MPLS and RSVP-TE
112
MPLS Guide
© 2022 Nokia.
Use subject to Terms available at: www.nokia.com
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The BFD wait-for-up-timer is started when BFD is first enabled on a path or when
an existing S-BFD session transitions from up to down. If this timer expires before
S-BFD is up, the path is torn down and the LSP retry timer is started.
In the S-BFD up-to-down case, if there is only one path, the LSP is torn down when
S-BFD fails and then deprogrammed when the wait-for-up-timer expires.
If all the paths of an LSP are operationally down due to S-BFD, the LSP is taken
operationally down and the BFD wait-for-up-timer is started for each path. If one or
more paths do not have S-BFD configured or are otherwise not down, the LSP is not
taken operationally down.
When an existing S-BFD session fails on a path and the failure action is failover-or-
down, the configured failure action is activated, the path is put into the operationally
down state, and a trap is raised. The state and reason code are displayed with the
show>router>bfd>seamless-bfd command.
3.12.6.3 S-BFD Operational Considerations
A minimum control packet timer transmit interval of 10 ms can be configured. To
maximize the reliability of S-BFD connectivity checking in scaled scenarios with short
timers, situations where BFD may go down due to normal changes of the next hop
of an LSP path at the head end must be avoided. It is therefore recommended that
LFA not be configured at the head-end LER when using S-BFD with sub-second
timers. When LFA is not configured, protection of the SR-TE LSP is still provided
end-to-end by the combination of S-BFD connectivity checking and primary or
secondary path protection.
Similar to LDP and RSVP functionality, S-BFD uses a single path for a loose hop;
multiple S-BFD sessions for each of the ECMP paths or spraying of S-BFD packets
across the paths is not supported. S-BFD is not down until all the ECMP paths of the
loose hop go down.
3.12.7 Static Route Resolution Using SR-TE LSPs
Static route packets can be forwarded to an indirect next hop over an SR-TE LSP
programmed in the TTM with the following static route tunnel binding command:
Note: With very short control packet timer values in scaled scenarios, S-BFD may bounce
if the next hop that the path is currently using goes down. This is because it takes time for
BFD to be updated to use another next hop in the ECMP set.
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