MPLS-TE LSP OOR
The MPLS-TE LSP OOR function adds capability for the RSVP-TE control plane to track the LSP scale of transit routers, so that
it can take a specific set of (pre-configured) actions when threshold limits are crossed, and inform other routers in the
network. MPLS-TE keeps track of the number of transit LSPs set up through the router. The limits do not apply to ingress and
egress LSP routers since they are driven by explicit configuration. In other words, the configuration determines how many
egress or ingress LSPs a router has. For midpoint routers, the number is a function of the topology, the links metrics, and
links’ bandwidth.
State Transition Triggers - The LSP OOR state transition is triggered by checking the total transit LSP count and the unprotected count. If either
count crosses the threshold, the state transition is triggered. If both counts cross the limit, the more critical state is
chosen. Each limit will have a value for the Yellow threshold and a value for the Red threshold. When these thresholds are crossed, the configured MPLS-TE LSP OOR actions take effect. Similarly, the transition
to Green state occurs when the LSP numbers drop.
LSP OOR State Dampening - The reason for LSP OOR State Dampening is that the number of accepted LSPs would be at the threshold and once an LSP is
deleted, the state goes back from Red to Yellow, and a new LSP is setup and the state goes back to Red.
The solution is to introduce dampening when there is a state transition from Red to Yellow or from Yellow to Green. Whenever
the transit number of LSPs crosses down a threshold, a timer is started for 10 seconds. After the timer expires, the new state
is computed and moved to it. The timer is stopped if the transit number threshold is crossed (up) again. The transition from
a state to a more severe state is not dampened.
Low and High Priority LSPs - When the LSP OOR is in yellow or red state, new high priority LSPs will not preempt low priority LSPs. Preemption can still
occur but only for bandwidth reasons. In other words, if the router is in Red state where one of the actions is to reject
any new LSP, the new high-priority LSPs are rejected even if there is an established low-priority LSP. The low-priority LSP
is not removed to make room for the high-priority one.
Configuration Limit - Setting the configured limit to a value that is smaller than the current number of LSPs will trigger state transition but
will not cause existing LSPs to be deleted or preempted. Setting the configured limit to a value that is larger than the current
number of LSPs takes the node out of LSP OOR state. When an LSP cannot be admitted due to LSP OOR, the LSRs send Path Error
messages to the LERs.
Event Logging - This is generated when the system transitions across OOR states, such as a resource change into an yellow or red state. Reporting level for Red is critical (1), and for yellow is warning (4). The following example shows that the count has crossed the threshold of 5000.
RP/0/RP1/CPU0:May 15 17:05:48 PDT: te_control[1034]: %ROUTING-MPLS_TE-4-LSP_OOR :
Transit LSP resources changed to Yellow.
Total transit: configured threshold 5000; actual count 5001;
Unprotected transit: configured threshold 4294967295; actual count 0
When the resource comes out of OOR, it will report as green.
Configuration Example
mpls traffic-eng
lsp-oor
green
action accept reopt-lsp
action flood available-bw 20
recovery-duration
action admit lsp-min-bw X -- > (in kbps, a lower limit than yellow and red state)
yellow
transit-all threshold 75000
action accept reopt-lsp
action flood available-bw 0
action admit lsp-min-bw Y
red
transit-all threshold 90000
action flood available-bw 0
action admit lsp-min-bw Z
The LSP OOR threshold values are set to yellow as 75000 and red as 90000. When these thresholds are crossed, corresponding
actions are applied to all the TE interfaces.
Note
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The default values of the above thresholds are infinite.
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When the LSP OOR yellow state is reached, the accept reopt-lsp action, flood available-bw 0 action and admit lsp-min-bw actions are activated. This allows headend routers to reoptimize existing LSPs through, but doesn’t allow new LSPs to get
established. Also, MPLS-TE advertises zero bandwidth out of all interfaces, making this transit router less preferable for
new LSPs. To handle a sudden burst of new LSPs that get signaled, the action admit lsp-min-bw function ensures only a small number of high bandwidth LSPs get provisioned through the affected router. When the red threshold
state is crossed, the flood available-bw 0 and admit lsp-min-bw actions prevent any additional or reoptimized transit LSPs from getting set up through the affected router.
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