# Tracking Resources for MPLS Traffic Engineering Overview

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 Documentation

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