Explicit Routing for MPLS Overview

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

Explicit Routing in an MPLS Domain

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