**6. The MPLS domain**

In [3] the MPLS domain is described as "a contiguous set of nodes which operate using MPLS routing and forwarding". This domain is typically managed and controlled by one administration. The MPLS domain concept is therefore similar to the notion of an AS (autonomous system), as the term is used in conventional IP routing i.e. a set of related routers that are usually under one administrative and management control.

The MPLS domain can be divided into MPLS core and MPLS edge. The core consists of nodes neighboring only to MPLS capable nodes, while the edge consists of nodes neighboring both MPLS capable and incapable nodes. The nodes in the MPLS domain are often called LSRs (Label Switch Routers). The nodes in the core are called transit LSRs and the nodes in the MPLS edge are called LERs (Label Edge Routers). If a LER is the first node in the path for a packet traveling through the MPLS domain this node is called the ingress LER, if it is the last node in a path it's called the egress LER. Note that these terms are applied according to the direction of a flow in the network, one node can therefore be both ingress and egress LER depending on which flow is considered. The terms upstream and downstream routers are also often used to indicate in which order the routers are traversed. If a LSR is upstream from another LSR, traffic is passed through that LSR before the other (downstream). A schematic view of the MPLS domain is illustrated in figure 2.
