About BFD for SRTE
BFD for SRTE is similar to MPLS OAM Monitoring for SRTE Policies. BFD for SRTE allows the switch on which one or more SRTE policies are configured to proactively detect if the active path or paths of an SRTE policy have failed. If the paths in the currently active preference have all failed, SRTE considers that preference down and so make the next highest preference of the policy active, if there is such a preference, or otherwise mark the policy as down.
BFD for SRTE performs the detection by sending BFD probes continuously along the SRTE path. Each probe is encapsulated in MPLS with the same label stack as would be imposed on traffic that follows the SRTE policy, making the probes take the same path. In addition, one more label is imposed innermost in the label stack of the probe that causes the probe to be returned to the sender by the data plane of the final node of the policy when it is reached. This differs from MPLS OAM Monitoring for SRTE Policies in which the probe is received by the final node, processed in the control plane, and a response sent back.
The probes are sent with a configurable interval between each probe, and a probe is expected to loop back to the sender within the interval. After a configurable number of failed intervals occur in sequence, the path is considered down. If all paths in a preference are down, then the preference is considered down.
Paths Monitored
Only when the commands are enabled to monitor a path using proactive monitoring will the path be monitored using BFD. Only the paths that are associated with a policy will be monitored. For example, if a segment list is created and is not associated with a policy, it is not monitored. As well, if the same path is used in multiple policies, only one monitoring session will be created for that path. This applies whether the path is a segment-list associated to a preference in a policy or is calculated using path completion on the headend. MPLS OAM monitoring may be enabled globally for all SRTE policies. If enabled globally, it can be selectively disabled per policy. If not enabled globally, it can be enabled selectively for individual policies. When a policy is monitored, SRTE will pick the highest viable preference as a primary preference and the next highest as a backup. This primary and backup are programmed to the forwarding plane so that when a failure in the primary path is detected in BFD, the forwarding layer can immediately switch to the backup path without requiring intervention from SRTE in the control plane. This reduces the time needed for failure recovery.
Index Limit
The index-limit X command is used to validate only an initial subset of the path rather than the entire path. Only indexes in the segment list that are less than or equal to the specified index-limit are part of the path to monitor. For example, if the segment list is the following:
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index 100 mpls label 16001
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index 200 mpls label 16002
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index 300 mpls label 16003
Then, if index-limit is not specified, the path to be validated will be 16001, 16002, 16003. If the index-limit is 250, then the path to be validated will be 16001, 16002. If the index-limit is 200, then the path to be validated will also be 16001, 16002.