MPLS is a packet forwarding technique that can carry any layer 3 protocol. It works by assigning labels to packets at the edge router. Subsequent routers use these labels to forward packets without looking at the layer 3 headers, making forwarding more efficient. MPLS provides benefits like traffic engineering, quality of service, and scalability compared to traditional IP routing. It works by assigning packets to forwarding equivalence classes, assigning labels to these classes, and using label switching to forward packets based on these labels rather than IP routing lookups.

1. Multiprotocol Label Switching
MPLS:
The key thing to remember about MPLS is that it’s a technique, not a service — so
it can be used to deliver anything from IP VPNs to metro Ethernet services, or
even to provision optical services. So although carriers build MPLS backbones, the
services that users buy may not be called “MPLS”. They could be called anything
from“IP VPN” to “metro Ethernet”
What is MPLS?
MPLS stands for Multi-protocol Label Switching. MPLS is a packet forwarding
technology that is capable of carrying any L3 protocol and here comes the word
multi-protocol. MPLS is capable of tunneling L3 packets inside the MPLS network
using MPLS labels. The MPLS label is pushed into the packet between the layer
two headers and the layer three header of the packetat the ingress router and is
used to switch the packets across thenetwork to its destination.
Basic Concept of MPLS:
The fundamental concept behind MPLS is that of labeling packets. In a traditional
routed IP network, each router makes an independent forwarding decision for

2. each packet based solely on the packet’s network-layer header. Thus, every time
a packet arrives at a router, the router has to “think through” whereto send the
packet next.
With MPLS, the firsttime the packetenters a network, it’s assigned to a specific
forwarding equivalenceclass (FEC), indicated by appending a shortbit sequence
(the label) to the packet. Each router in the network has a table indicating how to
handle packets of a specific FEC type, so once the packet has entered the
network, routers don’tneed to performheader analysis. Instead, subsequent
routers usethe label as an index into a table that provides them with a new FEC
for that packet.
How is MPLS faster/better than traditional routing?
Fact that MPLS doesn’trequire the router to do a lookup in the routing
table.
However, in terms of how MPLS works, it"sticks" labels to routes/FECs. Then
depending on the destination, the MPLS forwarding table is looked up and labels
are popped/pushed accordingly.
So in practice, a lookup is still performed - not in the routing table, rather in the
MPLS forwarding table (whereall the labels are held depending on the FEC)

3. Comparison between Traditional IP Network and MPLS Network
Traditional IP Networks/Routing MPLS network
In traditional IP networks, each router must
process every packet to determine the next hop
that the packet must take to reach its final
destination
In an MPLS network, only edge routers fully
process each packet. Labelswitches within the
network simply forward packets based on the
label. This decreases latency experienced by
traditional routed networks performing standard
IP routing.
There is no such separation. There is a separation of the control and data
planes in MPLS.
IP based networks lack the quality-of-service
features available in circuit-based networks, such
as ATM and Frame Relay
MPLS support quality of services QoS. MPLS
replaces the virtual circuits (VC) which reduces
the hardware components for connection between
routers in the ATM network. MPLS provides an
increase in the performance enhancements and
service creation capabilities to the network.
There is no such provision in traditional IP
network.
In MPLS, routing table for every customer is
separate from other routing table for another
customer.
Traditional IP routing/networks has poor support
for traffic engineering.
MPLS has good support for traffic engineering.
Traditional IP routing/networks has poor
integration support with Layer 2 backbones
already existing in large service provider
networks.
MPLS has good integration support with Layer 2
backbones.
Traditional IP routing/networks is not scalable as
compared to MPLS.
MPLS does not have any scalability issue.
Traditional IP routing/networks clearly fits in OSI
Model.
MPLS does not fit in OSI Model.
Poor IP over ATM integration Better IP over ATM integration
There is no provision of Overlapping Address
Pools in case of traditional IP routing/networks
Overlapping Address Pools can exists in MPLS
networks
So, with traditional IP routing you're bound to
the IP topology, because that's what comes
from the routing protocols.
With MPLS, you can create (with use of
different labels and label stacks) different
topologies & services (MPLS-TE, MPLS

4. VPNs)
MPLS L3 VPNs allow you to seperate the
traffic and give each customer a private
routing instance that spans the MPLS
network.
Why Do We Need MPLS?
1. Scalability of network layer routing
2. Greater flexibility in delivering routing services
3. Increased performance–
4. Simplify integration of routers with cell switched based technologies
What is the MPLS Label and How is it used?
The MPLS label is a fixed 4 byte identifier added to the packet by the ingress
router between the data-link layer (Layer2) and the network layer (Layer3) and is
used by all middle routers to switch the packet to its destination withoutthe need
for any routing table (Layer3) look-ups. MPLS is considered a layer 2.5 technology
and the MPLS header is called the shimheader.
The diagram below illustrates the structureof the label. Oneor more labels are
pushed on the packet at the ingress router forming a label stack. The firstlabel is
called the top label or the transportlabel; other labels are used by different MPLS
applications if needed.
MPLS Label Structure:
 Label: label value, 20 bits.
 EXP: Experimental bits, Name are currently changed to Traffic class, 3 bits.
 S: bottom of stack, 1 bit.

5.  TTL: Time to live, 8 bits.
A couple of definitions are important before moving to MPLS operation:
 Downstreamrouter: This is the router which advertises the prefix. In other
words the router that is the next hop to a specific prefixis the downstream.
 Upstreamrouter: This router receives the routing information fromits
downstreamrouter.
 Label Edge Router (LER): Operates at the edge of the MPLS network
(ingress/egress) and makeforwarding decisions based on the IP header
information of the packet.
 Label Switch router (LSR): the routers in the middle of the MPLS network
which forwards MPLS packets based on label information.
Routing information flows fromdownstreamrouters to upstream router.
While data flows fromupstreamrouters to downstream routers.
How MPLS works?
MPLS uses the concept of Forwarding EquivalenceClass (FEC). The FEC is of a set
of packets forwarded in the same manner by the label switching routers (LSR).
Each router assigns a label to a FEC and distributes this label to other routers
using label distribution protocols forming label switched paths or LSPs. When a
packet is received by the ingress router it determines the next hop and inserts
one or more labels to the packet. Then the labeled packets are passed to the
next-hop router (downstream). When the packets reach the downstreamrouter,
the top mostlabels are examined and used as a unique identifier to look into the
label forwarding tableto determine the next hop and label operation to be
performed on each MPLS packet. Finally the packet reaches the egress router, the
label is removed and the packet is forwarded using an IP lookup or another label
based on the MPLS application used. As you can see the provider routers do not
need to examine layer 3 information of the traversed packets, allowing for
protocol independent packet forwarding.

6. Example:
1. R1 advertises prefix 10.10.10.0/24 to the network using any IGP.
2. Routing information about the subnet flows away from R1.
3. An IP packet enters R4 (LER) with a destination of 10.10.10.0/24.
4. R4 looks in its label forwarding information base, determines the next hop (R3)
and pushes the label assigned by R3 (L4) for this FEC.
5. R3 receives labeled packet from R4 with a label L4. R3 examines the LFIB and
swaps L3 label to L2.
6. R2 receives the MPLS packet, looks up the LFIB and pops the label (penultimate
hop popping) before sending the packet to R1 as an IP packet.
7. R1 forward the packet to its destination based on IP header information.
MPLS network requirements:
The following elements mustexist in the network to be able to run MPLS
1. A layer 3 routing protocol(IS-IS, OSPF,EIGRP or RIP); preferably IS-ISor
OSPF for Traffic engineering.
2. Label distribution protocol (RSVP, LDP or BGP).
3. Network capable of handling MPLS traffic.
MPLS Benefits:
1. BGP free core in the serviceprovider.
2. MPLS Applications like MPLS VPNand Traffic Engineering.
3. Having unified network in the serviceprovider as you can provide IP, L3
VPN or L2 VPNover the same network.

7. The Function of Labels in MPLS Networks:
Multi-Protocol Label Switching (MPLS) converts your routed network to
something closer to a switched network. Instead of forwarding packets on a hop-
by-hop basis, paths are established for particular source-destination pairs. These
predetermined paths are called label-switched paths (LSPs). Therouters that
make up a label-switched network are called label-switching routers (LSRs).
Label basics:
As packets are forwarded in a label-switching framework, MPLS routers
encapsulate the packets with special headers called labels. A label basically tells
the router which LSP it belongs to. The router can then use the ingress portand
the LSP information to determine where the next hop in the LSP is.
A label connectiontable:
The MPLS packet arrives via port 1. The router examines the label and sees that it
has a numeric identifier that associates the packet with a particular LSP. Based on
the input portand the label value, the router can look up in its MPLS routing table
wherethe next hop in the LSP is.
In this case, the lookup reveals that the outbound port is port 4. The packet
forwards thetraffic out the correctport, and the process repeats at the next LSR.
Label operations:
An LSR’s responsibilities extend beyond justlooking at the label and forwarding
the packetto wherever it needs to go. LSRs are also responsiblefor managing and
assigning the label on the packet.

8. For example, when the packet arrives at the ingress router for a particular LSP,
that ingress router is responsiblefor examining the packet so that it can send the
packet through the LSP. However, it mustalso add the MPLS label so that the next
hop in the LSP can process the packetcorrectly.
The act of adding an MPLS label is called pushing. The following three label
operations formthe basis of all MPLS forwarding:
1. Push: Add a new MPLS label to a packet. When a normal IP packet
enters an LSP, the new label is the firstlabel on the packet.
2. Pop: Removes the MPLS label from a packet. This is typically done at
either the penultimate or the egress router.
3. Swap: Replaces the label with a new label. When an LSR performs an
MPLS lookup, that lookup yields the LSP next hop information as well
as the numeric identifier for the next segment in the LSP.
Reference: www.dummies.com/how.../the-function-of-labels-in-MPLS-
networks.html
Demand of MPLS:
Need of per customer routing in network.

9. For example: If we wantto connect two companies, one branch is in Karachiand
other one is in Quetta ,so we cannot connect both companies by leas link
because of cost.
So in above diagramthe provider routers areconnected by ptcl link, and both
company branch are connected with provider edge router. The provider edge
router are connected with provider router.so that is simply connectivity .if we
want to connectivity between both abc branch (customer edge ) and provider
edge router then we will not run MPLS over them. MPLS will be run over PE
(provider edge) and PR (provider router). So in this scenario per customer routing
is being running, every customer his own routing to send secure network. So
there is no need of leas link. Load will be reduced on serviceprovider if we are
using MPLS. We also use MBGP for normalrouter.
How network flow:
So in the abovefigure how networks areflowing between two customer edge
routers. IPv4 is running between CE and PE.so MPLS header is attach with IPv4
when reach at last PE router then PE router removethe MPLS header and give
IPv4 to customer edge so this is simple MPLS network.

10. How labels are swap:
For example: CE have network 20.1.1.1,when 20.1.1.1 reached atR3 ,R3 add label
341 with 20.1.1.1and next router R1 swap the label 342 and so on when 20.1.1.1
reached at router R6, R6 removed the label and give simple IP packetto CE router
.in this figure routers justunderstood labeling and do not look whatis being in
routing .MPLS areuseful when a CE router using IP 192.168.0.0 then other CE
customer can use sameIP address by the help of RD router distenshir .

11. There are two ways to terminate an LSP:
• ImplicitNull
• Also called “Penultimate Hop Popping” (PHP).
• Justa long way of saying “removethe label on the next-to-last hop”.
• Explicit Null
• Preservethe label all the way to the very last router.
• What’s the difference?
• Implicitnull is an optimization technique.
• Since the label is already removed on the next-to-last router, the last
Router can more easily begin to routethe packet after it exits the LSP.
• Otherwise, thepacket has to make “two trIPs” through thelast router.
• Onepasses through the forwarding path to pop the label.
• Another pass to route the packetbased on the underlying information.
Traditional IP routing:
Routing is the primary function of IP. IP datagrams areprocessed and forwarded
by routers which relay traffic through paths set up by various routing protocols.
Routing in todays fixed networks is based on network aggregation combined
with best matching. TCP/IP hosts usea routing table to maintain knowledgeabout
other IP networks and IP hosts. Networks areidentified by using an IP address and
a subnet mask, and routes to single hosts arerarely set up. When a packet is to be
forwarded, therouting table is consulted and the packet is transmitted on the
interface registered with a route containing the best match for the destination. If
no network matches are found, a default route is used if one exists.
When configuring a network interface with an IP address, a routeto the network
the address is a member of is usually registered on the interface automatically.
This route is not set up with a gateway (the next hop along the path to the host)
since hosts with addresses within this network are assumed to be reachable
directly fromthis interface. This shows thatthe traditional IP routing maintains an
idea of all hosts within the same subnet being on the same link. This means that
all hosts in a subnetare available on a single one-hop network segment, typically
via routers or switches. When working on wireless multi-hop networks this is not
the case. One needs to redefine the idea of nodes being available ``on the link''.

12. Drawbacks of Traditional IP forwarding:
As shown in the diagram, router does a routing lookup for each packet in a large
routing database. The destination based routing lookup is forward through
longest prefix match of the destination IP address. Each router has to do the same
job until the packet reaches the destination. Itmakes morelatency on packet
delivery, processing load for routers. Serviceprovider corenetwork is running
with much loaded traffic. Normal routers performpacketforwarding based on
traditional IP forwarding technology.
Routing protocols are used to distribute Layer3 routing Information. Regardless of
routing protocol, routers always forward packets based on the destination
address only. Destination based routing does not provideany mechanismfor load
balancing across unequalpaths. Routing lookups are performed on every hop.
This is much over head to every hop and it makes delay on forwarding packets
since, the routing table consists hundreds of thousands routes. Routing
complexity depends on the size of routing table.
Conclusion:
The comparison between traditional IP networks and MPLS is made on
focusing on QoS, Traffic Engineering (TE), Scalability, Overlapping IP addresses etc.
Based on the theoretical study it can be concluded that MPLS has significant
advantages over traditional IP networks and provides thebest solutions because
of the following reason:
MPLS takes less processing time in forwarding thepackets due to label switching.
Implementing MPLS with TE minimizes the congestion in the network and
provides the better utilizations of network links.
MPLS suffers minimumdelay and provides high throughoutcompared to
traditional/conventional IP network.
MPLS supportoverlapping IP addresses. Itmeans sameIP address schemecan be
given to two or more different VPNs.
MPLS VPNs are more scalable than traditional IP VPNs.
MPLS provides better IP over ATM integration.
MPLS provides better results when configured with Multicasting than traditional
IP networks with Multicasting.
Because of these few notable benefits of MPLS, the serviceproviders areadapting
MPLS in their networks.