MPLS Guide MPLS and RSVP-TE
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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, refer to draft-francois-rtgwg-segment-routing-ti-lfa-04 (Topology
Independent Fast Reroute using Segment Routing).
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