Tunnel Optimization
Tunnel Re-optimization
An MPLS TE tunnel can be automatically reestablished over a new optimal path (if one exists) if topology information is updated.
Background
MPLS TE tunnels are used to optimize traffic distribution on a network. An MPLS TE tunnel is configured using the initial bandwidth required for services and initial network topology. The network topology often changes, so the ingress node may not use the optimal path to forward MPLS packets, causing a waste of network resources. MPLS TE tunnels need to be optimized after being established.
Implementation
A specific event that occurs on the ingress can trigger optimization for a CR-LSP bound to an MPLS TE tunnel. The optimization enables the CR-LSP to be reestablished over the optimal path with the smallest metric.
Although re-optimization can be successfully configured for a CR-LSP that is established over an explicit path, the configuration does not take effect.
Automatic re-optimization
When the interval at which a CR-LSP is optimized elapses, Constraint Shortest Path First (CSPF) attempts to calculate a new path. If the calculated path has a metric smaller than that of the existing CR-LSP, a new CR-LSP is set up over the new path. After the CR-LSP is successfully set up, the ingress instructs the forwarding plane to switch traffic to the new CR-LSP and tear down the original CR-LSP. Re-optimization is then completed. If the CR-LSP fails to be set up, traffic is still forwarded along the existing CR-LSP.
Manual re-optimization
An administrator can run a reoptimization command to trigger reoptimization.
The Make-Before-Break mechanism is used to ensure uninterrupted service transmission during the re-optimization process. Traffic must be switched to a new CR-LSP before the original CR-LSP is torn down.
Automatic Bandwidth Adjustment
Automatic bandwidth adjustment enables the ingress of an MPLS TE tunnel to dynamically update tunnel bandwidth after traffic changes and to reestablish the MPLS TE tunnel using changed bandwidth values, all of which optimizes bandwidth resource usage.
Background
MPLS TE tunnels are used to optimize traffic distribution over a network. Traffic that frequently changes wastes MPLS TE tunnel bandwidth; therefore, automatic bandwidth adjustment is used to prevent this waste. A bandwidth is initially set to meet the requirement for the maximum volume of services to be transmitted over an MPLS TE tunnel, to ensure uninterrupted transmission.
Related Concepts
Automatic bandwidth adjustment allows the ingress to dynamically detect bandwidth changes and periodically attempt to reestablish a tunnel with the needed bandwidth.
Variable |
Notation |
Description |
|---|---|---|
Adjustment frequency |
A |
Interval at which bandwidth adjustment is performed. |
Sampling frequency |
B |
Interval at which traffic rates on a specific tunnel interface are sampled. |
Current bandwidth |
C |
Configured bandwidth. |
Target bandwidth |
D |
Updated bandwidth after adjustment. |
Threshold |
Threshold |
An average bandwidth is calculated after the sampling interval time elapses. If the ratio of the difference between the average bandwidth and actual bandwidth to the actual bandwidth exceeds a specific threshold, automatic bandwidth adjustment is triggered. |
Implementation
Samples traffic.
The ingress starts a bandwidth adjustment timer (A) and samples traffic at a specific interval (B seconds) to obtain the peak bandwidth during each sampling period. The ingress records the peak bandwidths.
Calculates an average bandwidth.
After timer A expires, the ingress uses the records to calculate an average bandwidth (D) to be used as a target bandwidth.
Calculates a path.
The ingress runs CSPF to calculate a path with bandwidth D and establishes a new CR-LSP over that path.
Switches traffic to the new CR-LSP.
The ingress switches traffic to the new CR-LSP before tearing down the original CR-LSP.
The preceding procedure repeats each time automatic bandwidth adjustment is triggered. Bandwidth adjustment is not needed if traffic fluctuates below a specific threshold. The ingress calculates an average bandwidth after the sampling interval time elapses. The ingress performs automatic bandwidth adjustment if the ratio of the difference between the average and existing bandwidths to the existing bandwidth exceeds a specific threshold. The following inequality applies:
[(D - C)/D] x 100% > Threshold
Other Usage
- The ingress only samples traffic on a tunnel interface, and does not perform bandwidth adjustment.
- The upper and lower limits can be set to define a range, within which the bandwidth can fluctuate.