MPLS (Multi Protocol Layer System) 

The reason of this post is same as BGP. Unanswered question in an interview. Question that should know due to my background.

Multi Protocol Layers System protocol is a generic Layer 2 packet switching protocol. It uses a mechanism that allows setting MPLS labels to data packets in order to indicate their destination. An MPLS label improves the efficiency of an IP network by helping the routers to steer a packet to its final destination over a network. MPLS can be implemented over both the IPv4 as well as the IPv6 networks. MPLS also helps in the integration of the data link layer information such as the bandwidth, latency and utilization parameters with the network layer. Since MPLS attempts to integrate layer 2 with the layer 3, it is often referred to as a “Layer 2.5 protocol”.

The Multi Protocol Label Switching (MPLS) can be used to carry a wide variety of traffic including the IP packets, ATM, SONET and Ethernet frames.

MPLS works by prefixing packets with an MPLS header, containing one or more “labels”. This is called a label stack. Each label stack entry contains four fields:

  1. A 20-bit label value.
  2. A 3-bit Traffic Class field for QoS (quality of service) priority (experimental) and ECN (Explicit Congestion Notification).
  3. A 1-bit bottom of stack flag. If this is set, it signifies that the current label is the last in the stack
  4. An 8-bit TTL (time to live) field.

Possible problems whith MPLS:

Scaling problems:

At layer 3, the ISP’s must manage a routing table for each VPN and store parts of that table at every site where the VPN Is accessed.

At layer 2, resolves the scaling problem by having customers manage their own routing tables.

Security problems:

At layer 3, has no encryption built in. Underlying MPLS architecture poses a risk for data splills.

At layer 2, has no encryption built in. Underlying MPLS architecture poses a risk for data splills.


MPLS capabilities have expanded massively, for example to support service creation (VPNs), traffic engineering, network convergence, and increased resiliency. MPLS is now the de-facto standard for many carrier and service provider networks and its deployment scenarios continue to grow.