Setting up Paths

A path is a list of router hops that specifies a route across an MPLS domain.

Once the user creates a path, the user can create signaled LSPs that sees the path. Paths are configured separately from LSPs so that a path may be specified once and then used by several LSPs that see the path by name. An LSP may specify a primary and one or more redundant paths.

A path is always configured at the ingress LER and assumes that the ingress LER is the beginning of the path. A path can contain any number of nodes, which correspond to MPLS-enabled routers in the network. Each node has one attribute: whether it is strict or loose. A strict node means that the router must be directly connected to the preceding node. A loose node means that there can be other routers in between.

Creating a path is not absolutely necessary when configuring an LSP. When the user configures a signaled LSP without naming a path, CSPF uses only information in the Traffic Engineering Database (TED), as well as the user-configured attributes and requirements of the LSP to calculate the path. When the LSP has been configured not to use CSPF, the path between the ingress and egress LERs is determined using standard hop-by-hop routing methods, as if the path consisted of a single loose node.

Complete the following steps to set up a path called sf_to_sj that has four nodes.

Configure the device.
```
device# configure
```
Enable the MPLS router.
```
device(config)# router mpls
```
Define the explicit route path.
```
device(config-router-mpls)# path sf_to_sj
```
In this example the explicit route path is sf to sj.
Configure the first hop.
```
device(config-router-mpls-path-sf_to_sj)# hop 10.150.1.1 strict
```
In this example the IP address of the hop is 10.150.1.1.

Configure insert node 3.3.3.3 before 7.7.7.7.

device(config-router-mpls-path-sf_to_sj)# insert 3.3.3.3 strict before 7.7.7.7

Configure the loose node.

device(config-router-mpls-path-sf_to_sj)# hop 10.1.1.1 loose

Configure the last node.

device(config-router-mpls-path-sf_to_sj)# hop 10.100.1.1 strict

Example

The following example shows how to set up a path called sf_to_sj that has four nodes.

device# configure
device(config)# router mpls
device(config-router-mpls)# path sf_to_sj
device(config-router-mpls-path-sf_to_sj)# hop 10.150.1.1 strict
device(config-router-mpls-path-sf_to_sj)# insert 3.3.3.3 strict before 7.7.7.7
device(config-router-mpls-path-sf_to_sj)# hop 10.1.1.1 loose
device(config-router-mpls-path-sf_to_sj)# hop 10.100.1.1 strict

The path is assumed to start from the local node. The user specifies the nodes in order from ingress to egress. Specifying the local node itself as the first node in the path is optional. Further, the final node does not necessarily have to be the egress LER in the LSP. (The egress LER is specified at the LSP configuration level with the to command.) When the final node in the path differs from the egress LER, the hop between the final node in the path and the egress LER is treated as a hop to a loose node; that is, standard IP routing is used to determine the path between the final node and the egress LER.

The IP address defines an LSR and can be any interface address or a loopback interface address on the LSR.

The strict and loose parameters are relative to the preceding node. In the sf_to_sj path defined above, LSR 10.150.1.2 is a strict node; it must be directly connected to LSR 10.150.1.1. LSR 10.1.1.1 is a loose node; this means there can be other routers between LSR 10.150.1.2 and 10.1.1.1. When specifying a strict node, the user must make sure that the LSR is actually directly connected to the preceding node.