GMPLS and CSPF

GMPLS introduces extra constraints for computing paths for GMPLS LSPs that use CSPF. These additional constraints affect the following link attributes:

• Signal type (minimum LSP bandwidth)
• Encoding type
• Switching type

These new constraints are populated in the traffic engineering database (TED) with the exchange of an interface-switching capability descriptor type length value (TLV) through an IGP.

The ignored constraints that are exchanged through the interface switching capability descriptor include:

• Maximum LSP bandwidth
• Maximum transmission unit (MTU)

The CSPF path computation is the same as in non-GMPLS environments, except that the links are also limited by GMPLS constraints.

Each link can have multiple interface-switching capability descriptors. All the descriptors are checked before a link is rejected.

The constraints are checked in the following order:

1. The signal type configured for the GMPLS LSP signifies the amount of bandwidth requested. If the desired bandwidth is less than the minimum LSP bandwidth, the interface-switching descriptor is rejected.
2. The encoding type of the link for the ingress and the egress interfaces should match. The encoding type is selected and stored at the ingress node after all the constraints are satisfied by the link and is used to select the link on the egress node.
3. The switching type of the links of the intermediate switches should match that of the GMPLS LSP specified in the configuration.

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