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aka Layer 2.5 networking protocol, Layer 2.5






  • Each router performs an IP lookup (“routing”), determines a next-hop based on its routing table, and forwards the packet to that next-hop.
  • Rinse and repeat for every router, each making its own independent routing decisions, until the final destination is reached.
  • MPLS does “label switching” instead:
  • The first device does a routing lookup, just like before:
  • But instead of finding a next-hop, it finds the final destination router.
  • And it finds a pre-determined path from “here” to that final router.
  • The router applies a “label” (or “shim”) based on this information.
  • Future routers use the label to route the traffic
  • Without needing to perform any additional IP lookups.
  • At the final destination router the label is removed.
  • And the packet is delivered via normal IP routing.
  • MPLS Label Switched Path (LSP): Essentially a unidirectional tunnel between a pair of routers, routed across an MPLS network. An LSP is required for any MPLS forwarding to occur.
  • MPLS Router Roles/Positions
  • Label Edge Router (LER) o ingress node: The router which first encapsulates a packet inside an MPLS LSP. Also the router which makes the initial path selection.
  • Label Switching Router (LSR) or transit node: A router which only does MPLS switching in the middle of an LSP. Egress Node. The final router at the end of an LSP, which removes the label.

MPLS router roles may also be expressed as “P” or “PE”

  • Terms which come from the description of VPN services.
  • P – Provider Router
  • A core/backbone router which is doing label switching only.
  • A pure P router can operate without any customer/Internet routes at all.

This is common in large service provider networks.

  • PE – Provider Edge Router
  • A customer facing router which does label popping and imposition.
  • Typically has various edge features for terminating multiple services:
  • Internet
  • L3VPN
  • L2VPN / Pseudowires
  • VPLS
  • CE is the “Customer Edge”, the customer device a PE router talks to.

Vegeu també

Enllaços externs