Once you enable MPLS on the interfaces between the routers and LDP neighbor adjacencies have been formed, a label will be advertised for each network. With LDP, however, we can configure filters to decide what networks should get a label and which ones shouldn’t be tagged. I’ll use the following topology to demonstrate this:

Above we have 3 routers, and each router has 2 loopback interfaces so that we have plenty of networks to play with. Before we enable MPLS, we’ll configure OSPF so that all networks are advertised:
R1,R2,R3:
(config)#router ospf 1
(config-router)#network 0.0.0.0 255.255.255.255 area 0
We’ll do this the easy way and activate OSPF on all interfaces. Now let’s enable MPLS on the FastEthernet interfaces:
R1(config)#interface fastEthernet 0/0
R1(config-if)#mpls ip
R2(config)#interface fastEthernet 0/0
R2(config-if)#mpls ip
R2(config-if)#exit
R2(config)#interface fastEthernet 0/1
R2(config-if)#mpls ip
R3(config)#interface fastEthernet 0/0
R3(config-if)#mpls ip
Let’s check if we have LDP neighbors:
R2#show mpls ldp neighbor | include Peer
Peer LDP Ident: 11.11.11.11:0; Local LDP Ident 22.22.22.22:0
Peer LDP Ident: 33.33.33.33:0; Local LDP Ident 22.22.22.22:0
So far, so good. Now let’s take a look at the LDP labels that have been generated:
R1#show 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 Fa0/0 192.168.12.2
17 17 33.33.33.33/32 0 Fa0/0 192.168.12.2
18 18 3.3.3.3/32 0 Fa0/0 192.168.12.2
19 Pop tag 22.22.22.22/32 0 Fa0/0 192.168.12.2
20 Pop tag 192.168.23.0/24 0 Fa0/0 192.168.12.2
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 Fa0/0 192.168.12.1
17 Pop tag 33.33.33.33/32 0 Fa0/1 192.168.23.3
18 Pop tag 3.3.3.3/32 0 Fa0/1 192.168.23.3
19 Pop tag 11.11.11.11/32 0 Fa0/0 192.168.12.1
R3#show mpls forwarding-table
Local Outgoing Prefix Bytes tag Outgoing Next Hop
tag tag or VC or Tunnel Id switched interface
16 Pop tag 192.168.12.0/24 0 Fa0/0 192.168.23.2
17 16 1.1.1.1/32 0 Fa0/0 192.168.23.2
18 Pop tag 2.2.2.2/32 0 Fa0/0 192.168.23.2
19 Pop tag 22.22.22.22/32 0 Fa0/0 192.168.23.2
20 19 11.11.11.11/32 0 Fa0/0 192.168.23.2
For all networks, a label has been generated by LDP. Now let’s configure filtering to only generate labels for the loopback 0 interfaces. This is how you do it:
- Unit 1: Introduction
- Unit 2: LDP (Label Distribution Protocol)
- Unit 3: MPLS VPN
- VRFs (Virtual Routing and Forwarding)
- MPLS L3 VPN Explained
- MPLS L3 VPN Configuration
- MPLS L3 VPN BGP Allow AS in
- MPLS L3 VPN BGP AS Override
- MPLS L3 VPN PE-CE RIP
- MPLS L3 VPN PE-CE EIGRP
- MPLS L3 VPN PE-CE OSPF
- MPLS L3 VPN PE-CE OSPF Default Route
- MPLS L3 VPN PE-CE OSPF Global Default Route
- MPLS L3 VPN PE-CE OSPF Sham Link
- VRF Lite Route Leaking
- MPLS VPN Extranet Route Leaking
- MPLS VPN VRF Export Map
- MPLS VPN VRF Import Map
- MPLS over FlexVPN
- Unit 4: MPLS L2 Encapsulation
- Unit 5: IPv6 MPLS
- Unit 6: MPLS Traffic Engineering (TE)
- Introduction to MPLS Traffic Engineering (TE)
- MPLS Traffic Engineering (TE) IS-IS Configuration
- MPLS Traffic Engineering (TE) OSPF Configuration
- MPLS TE RSVP-TE
- MPLS TE Static Routes
- MPLS TE Policy Based Routing (PBR)
- MPLS TE Autoroute Announce
- MPLS TE Autoroute Destination
- MPLS TE Autoroute Metric
- MPLS TE Unequal Cost Load Balancing
- MPLS TE Load Balancing between IGP and TE
- MPLS TE Forwarding Adjacency
- MPLS TE Path Options Explicit
- MPLS TE Class-Based Tunnel Selection (CBTS)
- MPLS TE Metric
- MPLS TE Setup and Hold Priority
- MPLS TE Attribute Flag and Affinity
- MPLS TE Reoptimization
- MPLS TE Fast Reroute (FRR)
- MPLS TE Fast Reroute Path Link Protection
- MPLS TE Fast Reroute Path Node Protection
- MPLS TE FRR RSVP Hello Support
- MPLS TE DiffServ Aware (DS-TE) Traditional
- MPLS TE Diffserv-Aware (DS-TE) IETF Mode
- MPLS VPN over MPLS TE Tunnels
- MPLS TE Per VRF TE tunnel