What Is MultiProtocol Label Switching (MPLS)? What Does It Work?

MPLS stands for Multiprotocol Label Switching. It is a data transport method for different protocols on the Packet Switched Network (PSN). It is a protocol-agnostic routing technique. We use Multiprotocol Label Switching to shape and speed up traffic flows across the enterprise-wide area. Also, we try to help service provider networks.

Multiprotocol Label Switching was first created in late 1990. You can rather address it as an efficient alternative to our traditional IP routing methods. It requires each of your routers to independently determine a packet’s next hop. First, it inspects the packet’s destination IP address. Well, before that it consults its own routing table. It is a time-consuming process. Also, it requires slightly heavy hardware resources. MPLS potentially degrades performance for real-time applications. For example, video and voice files.

The first router to obtain a packet. Then, it determines this packet’s entire route as a blueprint. In an MPLS network, all the identities are quickly carried to the subsequent routers. It happens in the packet header via a label.

Router hardware has been improving exponentially throughout the history of networking technology. But ever since MPLS was developed, everything else lost its pizzazz. Everything else is diminishing its importance. MPLS is gaining its market favor due to its many other advantages. It is very secure, flexible and ensures most traffic engineering.

There usually are many complex protocols defined at many different layers for the OSI model. Take, for example, Ethernet, SONET/SDH etc. All of these are defined at the 2nd layer. Like for, the data link layer. Many other protocols are defined at 3rd layers. Like, TCP and IP network. Whereas, the individual protocols have its encapsulation method. The router has to do a bunch of works right before it decides which router port to pick to forward to when the encapsulated packet arrives at the network layer.

This entire decision-making method includes reading the packet header too. This sources and target IP address, encapsulate the packet. Then, forward the packet to a suitable port. MPLS was designed in order to dodge all of these extra steps. MPLS supports multiple protocols. It evidences its own name. You can also say, MPLS works independently of any protocol.

Segments Of Multiprotocol Label Switching (MPLS)

Uses of labels are one of the defining features of MPLS. The letter L in MPLS resides between 2nd Layers and 3rd Layers. It is a 4-byte label. It comes as the 32-bit label. An identifier that conveys the calculated forwarding path of the packet in the MPLS network. These labels are capable of containing the information related to the Quality of Service. In short, it is known as QoS. This indicates the priority level of the packet.

MPLS labels consist of 4 parts:
Label-value: 20 Bits
Experimental: 3 Bits
Bottom of Stack: 1 bit
Time to Live: 8 bits

The Label-switched paths (LSPs) paths then help service providers to make decisions ahead of time. It does such in the best possible way for each type of traffic. This helps to flow within the public and private networks.

Let’s take a look at how MPLS works

Let’s check the whole process in short. After that, we will go through all of the steps in details.

Every single packet gets labeled on entrance into the network of the service provider in the MPLS network. Ingress router is in charge of this task. This is also recognized as a Label Edge Router. In short, it is called LER. This is the router that also decides the LSP. And, the packet will take its targeted address until it reaches its destination.

Just one more this, all of these succeeding label-switching routers (LSRs) forwards the packet based on only these MPLS labels. After all of these, the only obstacle never looks as far as the IP header. And at the end, the Egress router removes these labels. After that, it forwards an original IP packet to the last destination.

Then the Label Switch Router receives the packet. Also, it performs these following tasks:

Push: It adds a label. Usually, the Ingress router does this task.
Swap: It replaces the label. The LSRs between the egress and ingress routers does this task.
Pop: This step removes the label. The Egress router takes care of this task.

Let’s walk through These
Switching The Routing Process

Switching and routing take place at the 2nd layer and 3rd layer sequentially, we already have discussed it. On the other hand, MPLS is a unique combination of both switching and routing. This is why MPLS works on 2.5 layers. It is sandwiched between the network layer and data link layer.

What Happens When The Router Receives Any Packet?

When a router takes a packet, it then finds out which network this packet is to be forwarded to. Every single router has its own routing table. The packet receiving router then scans through its own routing table. It looks for the longest match to the IP address of the due packet. This whole concept of matching an IP address is widely known as Longest Prefix Matching. Or, in short, LPM. Are you interested to know more about LPM? Write down your requests or queries I will try to address and answer them in the upcoming post.

Do you know finding the right or appropriate forwarding address is a time-consuming job? Well, it leads to congestion sometimes also for the router. Let us look at the concept of Switching, shall we?

How does Switching operate?

Switching uses of the Hardware address of the host to send the data. Or in many cases, it uses the MAC address. The sender finds the host’s MAC address using an ARP protocol. Every single Mac Address is usually assigned to a port. As there is no matching at the ethernet level switching. Switching is very fast unlike the routing.

How MPLS Works?

MPLS uses the latest technology routing and switching. It uses a special device to operate smoothly. This device is known as Label Switch Router. In short, it is known as LSR. Clearly, it took its name from the switch and router.

Two kinds of routers are used in an MPLS domain:

1. LSR (Label Switch Router) and
2. LER (Label Edge Router).

Label Switch Routers work at the very core level of the MPLS domain. Whereas, the Labeled Edge Routers performs at the edge of the network. Here, the MPLS network and IP networks meet each other.

How Does Label Edge Router Work In MPLS?

LER lies right in between the MPLS domain and IP network domain. LER is capable of either working as an egress router or as an ingress router. When the LER receives any packet from an IP domain, it triggers as an ingress router. Then, it starts checking the 3rd layer’s information. After this, it designates a label to the due packet before delivering the packet into the MPLS domain basing on the IP address of the due packet.

This process of designating a label to the packet is known as Pushing. A special header, known as “Shim” header then get inserted right in between the 2nd layer and 3rd layer during Pushing process. It then carries a label information of the due packet.


On the other hand, Ingress Label Edge Router sits at the edge of the network. The LSR checks into Label Info Base (LIB) when the due packet arrives at the Label Switch Router (LSR). Then, it swaps this label with another label. It is to make sure that the due packet can remain on the right path. Also, keep it in mind that we call this replacing process of the label of a packet with the brand new label, the “Swapping”.

Egress Label Edge Router (LER) is subjected for transmitting the packet from an MPLS domain back to the IP domain. But, Egress LER takes out the Shim Header label before sending that packet back into the IP network. And, then it hands that packet back to the earlier IP network. We call this taking out the labeling process, the “Popping”.

Label Switched Path (LSP)

The route that the particular labels packet follows in an MPLS domain is widely known as Label Switched Path (LSP). Well, now you must be realizing how MultiProtocol Label Switching (MPLS) is independent of other protocols. MultiProtocol Label Switching (MPLS) is a mere data transport tool. It can actually receive data frames from its layer using practically any protocol. Like for example, Ethernet or ATM. Yet, it does not really affect the working process of the MultiProtocol Label Switching (MPLS). Simply it operates before it hands. And, it does such over the subjected packet to the layer of the network.

What Are The Advantages Of The MPLS

Enterprises and the service providers use MPLS to complete the Quality of service. They define the LSPs. Usually, we do it by specifying the service-level of agreements on the packet loss, traffic latency, downtime, and jitter. Like, as an example, a network that has 3 service levels. It prioritizes various types of traffic. For example, 1level for the best effort traffic, level for time-sensitive traffic, it has 1level for voice.

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Future Of Multiprotocol Label Switching (MPLS)

MultiProtocol Label Switching (MPLS) is measurable to IPv6 too. Still, the main motive behind MultiProtocol Label Switching (MPLS) is to upscale the routing speed. Well, it is no more a constraint, thanks to switchings like ASICs and CAM-table. Because of these, MultiProtocol Label Switching (MPLS) fixes the usage system these days. Good news is, there are some improvements taking places in the MPLS over the years. For example, MPLS-TP. It is a variant of MPLS. Still, the usage stays just the same. We would like to invite all our readers to check out our other articles for more information regarding IP telephone service in Bangladesh.