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MPLS Basic Concept And Applications

Posted By: technopediasite


Multiprotocol Label Switching (MPLS) is now a widely deployed technology, which addresses a variety of issues, including traffic engineering, Quality of Service, Virtual Private Networks, and IP/ATM integration. MPLS: Technology and Applications is the first book that provides a detailed analysis of the architecture, protocols, and application of MPLS. MPLS was created in the late 1990s as a more efficient alternative to traditional IP routing, which requires each router to independently determine a packet's next hop by inspecting the packet's destination IP address before consulting its own routing table. This process consumes time and hardware resources, potentially resulting in degraded performance for real-time applications such as voice and video.

MPLS concepts Label Switch Routers

Image result for image of IGP domain with a label distribution protocol
IGP domain with a label
distribution protocol 

•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

Image result for image of 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.

Label Switch Path (LSP)

Image result for image of 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

LSP tunnels (explicit routing) with Traffic Engineering.

MPLS concepts Labels

Image result for image of MPLS concepts Labels
Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits

•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

Image result for image of MPLS layers 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

•Several protocols for label exchange
Maps unicast IP destinations into labels
Used for traffic engineering and resource reservation
External labels (VPN)

Upstream and Downstream LSRs

Image result for image of 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

Unsolicited Downstream distribution
Image result for image of Unsolicited Downstream distribution
•LSRs assign a label to each FEC
•LSRs distribute labels to the upstream neighbors  

Downstream on demand distribution

LSRs assign a label to each FEC
•Upstream LSRs request labels to downstream neighbors
•Downstream LSRs distribute labels upon request.

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.

LDP Concepts

•One of several standardised label distribution protocol
•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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