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# MPLS Traffic Engineering

MPLS traffic engineering (TE) is the ability to establish LSPs according to particular criteria (constraints) in order to meet specific traffic requirements rather than relying on the path chosen by the conventional IGP. The constraint-based IGP examines the available network resources and calculates the shortest path for a particular tunnel that has the resources required by that tunnel. Traffic engineering enables you to make the best use of your network resources by reducing overuse and underuse of certain links.

Constraint-based routing (CR) makes traffic engineering possible by considering resource requirements and resource availability rather than merely the shortest path calculations. Constraints are determined at the edge of the network and include criteria such as required values for bandwidth or required explicit paths. You can use RSVP-TE as the label distribution protocol for traffic engineering. The IGP propagates resource information throughout its network. RSVP-TE employs downstream-on-demand, ordered control for label mapping and distribution.

Explicit routing specifies a list or group of nodes (hops) that must be used in setting up the tunnels. CR explicit paths can be strict or loose. Strict paths specify an exact physical path, including every physical node. Loose paths include hops that have local flexibility; the hop can be a traditional interface, an autonomous system, or an LSP.

## LSP Backup

You can configure multiple LSPs to the same destination. By configuring different tunnel metrics for these LSPs, you can force a ranking or priority of use for the LSPs. In this scenario, all the configured LSPs are up and active. If the LSP in use develops problems and goes down, traffic is diverted to the LSP having the next best metric.

## Path Option

You can configure multiple paths for an LSP with the tunnel mpls path-option command. Each path option has an identifying number; the lower the number the higher the preference for that path option. In this scenario, only a single LSP is up and active at a time. If the path option currently in use by an LSP goes down, MPLS tries to reroute the tunnel using the path option with the next highest preference. In certain circumstances—for example, when a tunnel is preempted by another—MPLS first attempts to reroute the tunnel with the current path option.

## Reoptimization

You can use the traffic-engineering reoptimization capability to ensure that the best path is being used. Suppose the current path goes down and MPLS switches to an alternate path that is not as good as the failed path. You can have MPLS periodically search—according to a specified schedule—for a path better than the alternate by configuring the reoptimization timer. For example, you might configure MPLS to search for a better path every 10 minutes; if it finds a better path, it switches.

On the other hand, you might be concerned about route flapping. If a path goes down and then comes back up, perhaps it will continue to do so. In this case, you might not ever want to go back to a path that goes down. To accomplish this, you can configure reoptimization to never occur.

When you do not want the initial path to change—that is, when you want to pin the route—you can disable reoptimization globally by setting the timer to 0. Alternatively, you can disable reoptimization on a per-tunnel basis by using the lockdown option with the tunnel mpls path-option command. LSP paths are always pinned until the next reoptimization.

Finally, you can manually force an immediate reoptimization. See MPLS Global Configuration Tasks in Configuring MPLS in the JUNOSe BGP and MPLS Configuration Guide for information about configuring reoptimization.

## Methods for Configuring RSVP-TE Tunnels

You can use either of the following methods to configure RSVP-TE tunnels:

- Configure individual tunnels with the interface tunnel mpls: tunnelName command.
- Configure multiple tunnels with the same set of parameters with the mpls tunnels profile command.

## Tracking Resources for MPLS Traffic Engineering

MPLS traffic engineering uses admission control to keep track of resource information. Admission control has an accounting feature that ensures that requests are not accepted when the router does not have sufficient resources to accommodate them.

Currently, bandwidth (BW) and bandwidth-related information are the only resources tracked and used for traffic engineering. Admission control determines whether a setup request can be honored for an MPLS LSP with traffic parameters.

Admission control provides bandwidth information to the IGP protocols, ISIS and OSPF. As new LSPs are created, the available bandwidth decreases. The IGPs can subsequently advertise this information and use it for SPF calculations to determine paths that satisfy the traffic requirements. You can configure readvertisement to occur periodically or when the change crosses some threshold.

### Starting Admission Control

Admission control operates on a router-wide basis rather than a per-virtual-router basis. Admission control of resources begins when either of the following occurs:

- You configure resource-related information about an interface, including bandwidth (either total bandwidth or MPLS reservable bandwidth), flooding frequency, flooding threshold, administrative weight, or attribute flags.
- MPLS begins to use admission control services; for example, by attempting to set up a constraint-based LSP.

### Admission Control Interface Table

Configuring bandwidth on an interface creates an entry for the interface in the admission control interface table. Each entry in the table stores the following information per interface:

- Maximum (physical or line-rate) bandwidth
- Maximum reservable bandwidth
- The following information per IP class (currently a single, default class)
  - Total available (unreserved) bandwidth
  - Available bandwidth at each MPLS priority level
- Resource flooding threshold and period

The resource flooding threshold and period together control the flooding of the resource information by the IGP protocols, IS-IS and OSPF.

### Configuring Traffic-Engineering Resources

You can configure the following resource-related information about an MPLS interface (at either the major interface or subinterface level):

- Bandwidth—Total bandwidth that can be reserved on the interface
- Flooding thresholds—Sets of absolute percentages of total reservable bandwidth that trigger the new bandwidth value to be flooded throughout the network; flooding is triggered when bandwidth increases past any up threshold value or decreases past any down threshold value
- Flooding frequency—Periodicity with which the bandwidth value is flooded, apart from any flooding due to value changes
- Administrative weight—Weight assigned to the interface that supersedes any assigned by the IGP
- Attribute flags—32-bit value that assigns the interface to a resource class and enables a tunnel to discriminate among interfaces by matching against tunnel affinity bits

## LSP Preemption

You can develop a preemption strategy whereby a new LSP can claim resources from an existing LSP. Each tunnel can be configured with a setup priority and a hold priority. Priority levels range from 0 (highest priority) through 7 (lowest priority).

If traffic engineering admission control determines that there are insufficient resources to accept a request to set up a new LSP, the setup priority is evaluated against the hold priority of existing LSPs. An LSP with a hold priority lower than the setup priority of the new LSP can be preempted. The existing LSP is terminated to make room (free resources) for the new LSP. You must assign priorities according to network policies to prevent resource poaching and LSP thrashing.

#### Related Topics

- See Configuring MPLS, for information about displaying information related to traffic-engineering resources

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