The MPLS Traffic Engineering—LSP Attributes provides an LSP Attribute List feature and a Path Option for Bandwidth Override feature. These features have the following benefits:
- The LSP Attributes List feature provides the ability to configure values for several LSP–specific path options for TE tunnels.
- One or more TE tunnels can specify specific path options by referencing an LSP attribute list.
- The Path Option for Bandwidth Override feature provides a single command that allows a TE tunnel to fall back temporarily to path options that can reduce bandwidth constraints.
You can configure LSP attributes for path options associated with MPLS TE tunnels only through Cisco IOS commands and not through CTC.
Several LSP attributes can be applied to path options for TE tunnels using an LSP attribute list. If bandwidth is the only LSP attribute you require, then you can configure a path option for bandwidth override.
Before configuring either an LSP Attribute List or a Path Option for Bandwidth Override feature, you must configure a MPLS TE tunnel.
Traffic Engineering Bandwidth and Bandwidth Pools
MPLS TE allows constraint–based routing (CBR) of IP traffic. One of the constraints satisfied by CBR is the availability of required bandwidth over a selected path. Regular TE tunnel bandwidth is called the global pool.
You can configure the LSP Attributes bandwidth path option to use the global pool bandwidth. The bandwidth value for the path option may be any valid value and the pool does not have to be the same as that configured on the tunnel.
Autobandwidth and Path Option for Bandwidth Override
If Traffic Engineering automatic bandwidth (autobandwidth) adjustment is configured for a tunnel, traffic engineering automatically adjusts the bandwidth allocation for the traffic engineering tunnel based on its measured usage of the bandwidth of the tunnel.
Traffic engineering autobandwidth samples the average output rate for each tunnel marked for automatic bandwidth adjustment. For each marked tunnel, it periodically adjusts the allocated bandwidth for the tunnel to be the largest sample for the tunnel since the last adjustment. The default reoptimization setting in the MPLS AutoBandwidth feature is every 24 hours.
The frequency at which the tunnel bandwidth is adjusted and the allowable range of adjustments is configured on a per–tunnel basis. In addition, the sampling interval and the interval over which to average tunnel traffic to obtain the average output rate is user–configurable on a per–tunnel basis.
The automatic bandwidth feature allows you to configure and monitor the bandwidth for MPLS TE tunnels. If automatic bandwidth is configured for a tunnel, TE automatically adjusts the tunnel bandwidth.
The Path Option for Bandwidth Override feature allows you to override the bandwidth configured on a TE tunnel. This feature also overrides bandwidth configured or recalculated by automatic bandwidth adjustment if the path option in effect has bandwidth override enabled.
Constraint–Based Routing and Path Option Selection
MPLS traffic engineering automatically establishes and maintains LSPs across the network by using the RSVP. The path that an LSP uses is determined by the LSP resource requirements and network resources, such as bandwidth. Traffic engineering tunnels are calculated at the LSP head based on a fit between required and available resources (constraint–based routing).
Without the Path Option for Bandwidth Override feature, a TE tunnel establishes an LSP based on dynamic or explicit path options in order of preference. However, the bandwidth and other attributes configured on the TE tunnel allow the setup of an LSP only if LSP path options satisfy the constraints. If a path that satisfies the configured path options cannot be found, then the tunnel is not set up.
The Path Option for Bandwidth Override feature provides a fallback path option that allows overriding the bandwidth configured on the TE tunnel interface. For example, you can configure a path option that sets the bandwidth to zero effectively removing the bandwidth constraint imposed by the constraint–based routing calculation.
Tunnel Reoptimization and Path Option Selection
Reoptimization occurs when a device with traffic engineering tunnels periodically examines tunnels with established LSPs to learn if better LSPs are available. If a better LSP is available, the device attempts to signal the better LSP. If the signaling is successful, the device replaces the older LSP with the new LSP.
Reoptimization can be triggered by a timer, the mpls traffic–eng reoptimize command, or a configuration change that requires the resignalling of a tunnel. The MPLS AutoBandwidth feature, for example, uses a timer to set the frequency of reoptimization based on the bandwidth path option attribute. The Path Option for Bandwidth Override feature allows for the switching between bandwidth configured on the TE tunnel interface and bandwidth configured on a specific path option. This increases the success of signaling an LSP for the TE tunnel.
With bandwidth override configured on a path option, traffic engineering attempts to reoptimize the bandwidth every 30 seconds to reestablish the bandwidth configured on the tunnel.
Path Option Selection with Bandwidth Override
The Path Option for Bandwidth Override feature allows you to configure bandwidth parameters on a specific path option using the bandwidth keyword in the tunnel mpls traffic–eng path–option command. When an LSP is signaled using a path option with a configured bandwidth, the bandwidth associated with the path option is signaled instead of the bandwidth configured directly on the tunnel.
This feature provides you with the ability to configure multiple path options that reduce the bandwidth constraint each time the headend of a tunnel fails to establish an LSP.
Explicit and Dynamic Path Options
You can configure multiple path options for a single tunnel. For example, there can be several explicit path options and a dynamic option for one tunnel.
If you specify the dynamic keyword, the physical bandwidth of the interface and the available TE bandwidth are checked to ensure that the requested amount of bandwidth does not exceed the physical bandwidth of any link. To oversubscribe links, you must specify the explicit keyword. If you use the explicit keyword, the amount of bandwidth that is available on the link for TE is only checked; the amount of bandwidth you configure is not limited to how much physical bandwidth is available on the link.