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

MPLS Network Applications

Application of MPLS

MPLS concepts: Label Switch Routers

MPLS Concept Label Switch Routers

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 treatment⏩Packet 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

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)

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

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

MPLS concepts Labels

⏩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

MPLS Penultimate Hop Popping

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