPLS LDP uses implicit or explicit null label 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 (PHP) mechanism. When
this is not desirable, you can configure explicit-null label that allows LDP control plane to implement ultimate
hop popping (UHP) mechanism. You can configure explicit-null feature on the ultimate hop LSR. Access-lists
can be used to specify the IP prefixes for which PHP is desired.
You can enforce implicit-null local label for a specific prefix by using the implicit-null-override command
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 the
implicit-null-override command.
Configuration Example: Explicit Null
This example shows how to configure explicit null label.
RP/0/RP0/CPU0:Router(config)# mpls ldp
RP/0/RP0/CPU0:Router(config-ldp)# address-family ipv4
RP/0/RP0/CPU0:Router(config-ldp-af)# label local advertise explict-null
RP/0/RP0/CPU0:Router(config-ldp-af)# commit
Configuration Example: Implicit Null Override
This example shows how to configure implicit null override for a set of prefixes.
RP/0/RP0/CPU0:Router(config)# mpls ldp
RP/0/RP0/CPU0:Router(config-ldp)# address-family ipv4
RP/0/RP0/CPU0:Router(config-ldp-af)# label local advertise implicit-null-override for acl-1
RP/0/RP0/CPU0:Router(config-ldp-af)# commit

Label Distribution Protocol Auto-configuration

LDP auto-configuration allows you to automatically configure LDP on all interfaces for which the IGP protocol
is enabled. Typically, LDP assigns and advertises labels for IGP routes and must often be enabled on all active
interfaces by an IGP. During LDP manual configuration, you must define the set of interfaces under LDP
which is a time-intensive procedure. LDP auto-configuration eliminates the need to specify the same list of
interfaces under LDP and simplifies the configuration tasks.
Configuration Example: Enabling LDP Auto-Configuration for OSPF
This example shows how to enable LDP auto-configuration for a specified OSPF instance.
RP/0/RP0/CPU0:Router(config)# router ospf 190
RP/0/RP0/CPU0:Router(config-ospf)# mpls ldp auto-config
RP/0/RP0/CPU0:Router(config-ospf)# area 8
RP/0/RP0/CPU0:Router(config-ospf-ar)# interface HundredGigE 0/0/0/5
RP/0/RP0/CPU0:Router(config-ospf-ar-if)# commit
MPLS Configuration Guide for Cisco NCS 5500 Series Routers, IOS XR Release 6.2.x
8
Implementing MPLS Label Distribution Protocol