LSR forwards the packet based on:
Top Label value of the received packet
Corresponding entry in LFIB (LABEL <=> INTERFACE)
#show mpls forwarding-table - will show:
local label
outgoing label
prefix (network)
outgoing interface
next-hop
cisco#show mpls forwarding-table Local Outgoing Prefix Bytes tag Outgoing Next Hop tag tag or VC or Tunnel Id switched interface 16 Pop tag 10.1.1.12/30 636 Se3/0 point2point 17 Pop tag 10.10.10.1/32 0 Se3/0 point2point 18 21 10.10.10.4/32 0 Se3/0 point2point 19 Pop tag 10.1.1.0/30 0 Se4/0 point2point Pop tag 10.1.1.0/30 0 Se3/0 point2point 20 Pop tag 10.10.10.6/32 612 Se2/0 point2point 21 Pop tag 10.1.1.16/30 0 Se3/0 point2point 22 16 10.10.10.5/32 0 Se3/0 point2point 23 Pop tag 10.10.10.2/32 0 Se4/0 point2point
LSR expects packet to come with "top" label being "Local" (from show mpls forwarding-table). If Outgoing label is "Aggregate", then that means that this is a summary route and more specific lookup is performed.
If LSR cannot find label/interface mapping in LFIB, then it drops the packet. There are several "RESERVED" labels numbered from 0 to 15:
0 - explicit NULL - is used to preserver QoS info through EXP bits. It copies 'ip prec' or DiffServ. 1 - Router alert label - forces LSR to software switch the packet. 3 -Implicit NULL - this label is used for "connected" or "summary" routes. This way LSR signals its neighbors to execute "POP label" operation on "connected" routes.
It is called PHP, Penultimate Hop Popping, and is used to make sure that LSR does not perform 2 lookups (label + ip). 14 - OEM alert label - is used for monitoring purpose. In Cisco IOS, the default range is 16 through 100,000, but can be expanded by using mpls label range command.