The figure below illustrates how LDP neighbor relationships are built. First each router sends a discovery to a destination multicast address (TTL=1) 224.0.0.2 on port 646. This discovery contains the router-id and the transport IPv4 address the router wants to use. The second stage is building the TCP peering session using the specified transport IP addresses. This is normally loopback to loopback.

Examples

• The show mpls ldp neighbor command on the North Edge router displays more detail on TCP session establishment, and the local addresses of the LDP neighbor for which it is binding a label.
  Note: All connected interfaces are advertised as bound. However, eos currently advertised labels for /32 addresses, and FEC filter is configured to install only x.x.x.200/32 prefixes.

  North Edge.17:51:17# show mpls ldp neighbor
  Peer LDP ID: 2.2.2.200:0; Local LDP ID: 1.1.1.200:0
     TCP Connection: 2.2.2.200:38395 - 1.1.1.200:646
     State: oper; Msgs sent/rcvd: 46/46; downstream unsolicited
     Uptime: 0:06:17
     KeepAlive expires in: 20.27 sec
     LDP discovery sources:
        Ethernet1/1
     Addresses bound to peer:
        2.2.2.200           2.2.2.2             192.168.1.177    192.168.62.11
        192.168.1.181       192.168.58.12       192.168.60.11    192.168.61.11
  Peer LDP ID: 3.3.3.200:0; Local LDP ID: 1.1.1.200:0
     TCP Connection: 3.3.3.200:38510 - 1.1.1.200:646
     State: oper; Msgs sent/rcvd: 42/42; downstream unsolicited
     Uptime: 0:05:51
     KeepAlive expires in: 20.02 sec
     LDP discovery sources:
        Ethernet2/1
     Addresses bound to peer:
        192.168.65.11       192.168.59.12       3.3.3.200        192.168.60.12
        192.168.63.11       3.3.3.3             192.168.64.11

• The show mpls lfib route 116384 command on the North Edge router displays the label POP and swap operations for any traffic traversing North Edge. If traffic came in with label 116384 it would be swapped to the labels seen in the tunnel table.

  North Edge.23:38:28(config)# show mpls lfib route 116384
  MPLS forwarding table (Label [metric] Vias) - 1 routes
  MPLS next-hop resolution allow default route: False
  Via Type Codes:
            M - Mpls Via, P - Pseudowire Via,
            I - IP Lookup Via, V - Vlan Via,
            VA - EVPN Vlan Aware Via, ES - EVPN Ethernet Segment Via,
            VF - EVPN Vlan Flood Via, AF - EVPN Vlan Aware Flood Via
  Source Codes:
            S - Static MPLS Route, B2 - BGP L2 EVPN,
            B3 - BGP L3 VPN, P - Pseudowire,
            L - LDP, IP - IS-IS SR Prefix Segment,
            IA - IS-IS SR Adjacency Segment, IL - IS-IS SR Segment to LDP,
            LI - LDP to IS-IS SR Segment, BL - BGP LU,
            DE - Debug LFIB
  
   L   116384   [1], 6.6.6.200/32
                  via M, 192.168.58.12, swap 132768
                      payload autoDecide, ttlMode autoDecide, apply egress-acl
                      interface Ethernet1/1
                  via M, 192.168.59.12, swap 100000
                      payload autoDecide, ttlMode autoDecide, apply egress-acl
                      interface Ethernet2/1

The following figure illustrates how ISIS-SR distributes the SID index information in the ISIS TLVs and sub-TLVs

The Prefix SID index, SRGB, and ADJ SID values are populated in the sub-TLVs in the ISIS neighbor updates. Each router then builds its database of Node (Prefix) segments (Labels) and locally assigned ADJ labels.

BGP-LU Label Distribution illustrates how BGP-LU distributes the label information in BGP.

BGP-SR Index and SRGB Distribution illustrates how BGP-LU distributes the Label SRGB and SID index information in BGP. This is known as BGP-SR.

The Prefix SID index and SRGB values are populated in the TLVs in the BGP neighbor updates. Each router then builds its own database of Node (Prefix) segments (Labels).