If a packet forwarded in a service or a shortcut application has resulted in the net label stack size being pushed on the packet to exceed the maximum label stack supported by the router, the packet is dropped on the egress. Each service and shortcut application on the router performs a check of the resulting net label stack after pushing all the labels required for forwarding the packet in that context.
To that effect, the MPLS module populates each SR-TE LSP in the TTM with the maximum transport label stack size, which consists of the sum of the values in max-sr-labels label-stack-size and additional-frr-labels labels.
Each service or shortcut application adds the additional, context-specific labels, such as service label, entropy/hash label, and control-word, required to forward the packet in that context and to check that the resulting net label stack size does not exceed the maximum label stack supported by the router.
If the check succeeds, the service is bound or the prefix is resolved to the SR-TE LSP.
If the check fails, the service does not bind to this SR-TE LSP. Instead, it either finds another SR-TE LSP or another tunnel of a different type to bind to, if the user has configured the use of other tunnel types. Otherwise, the service goes down. When the service uses a SDP with one or more SR-TE LSP names, the spoke SDP bound to this SDP remains operationally down as long as at least one SR-TE LSP fails the check. In this case, a new spoke SDP flag is displayed in the show output of the service: "labelStackLimitExceeded". Similarly, the prefix does not get resolved to the SR-TE LSP and is either resolved to another SR-TE LSP or another tunnel type, or becomes unresolved.
The value of additional-frr-labels labels is checked against the maximum value across all IGP instances of the parameter frr-overhead. This parameter is computed within an IGP instance as described in Table: frr-overhead parameter values .
| Condition | frr-overhead parameter value |
|---|---|
segment-routing is disabled in the IGP instance |
0 |
segment-routing is enabled but remote-lfa is disabled |
0 |
segment-routing is enabled and remote-lfa is enabled |
1 |
When the user configures or changes the configuration of additional-frr-labels, MPLS ensures that the new value accommodates the frr-overhead value across all IGP instances.
Example:
The user configures the config>router>isis>loopfree-alternates remote-lfa command.
The user creates a new SR-TE LSP or changes the configuration of an existing as follows: mpls>lsp>max-sr-labels 10 additional-frr-labels 0.
If the check is successful, MPLS adds max-sr-labels and additional-frr-labels and checks that the result is lower or equal to the maximum label stack supported by the router. MPLS then populates the value of {max-sr-labels + additional-frr-labels}, along with tunnel information in TTM, and also passes max-sr-labels to the PCEP module.
Conversely, if the user tries a configuration change that results in a change to the computed frr-overhead, IGP checks that all SR-TE LSPs can properly account for the overhead or the change is rejected. On the IGP, enabling remote-lfa may cause the frr-overhead to change.
Example:
An MPLS LSP is administratively enabled and has mpls>lsp>max-sr-labels 10 additional-frr-overhead 0 configured.
The current configuration in IS-IS has the loopfree-alternates command disabled.
The user attempts to configure
isis>loopfree-alternates remote-lfa. This changes frr-overhead to 1.
This configuration change is blocked.