MPLS advertise lable

When enable mpls in an interface, the router tags all prefixes it has learnt from the routing table and advertises out via ldp message to its neighbors. This behavior is enabled by default. However, we can tune this behavior by specifying prefixes in an access-list and configure the router to advertise them only.

Configuration Example

In normal condition which mpls ldp advertise-lables enable by default. Prefixed learnt by R1 are all tagged.

R1#sh mpls forwarding-table
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface
16     Pop tag     2.2.2.2/32        0          Fa0/0      192.168.1.2
17     Pop tag     4.4.4.4/32        0          Fa0/0      192.168.1.2
R1#

Configuration
Configure Access-list to allow specific prefix being ldp advertised by the router.

R2
mpls ldp advertise-labels for 1

access-list 1 permit 2.2.2.0 0.0.0.255 

R1

mpls ldp advertise-labels for 1

access-list 1 permit 1.1.1.0 0.0.0.255 

R2#show mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop

tag    tag or VC   or Tunnel Id      switched   interface

16     Pop tag     1.1.1.1/32        0          Fa0/0      192.168.1.1

17     Untagged    3.3.3.3/32        0          Fa0/0      192.168.1.1

R1#sh mpls forwarding-table

Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop

tag    tag or VC   or Tunnel Id      switched   interface

16     Pop tag     2.2.2.2/32        0          Fa0/0      192.168.1.2

17     Untagged    4.4.4.4/32        0          Fa0/0      192.168.1.2

Now, after the configuration, the prefix 4.4.4.4 and 3.3.3.3 are not tagged anymore. They will be forwarded by the IP lookup as in the normal network.