MPLS offers two options for selecting routing paths:
In explicit routing, the route the LSP takes is defined by the ingress node. The path consists of a series of hops defined by the ingress LSR. Each hop can be a traditional interface, an autonomous system, or an LSP. A hop can be strict or loose.
A strict hop must be directly connected (that is, adjacent) to the previous node in the path. A loose hop is not necessarily directly connected to the previous node; whether it is directly connected is unknown.
The sequence of hops comprising an explicit routing LSP may be chosen in either of the following ways:
Consider the MPLS domain shown in Figure 55. Without explicit path routing, the tunnel is created hop by hop along the following path:
LSR 1 –> LSR 3 –> LSR 4 –> LSR 7
Suppose LSR 5 and LSR 6 are underused and LSR 4 is overused. In this case you might choose to configure the following explicit path because it forwards the data better than the hop-by-hop path:
LSR 1 –> LSR 3 –> LSR 5 –> LSR 6 –> LSR 7
Figure 55: Explicit Routing in an MPLS Domain