17-32 Riverstone Networks RS Switch Router User Guide Release 8.0
Configuring L3 Label Switched Paths MPLS Configuration
information to each router in the LSP. In non-MPLS networks, explicit routing of packets requires
the packet to carry the identity of the explicit route. With MPLS, it is possible to have packets follow
an explicit route by having the label represent the route.
With an explicit LSP, each LSR in the path does not independently choose the next hop. Explicit LSPs are useful
for policy routing or traffic engineerin g.
•Constraint-based: You configure the LSP path constraints on the ingress router. The ingress router
calculates the entire path or part of the LSP. RSVP is then used to establish the LSP. All routers must
be running either the IS-IS or OSPF routing protocol with traffic engineering extensions enabled.
For information on configuring constraint-based LSPs, see Section17.7, "Traffic Engineering."
17.5.3 Configuring an Explicit LSP
As mentioned previously, all LSRs in a dynamic LSP use RSVP to establish and maintain the LSP. Therefore, all LSRs
must enable RSVP in addition to MPLS. See Section17.2, "Enabling and Starting MPLS on the RS."
You configure an explicit LSP only on the ingress router. Configuring an explicit LSP on the ingress router is a
two-step process:
1. Create one or more explicit paths. You can specify some or all transit LSRs in the path. For each
transit LSR you specified, you designate whether the route from the previous router to this router is
direct and cannot include other routers (strict route) or whether the route from the previous router to
this router can include other routers (loose route).
2. Create the LSP, specifying the previously created explicit paths as either the primary or secondary
path. The secondary path is the alternate path to the destination and is used if the primary path can
no longer reach the destination. If the LSP switches from the primary to the secondary path, the LSP
will revert back to the primary path when it becomes available.

Configuring an Explicit Path

Use the mpls create path and mpls set path commands to configure an explicit path. When configuring an
explicit path, you specify the following:
•maximum number of hops for the path
•hop number and IP address of transit routers in the path
•whether the route to the transit router is strict or loose
The following is an example of configuring an explicit path on the RS:
The mpls create path command shown above creates an explicit path 567 with a total of 3 hops. The mpls set
path commands identify each of the three hops in the explicit path. By default, the path is strict—the path must go
through the specified hop addresses. (To specify a loose route, include the option type loose.)
mpls create path 567 num-hops 3
mpls set path 567 hop-num 1 ip-addr 30.1.1.1
mpls set path 567 hop-num 2 ip-addr 30.1.1.2
mpls set path 567 hop-num 3 ip-addr 31.1.1.2