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. 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 then adds the additional, context-specific labels, such as service label and NGE label, required to forward the packet in that context, and checks 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 will not bind to this SR-TE LSP. Instead, the service will either find another SR-TE LSP or another tunnel of a different type to bind to, if the user configured the use of other tunnel types. Otherwise, the service will go down.
When the service uses an SDP with one or more SR-TE LSPs (up to eight), the spoke SDP bound to this SDP will remain operationally down as long as at least one SR-TE LSP fails the check. In this case, the spoke SDP flag ‟labelStackLimitExceeded” will be displayed in the show output of the service. As well, the prefix will not get resolved to the SR-TE LSP and will either be resolved to another SR-TE LSP or another tunnel type or become unresolved.
The value of additional-frr-labels labels is checked against the maximum value across all IGP instances of the parameter frr-overhead. The frr-overhead parameter value is computed within an IGP instance as shown in Table: Parameter Values for frr-overhead. For more information about FRR overhead, see the Segment Routing in Shortest Path Forwarding section in the 7705 SAR Routing Protocols Guide.
Condition |
Parameter Value |
|---|---|
segment-routing is disabled in the IGP instance |
0 |
segment-routing is enabled but remote-lfa is disabled and ti-lfa is disabled |
0 |
segment-routing is enabled and remote-lfa is enabled but ti-lfa is disabled |
1 |
segment-routing is enabled and ti-lfa is enabled, regardless of whether remote-lfa is enabled or disabled |
ti-lfa max-sr-frr-labels label |
When the user configures or changes the configuration of additional-frr-labels, MPLS ensures that the new value accommodates the frr-overhead parameter value across all IGP instances.
For 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 SR-TE LSP as follows: mpls>lsp>max-sr-labels 10 additional-frr-labels 0.
Performing a no shutdown of the new LSP or changing the existing LSP configuration will be blocked because the IS-IS instance enabled remote LFA, which requires one additional label on top of the 10 SR labels of the primary path of the SR-TE LSP.
If the check is successful, MPLS adds max-sr-labels and additional-frr-labels and checks that the sum is lower than 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 the 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, the IGP will check that all SR-TE LSPs can properly account for the overhead; otherwise, the change is rejected. On the IGP, enabling remote-lfa may cause the frr-overhead value to change.
For 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 or OSPFv2 has the loopfree-alternates command disabled.
The user attempts to configure loopfree-alternates>remote-lfa for IS-IS or OSPFv2. This changes frr-overhead to 1.
This configuration change would be blocked.
When the user configures the ti-lfa command, the max-sr-frr-labels value parameter is used to limit the search for the LFA backup next hop, as follows:
0 — the IGP LFA SPF restricts the search to the TI-LFA backup next hop that does not require a repair tunnel, meaning that the P node and Q node are the same and match a neighbor. This is also the case when both P and Q nodes match the advertising router for a prefix. For information on P nodes and Q nodes, see draft-francois-rtgwg-segment-routing-ti-lfa-04 (Topology Independent Fast Reroute using Segment Routing).
1 to 3 — the IGP LFA SPF widens the search to include a repair tunnel to a P node that is connected to the Q nodes with zero to two hops for a total of three labels maximum: one node SID-to-P node and two adjacency SIDs from the P node to the Q node. If the P node is a neighbor of the computing node, its node SID is compressed, meaning that up to three adjacency SIDs can separate the P and Q nodes.
2 (default) — this corresponds to a repair tunnel to a non-adjacent P node that is adjacent to the Q node. If the P node is a neighbor of the computing node, the node SID of the P node is compressed and the default value of two labels corresponds to two adjacency SIDs between the P and Q nodes.
When the user attempts to change the max-sr-frr-labels parameter to a value that results in a change to the computed FRR overhead, the IGP checks that all SR-TE LSPs can properly account for the overhead based on the configuration of the LSP max-sr-labels and additional-frr-labels values; otherwise, the change is rejected.
The FRR overhead is computed by the IGP and its value is shown in Table: Parameter Values for frr-overhead.
The above LFA commands allow the user to enable the base LFA feature with the loopfree-alternates command, and to add remote LFA with the remote-lfa command and TI-LFA with the ti-lfa command. For more information, see the Segment Routing in Shortest Path Forwarding section in the 7705 SAR Routing Protocols Guide.