Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 54)

30 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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RSVP soft preemption
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Only adaptive and non-FRR LSPs could be enabled for soft preemption. LSPs which are adaptive
and without FRR configuration have the facility to enable or disable the soft preemption feature
without disabling the LSP. When the soft preemption configuration is changed, RSVP is notified for
this change and a new Path message is triggered with the soft preemption desired flag bit (0x40)
set in session attribute for signaling.
Frequently used terms
Point of preemption - The midpoint (transit) or ingress LSR which, due to RSVP provisioning levels,
is forced to either hard preempt or under-provision and signal soft preemption.
Hard preemption - The (typically default) preemption process in which lower priority TE LSPs are
intrusively displaced at the point of preemption by high priority TE LSP. In hard preemption, the TE
LSP is torn down before reestablishment.
Soft preemption - Soft preemption attempts to establish a new path for a preempted LSP before
tearing down the original LSP.
Under-provisioned bandwidth (BW) - Amount of BW which is released as a result of preemption but
not yet actuated from the data plane point of view. It is actuated when the preempted LSP is torn
down as a result of the new MBB setup or hard preemption is triggered by point of preemption.
Soft preemption wait timer value - The soft preemption wait timer value is the time in seconds for
which the point of preemption router waits to receive the Path tear message for the preempted
session. In case the Path Tear is not received from the ingress and the wait timer expires, point of
preemption router again initiates the path error message intimating hard preemption of this
session with error code two, error value zero.
Configuration considerations
• Only adaptive LSPs could be configured with soft preemption capability.
• No FRR enabled LSP can be enabled for soft preemption. Vice versa is also true.
• No guarantee of preempting soft preempt-able LSP first. The protected FRRLSP is preempted
first and then the unprotected LSPs. The behavior remains the same except, in unprotected
LSPs which requests soft preemption would be soft preempted.
• Bypass LSPs cannot be enabled for soft preemption.
• In the case of an LSP passing through a series of routers running mixed releases (NetIron
R05.4.00 and lower), with soft preemption enabled, the user may see delayed propagation of
the soft preemption bit downstream from the first router running lower than NetIron R05.4.00.
• When bit 0x40 is not propagated to all downstream routers, soft preemption behavior does not
occur. The same is true when de-configuring soft preemption. Soft preemption may occur when
the 0x40 bit reset signaling has not propagated to the point of preemption on a downstream
router.
Upgrade and downgrade considerations
• Downgrading Ingress router to lower release, soft preemption configuration is lost.
• Downgrading transit-router to lower releases where MPLS soft preemption functionality is not
supported, the user will see hard preemption behavior for all LSPs irrespective of soft
preemption desired request, where this router is acting as point of preemption.