Explicit Routing for MPLS

MPLS offers two options for selecting routing paths:

• Hop-by-hop routing
• Explicit routing

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:

• Through a user-defined configuration, resulting in configured explicit paths. When you create the explicit route, you must manually configure each hop in the path.
• Through a routing protocol–defined configuration, resulting in dynamic explicit paths. When the routing protocol (IS-IS or OSPF) creates the explicit path, it makes use of the topological information learned from a link-state database in order to compute the entire path, beginning at the ingress node and ending at the egress node.

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

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