Implement and Troubleshoot MPLS Operations

Multi-Protocol Label Switching

Requires CEF to be enabled on all devices running MPLS

• Mpls ip
• Mpls label-distribution [ldp, tdp, both]

Packet forwarding based on labels to make forwarding decisions

• Label is 4 bytes, fixed length
  • Label – 20bits
  • Exp – 3 bits, COS
  • S – Bottom Stack, 1 bit
  • TTL – 8 bits
• Locally significant ID
• Forwarding Equivalence Class (FEC)
  • Group of IP packets which are forwarded in the same manner
• Label is imposed between layer 2 (data link) header and layer 3 (network) header

Tag Distribution Protocol (TDP)

• Cisco proprietary

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Label Stack, LSR, LSP

LSR – Label Switch Router

• Any router or switch that implements label distribution
• Forward packets based on labels
• Edge-LSR
  • Performs label imposition (push)
    • Prepending a label or stack of labels to a packet in the ingress point of the MPLS domain
  • Performs label disposition (pop)
    • Removing last label from a packet at the egress point before sending to neighbor outside the MPLS domain
• Maintains a LIB table (Label Information Base)
  • Holds label mappings assigned by the LSR and mappings of these labels to labels received by neighbors
• LFIB – Label Forwarding Information Base
  • MPLS forwarding tabel
  • Built from the LIB

LSP – Label Switched Path

• Packets entering and exiting an MPLS network
• Describes the set of LSR’s a labeled packet must traverse to reach the egress-LSR for a particular FEC
• Unidirectional
• Connection oriented scheme
  • Setup prior to any traffic flow
  • Based on topology information

FEC – Forwarding Equivalence Class

• Grouping IP packets that are forwarded in the same manner over the same path with the same forwarding treatment
• Might correspond to a destination IP subnet
• After LIB is built – Labels get assigned to every FEC known by the router

Penultimate Hop Popping (PHP)

Label Stack

• Inserted between L2 header and L3 contents of L2 frame
• Shim header
• 20 bits label
• 3 bit COS / Experimental bit
• 8 bit TTL
• 1 bit bottom-of-stack
  • Combines 2 or more label headers attached to a single packet
• Frame mode MPLS Actions
  • Pop – Remote top label
  • Swap – Replace top label with another value
  • Push – Replace top label with a set of labels
  • Aggregate – Remove top label and does L3 lookup of underlying IP packet
  • Untag – Remote top label and forward the underlying IP packet to next hop

 

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LDP

Label Distribution Protocol

IETF Standard

Enables LSR to inform other LSR’s about label bindings that have been made.

Dynamically assign labels on a hop by hop basis

https://www.cisco.com/c/en/us/td/docs/ios/12_0s/feature/guide/fsldp22.html#wp1517250

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MPLS Ping, MPLS Traceroute

mtrace

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Implement and Troubleshoot Basic MPLS L3VPN

L3VPN

CE – Customer Equipment

• Located on customer site
• Exchanges routes with PE device

PE – Provider Edge

• Exchanged routes with CE and other PE devices
• Connect to both CE and P devices
• Is part of the MPLS domain
• Exchanges labels

P – Provider

• Inside the MPLS cloud
• Exchanges labels with P and PE devices
• Does not need to know about CE routes

 

Extranet (route leaking)

Routes can be leaked between different VRF’s (customers) by using different route target import and exports.

Service providers can utilize this to help provide Internet or some other shared service connectivity from within their MPLS cloud

ip vrf VRF
rd 10:10
route-target export 100:100
route-target import 200:200
router-target import 100:100