MPLS – Write UP



MPLS technology was developed around in 2000 but it is very much adapted by all the Service providers along with the customer because of its N number of uses or benefits. Full form of MPLS is Multi Protocol Label Switching which means it is not protocol independent technology (Compatible with most of existing Routing Protocols) and labels will be switched between the routers to reach the destination instead of being Packet.  MPLS is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

The exponential growth of the Internet over the past several years has placed a tremendous strain on the service provider networks. Not only has there been an increase in the number of users but there has been a multifold increase in connection speeds, backbone traffic and newer applications. Initially ordinary data applications required only store and forward capability in a best effort manner. The newer applications like voice, multimedia traffic and real-time e-commerce applications are pushing toward higher bandwidth and better guarantees, irrespective of the dynamic changes or interruptions in the network.

To honor the service level guarantees, the service providers not only have to provide large data pipes (which are also costlier), but also look for architectures which can provide & guarantee QoS guarantees and optimal performance with minimal increase in the cost of network resources.

MPLS technology enables Service Providers to offer additional services for their customers, scale their current offerings, and exercise more control over their growing networks by using its traffic engineering capabilities. On the other hand, Diffserv using its scalable differentiation enables differential pricing scheme for providing differential QoS. Putting it simply, MPLS is a switching technology used to get packets from one place to another through a series of hops while DiffServ governs what happens to the packet at each hop. The marriage of these two technologies, Diffserv over MPLS, provides another interesting alternative solution to the bag of existing ones.

On the other hand, compared to Diffserv over MPLS which is still an evolving technology, SNMP is a widely deployed network management protocol capable of catering to a large number of device classes. SNMP, being such a widely accepted and understood management protocol standard, can be effectively used to gather traffic characteristics from the routers in the core network. The information gathered can be used for provisioning and for traffic engineering by the network operators at the Network Operating Centre (NOC). In this section, we will cover MPLS basics and other details related to MPLS.



Draw backs of traditional IP Routing


1. Every Router may need full Internet routing table .

2. Destination Based Routing.

3. Routing lookups are performed on every Router.

4. Independently decision


Control Plane – Take cares of the routing information exchange and the routing exchange between adjacent devices – e.g – RIP, OSPF, BGP, Static, LDP, TDP.

Data Plane – Take cares of the forwarding information based on either destination address or Label.                                                                                                                             E.g – LFIB and FIB.

MPLS Label Format – Layer 3 routing protocol is still needed to propagate layer3 information. This is an add on label on top of it to propagate labels that are used for Layer 3 destination.

E.g- RIP, OSPF, BGP, ISIS –Add on–> LDP or TDP —–à LIB or LFIB or FIB.


FEC: Forwarding Equivalence Class is a set of packets which receive the same treatment in the forward direction or packets that are forwarded based on common characteristics. The treatment may be dependent on the destination IP address, source IP address, DSCP value etc.

LSR: Label Switch Router is any router in the network which can process MPLS labels. Processing includes PUSH (add a new label to a frame), POP (remove a label from a frame), SWAP a label. An Edge LSR is a device which can process unlabeled packet, use Layer 3 lookup and assign a label.

LSP: Label Switched Path can be thought of as a virtual circuit from one end point (Edge LSR) to another end point (Edge LSR). Sequence of LSR’s that forward labeled packets for a particular FEC. It is always unidirectional. LSP is setup before the actual data flow.

PHP – Penultimate Hop Popping – In MPLS the last Router before egress Router removes the TOP label reduces the load on the LER. If this process didn’t happen, the LER would have to perform at least 2 label lookups. In a large network this can result in the CPU load on the LER reaching unacceptable levels. By having PHP for an LER done on the LSRs connected to it, the load is effectively distributed among its neighbor routers and optimizes MPLS Performance.

TTL Propagation – When IP packet is labeled, the TTL value from the IP header is copied into TTL field in the Label and whenever you do the trace root into MPLS domain network it shows core MPLS router which is used for transport MPLS label( P Router). Disabling TTL propagation cause routers to set value 255 into TTL field of the label when IP packet is labeled. It will hide the Core MPLS routers information from the end users.

Per-Interface Label space – Unique for specific Interfaces.

Per-platform Label space – Unique for entire Router.

Unsolicited downstream distribution – Routers can asynchronously generate local label and propagate those label to adjacent routers.

Independent control mode – All Routers   can start propagation Labels independently of one another label.

Liberal Label retention mode – Multiple labels being received but only on being used. Used labels are kept in LIB table.

Hello Message – UDP – Used for hello packet. Multicast (                                                                                                                                                                                                                                                                                                                                                                                                                           TCP- Establish session

LDP – Label distribution protocol – port no – 646.

TDP – Tag distribution protocol – port no – 711.




CEF Switching – CEF (Cisco Express Forwarding) uses a complete IP switching table, FIB table and the generations of entries are change triggered. FIB Table contain adjancy table which stores outgoing interface and the corresponding layer2 Header. This table is also known as ARP table.

MPLS VPN Architecture – PE Routers participate in customer routing and carry a separate set of routes for each customer. By this customers can use overlapping addresses into the network.

RD – Router Distinguisher – The 64 bit value is prepended to an IPV4 address to make it globally unique when PE Router propagates to PE Egress via MP-iBGP. The resulting address is called VPNV4 address in MPLS Technology.

MP-iBGP – Multi protocol interior gateway border protocol – BGP that supports address family other than IPV4 address is called MP-iBGP.                                                                                                                                               VPN4 Address + Extended BGP Community (RT + SSO) + Second Label.

RT – Route Target –Route targets can be used to share routes among them. We can apply route targets to a VRF to control the import and export of routes among it and other VRFs. A set of VPN identifier can be attached to a route to indicate its membership in several VPN.

CE-PE – Routing Protocol – eBGP + OSPF + RIPv2 + EIGRP + Static.




Akshay Sharma

AKshay has a rich experience of 10+ years in data and telecom domains. He is working as a solution architect in a reputed Telecom in India from the last 5 yrs and has diversified experience in providing robust network solution to SMB and enterprise segment. His core expertise is in DSL broadband/IP/MPLS/Routing and switching with hands on multiple telecom equipment’s and had done multiple certifications into his long career.

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2 Responses

  1. How many Hubs can we augment in a MPLS network or say is there any restriction in adding or having individual hubs for a single MPLS network..

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