This section gives an overview of the mechanisms underpinning MPLS. Readers who are familiar with these may wish to skip this section.

A fundamental property of an MPLS network is that it can be used to tunnel multiple traffic types through the core of the network. Tunneling is a powerful tool because only the routers at the ingress and the egress of the tunnel need to understand the 'context' of the underlying traffic carried over the tunnel (e.g. the protocol that the traffic belongs to and the reachability information required to route and forward it in its native form). This detail is hidden from routers in the core of the network. As a consequence, core devices only need to carry sufficient state to enable them to switch MPLS-encapsulated packets without regard to their underlying content. Besides these aggregation properties, which apply to tunnels in general, MPLS tunnels have the following particular properties:

  1. Traffic can be explicitly routed, depending on which signaling protocol is used.

  2. Recursion is provided for; hence tunnels can exist within tunnels.

  3. There is protection against data spoofing, as the only place where data can be injected into an MPLS tunnel is at the head end of that tunnel. In contrast, data can be injected into an IP tunnel from any source that has connectivity to the network that carries the tunnel.

  4. The encapsulation overhead is relatively low (4 bytes per MPLS header).

An MPLS network consists of edge devices known as Label ...

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