MPLS Building Blocks

IP routers make forwarding decision based on IP packet header, and local CEF and FIB table.
MPLS routers make forwarding decision based on the MPLS label and the LFIB .
MPLS is great technology these days not because it forwards the packets faster , but because of applications and solutions we can provide for our customers like MPLS VPN’s .

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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
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Author: Mo Moghaddas

Traveler. Casual investor. Hobbyist landscape photographer loving Networks. Blockchain/Cryptocurrency enthusiast. Internet citizen!

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