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Enabling explicit-null on an egress LSR causes that LSR to advertise the explicit-null label to all adjacent
MPLS routers.
Router# configure terminal
Router(config)# mpls ldp explicit-null
If you issue the show mpls forwarding-table command on an adjacent router, the output shows that MPLS
packets are forwarded with an explicit-null label (value of 0). In the following example, the second column
shows that entries have outgoing labels of 0, where once they were marked “Pop label”.
Router# show mpls forwarding-table
Local Outgoing Prefix Bytes label Outgoing Next Hop
label label or VC or Tunnel Id switched interface
19 Pop tag 10.12.12.12/32 0 Fa2/1/0 172.16.0.1
22 0 10.14.14.14/32 0 Fa2/0/0 192.168.0.2
23 0 172.24.24.24/32 0 Fa2/0/0 192.168.0.2
24 0 192.168.0.0/8 0 Fa2/0/0 192.168.0.2
25 0 10.15.15.15/32 0 Fa2/0/0 192.168.0.2
26 0 172.16.0.0/8 0 Fa2/0/0 192.168.0.2
27 25 10.16.16.16/32 0 Fa2/0/0 192.168.0.22
28 0 10.34.34.34/32 0 Fa2/0/0 192.168.0.2
Enabling explicit-null and specifying the forkeyword with a standard access control list (ACL) changes all
adjacent MPLS routers' tables to swap an explicit-null label for only those entries specified in the access-
list. In the following example, an access-list is created that contains the 10.24.24.24/32 entry. Explicit null
is configured and the access list is specified.
Router# configure terminal
Router(config)# mpls label protocol ldp
Router(config)# access-list 24 permit host 10.24.24.24
Router(config)# mpls ldp explicit-null for 24
If you issue the show mpls forwarding-table command on an adjacent router, the output shows that the
only the outgoing labels for the addresses specified (172.24.24.24/32) change from Pop label to 0. All other
Pop label outgoing labels remain the same.
Router# show mpls forwarding-table
Local Outgoing Prefix Bytes label Outgoing Next Hop
label label or VC or Tunnel Id switched interface
19 Pop tag 10.12.12.12/32 0 Fa2/1/0 172.16.0.1
22 0 10.14.14.14/32 0 Fa2/0/0 192.168.0.2
23 0 172.24.24.24/32 0 Fa2/0/0 192.168.0.2
24 0 192.168.0.0/8 0 Fa2/0/0 192.168.0.2
25 0 10.15.15.15/32 0 Fa2/0/0 192.168.0.2
26 0 172.16.0.0/8 0 Fa2/0/0 192.168.0.2
27 25 10.16.16.16/32 0 Fa2/0/0 192.168.0.22
28 0 10.34.34.34/32 0 Fa2/0/0 192.168.0.2
Enabling explicit null and adding the to keyword and an access list enables you to advertise explicit-null
labels to only those adjacent routers specified in the access-list.To advertise explicit-null to a particular
router, you must specify the router's LDP ID in the access-list.
In the following example, an access-list contains the 10.15.15.15/32 entry, which is the LDP ID of an
adjacent MPLS router. The router that is configured with explicit null advertises explicit-null labels only to
that adjacent router.
Router# show mpls ldp discovery
Local LDP Identifier:
10.15.15.15:0
Discovery Sources:
Interfaces:
Ethernet4 (ldp): xmit/recv
TDP Id: 10.14.14.14:0
MPLS Label Distribution Protocol (LDP)
How to Configure MPLS LDP
MPLS LDP Configuration Guide, Cisco IOS Release 12.4
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