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Basic Concept and Practical details of MPLS

Posted By: technopediasite

Introduction

Multiprotocol Label Switching (MPLS) is a protocol-agnostic routing technique designed to speed up and shape traffic flows across enterprise and business wide area and service provider networks.

Application Of MPLS

Application of MPLS Network
MPLS Network Applications

MPLS concepts: Label Switch Routers
MPLS Label Switch Routers
MPLS Concept Label Switch Routers
An IP routing protocol is used within the routing domain (e.g.OSPF,ISIS)
A label distribution protocol is used to distribute address/label mappings between adjacent neighbors
The ingress LSR receives IP packets, performs packet classification (into a FEC), assign a label, and forward the labelled packet into the MPLS network
Core LSRs switch packets/cells based on the label value (no packet classification in the core)
The egress LSR removes the label before forwarding the IP packet outside the MPLS network


Packet forwarding: FEC and Next-Hop
IP packets are classified in FECs
Forwarding Equivalence Class

A group of IP packets which are forwarded in the same manner 
Over the same path With the same forwarding treatmentPacket forwarding consists on
Assign a packet to a FEC
Determine the next-hop of each FEC

MPLS make use of FECs
MPLS nodes assign a label to each FEC
Packet classification (into a FEC) is done where the packet enters the core
No sub-sequent packet classification in the MPLS network
Packet forwarding: FEC and Next-Hop
Packet forwarding: FEC and Next-Hop
Label Switch Path (LSP)

LSP is the unidirectional sequence of LSRs through which the labelled packets have to go through in order to reach the egress LSR
FEC is determined in LSR-ingress
MPLS Label Switch Path (LSP)
MPLS Label Switch Path (LSP)
LSPs derive from IGP routing information
LSPs may diverge from IGP shortest path
LSP tunnels (explicit routing) with Traffic Engineering


MPLS concepts Labels


Generic: can be used over Ethernet, 802.3, PPP links, Frame Relay, ATM PVCs, etc. Uses new Ethertypes/PPP PIDs/SNAP values/etc. 
Can have label stacking of 4 octets each 
draft-ietf-mpls-label-encaps-07.txt
Labels have local significance
Each LSR binds his own label mappings 

Each LSR assign labels to his FECs
Labels are assigned and exchanged between adjacent LSRs
Downstream to Upstream

Applications may require non-adjacent neighbors  e.g VPN

Label Distribution Protocols

Several protocols for label exchange
LDP - Maps unicast IP destinations into labels
RSVP, CR-LDP - Used for traffic engineering and resource reservation
BGP - External labels (VPN)

Upstream and Downstream LSRs

Rtr-C is the downstream neighbor of Rtr-B for destination 171.68.10/24 
Rtr-B is the downstream neighbor of Rtr-A for destination 171.68.10/24
LSRs know their downstream neighbors through the IP routing protocol
Next-hop address is the downstream neighbor
LSRs assign a label to each FEC
Label distribution may be upstream or downstream driven 
Most implementations use downstream with two variants
Unsolicited Downstream
Downstream on demand

Penultimate Hop Popping

The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR
The egress LSR requests the “popping” through the label distribution protocol
Egress LSR advertises implicit-null label
The egress LSR will not have to do a lookup and remove the label itself
One lookup is saved in the egress LSR
Egress LSR needs to do an IP lookup for finding more specific route
Egress LSR need NOT receive a labelled packet-label will have to be popped anyway
Penultimate Hop Popping
MPLS Penultimate Hop Popping

LDP Concepts
One of several standardised label distribution protocol
draft-ietf-mpls-ldp-09.txt
A set of procedures and messages to distribute mappings between labels and FECs
Two LSRs which use LDP to exchange label/FEC mapping information are known as "LDP Peers"
Peers exchange LDP messages
Uses TLV encoded message structure

Discovery messages
Used to discover and maintain the presence of new peers
Hello packets (UDP) sent to all-routers-in-subnet multicast address
Once neighbor is discovered, the LDP session is established over TCP

Session messages
Establish, maintain and terminate LDP sessions

Advertisement messages
Create, modify, delete label mappings

Notification messages
Error signalling


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