MPLS (Multi-Protocol Label Switching) Types and Architecture
MPLS technology can be of three types. These are:
1. Layer 2 point-to-point
Layer 2 point-to-point is a type of MPLS suitable for companies that need high bandwidth connections connecting a few locations together while maintaining cost-effectiveness. Examples of practical use of layer 2 point-to-point include several network operations with their primary network infrastructure built using Ethernet and layer 2.
Layer 2 point-to-point is an excellent alternative to high bandwidth leased lines. It is not bound by internet protocol and can send data running in the Local Area Network (LAN) directly to the WAN without needing routers to change the packets to be compatible with layer 3 of the OSI model. Here are its pros and cons:
Pros: With this type of MPLS, the need to manage complex routing tables has been eliminated. Also, it is cost-effective, as WAN connections can be directly linked with layer 2 switches, eliminating the need for expensive routers.
Cons: It is challenging to get circuits of less than 10Mbps in bandwidth as providers only sell high bandwidth circuits. Further, it does not support point-to-multipoint connections.
2. Layer 2 Virtual Private LAN Services (VPLS)
Layer 2 Virtual Private LAN Services (also known as Layer 2 VPLS) is now becoming more sought after for its ability to provide Ethernet services. Layer 2 VPLS combines the Multi-Protocol Label Switching with the Ethernet and extends the benefits to end customers and carriers.
For over 20 years, LAN has predominantly used Ethernet switching for connectivity, while the carrier network relies on internet protocol routing. Internet protocol not only provides internet access but also provides virtual private network (VPN) access.
Ethernet, however, has continued to be widely used over various bandwidths because it requires little technical knowledge and remains more affordable. Ethernet is now the infrastructure of choice in both LAN and WAN. Virtual Private LAN Services (VPLS) is an ideal protocol that can provide its users with Multi-Protocol Label Switching and Ethernet, therefore diverting all the traffic in Layer 2 directly to the wide area network. In addition, VPLS remains simple, easy, affordable, and highly scalable. Here are its pros and cons:
Pros: It provides a transparent interface that does not require investment in hardware such as routers to upgrade bandwidth. Traffic is labeled with a MAC address as opposed to an IP address, and like all switched networks, Layer 2 VPLS offers lower latency periods than a router network will offer. Configuration and deployment are straightforward, even for newly added sites.
Cons: Layer 2 VPLS is still being used only in some parts of the world and has not attained global reach. Therefore this limits the applicability of any feature. The absence of routers as part of the hardware infrastructure places the layer 2 VPLS at higher risk of storm damage. Monitoring is complex due to a lack of visibility from the providers.
Layer 3 IP/VPN is a type of MPLS network most suitable for large enterprises covering multiple branches over a vast land mass. This includes corporations with offices spread across the globe, industries located in more than one country, etc.
Layer 3 IP/VPN is a service that is naturally a continuation of the ATM and legacy frame relay models. Layer 3 IP/VPN transports data packets based on labels attached as the packets enter the ingress nodes. Therefore, it is highly suitable for companies that are merging for easy scalability and rapid deployment.
It is also a good fit for companies migrating from the ATM to IP or from the inflexible frame relay to IP, and also for those preparing for voice and data convergence. Layer 3 IP/VPN makes it possible for all the sites in the network to have a blanket class of service prioritization based on the type of traffic (e.g., VoIP). Here are its pros and cons:
Cons: Changing the network settings like QOS takes time and involves sending requests. Layer 3 IP/VPN is not suitable for small businesses. It offers only IP services, and must convert data from layer 2 to layer 3 before you can use it on the network.
MPLS architecture comprises a combination of 2 OSI layers – i.e., the second and third layers. This means that in an MPLS network, there are unique steps that a data packet must follow to get it across the MPLS domain. These steps include:
Creation of tables at each router using the Label Forwarding Information Base (LFIB). The LFIB can be regarded as analogous to the routing table employed in the IP network.
Label switched path creation.
Label insertion/table lookup of data packets entering the ingress router.
Packet forwarding occurs at every router by swapping the labels until the bottom stack label is reached at the egress router. The primary architectural point of Multi-Protocol Label Switching is that one can add labels carrying additional information to data packets for transfer above what the routers previously had to use.
Apart from this, you must understand the five elements of MPLS to grasp the architecture of the network.
The ingress label edge router is located on the periphery and indicates a point of entry for the data packet from its source. Ingress label router imposes a label and forwards the packets to a destination. Therefore, the ingress edge router is responsible for initiating the packet forwarding operation and does this just after setting up the label switched path (LSP) and assigning proper labels.
The Forward Equivalence Class is a group of data packets related to one application that is forwarded in its switch path, applying the same treatment and across the same route. Therefore, all the packets of that class bear the same service requirement. Each type of data traffic is given a new forward equivalence class, which is done immediately when the packet enters the MPLS cloud.
The Label Switch Router is a part of the MPLS that exchanges inbound packets with outbound ones. It also performs functions such as label removal or disposition, label addition or imposition, and label swapping. In label swapping, the label switch router replaces the topmost label in a stack with the value of an outgoing label. This router also separates data streams from the access network into the core of the MPLS, into different FECs.
4. Label Switch Path (LSP)
The Label Switch Path (LSP) is a direct pathway in the Multi-Protocol Label Switching (MPLS) enabled network that is used by a packet moving from its source to the destination. LSP is a unidirectional path that allows packets to move in only one direction. The packet passes through several intermediate routers between the origin and destination.
A labeled switched path is necessary for every MPLS network for data transfer to occur. A typical scenario involves a data packet coming in from the ingress node (LER) and migrating through different nodes through the shortest possible path, using an established LSP before getting to the egress node.
Like the ingress LER, the Egress Label Edge Router (LER) is a router located on the MPLS network’s periphery. It serves as a point of exit for data packets that have arrived at their destination. Therefore, it removes labels (label disposition) and forwards the IP packet to the final destination. The egress LER uses a bottom-of-stack indicator to guide its function. This means it will only dispose of a label if the label on top of the stack is identified as a bottom label.
Multi-Protocol Label Switching is also separated into the control and forwarding planes:
MPLS forwarding plane: The forwarding plane directs packets through routers based on their labels. It uses the information in the LFIB.
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