MPLS Building Blocks

MPLS uses the IP routing information to determine the direction and next hop to forward a labeled packet .

Does this before the first packet even arrives.

Common MPLS Terminologies

• LSP: Label Switch Path , Data Plane for a labeled Packet through the LSRs – LSP is Unidirectional
• LSR: Label Switch Router
• Ingress LSR: PE (Provider Edge First hop)
• Intermediate  LSR: P (Provider)
• Egress LSR: PE (Provider Edge last hop)
• UpStream: Beginning at the network that is being advertised , this is the Control Plane for LDP
• DownStream: Transit path is downstream towards the network packets are trying to reach , Also called data plane .

MPLS Control Plane:

• LDP : Label Distribution Protocol
• LIB : Label Information Base

MPLS Data Plane:

LFIB: MPLS Label forwarding Information base

#show mpls ldp bindings
#show mpls forwarding-table
#show mpls ldp neighbor

• Labels live between L2 and L3 of the OSI
• A single Label corresponds to a single route ,and is locally significant to the router
• Labels are 4 bytes long
• Each router generates a local label for each route and then shares them via LDP with LDP neighbors
• Labels identify a Forward equivalence Class ( FEC ) . RFC 3031
• Implicit Null  : Reserved Label #3
• PHP : Penultimate Hop Pop : Next to last LSR , Removes top label , so that egress LSR ( PE ) doesn’t have to .

In Frame Mode:

• The label inserted between L2 and L3
• L2 protocol identifier ( PID ) is changed to indicate that the packet has an MPLS label
  • Unlabeled IP Unicast  PID : 0x0800
  • Labeled IP Unicast PID : 0x8847
  • Labeled IP Multicast PID : 0x8848

In Cell-Mode (ATM Switches as LSRs) The fields of the ATM header are used as the labels.

• LDP: uses 646 TCP
• TDP: uses 711 TCP
• RSVP is used only for TE