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Table Of Contents
Restrictions for IP Event Dampening
Information About IP Event Dampening
Benefits of IP Event Dampening
How to Configure IP Event Dampening
Configuration Examples for IP Event Dampening
Configuring IP Event Dampening: Example
Verifying IP Event Dampening: Example
IP Event Dampening
The IP Event Dampening feature introduces a configurable exponential decay mechanism to suppress the effects of excessive interface flapping events on routing protocols and routing tables in the network. This feature allows the network operator to configure a router to automatically identify and selectively dampen a local interface that is flapping.
Feature History for the IP Event Dampening feature
Finding Support Information for Platforms and Cisco IOS Software Images
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Contents
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Restrictions for IP Event Dampening
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Restrictions for IP Event Dampening
Subinterface Restrictions
Only primary interfaces can be configured with this feature. IP Event Dampening does not track the flapping of individual subinterfaces on an interface.
Virtual Templates Not Supported
Copying a dampening configuration from virtual templates to virtual access interfaces is not supported because dampening has limited usefulness to existing applications that use virtual templates. Virtual access interfaces are released when an interface flaps, and new connections and virtual access interfaces are acquired when the interface comes up and is made available to the network. Since dampening states are attached to the interface, the dampening states would not survive an interface flap.
IPX Routing Protocols Not Supported
IPX protocols are not supported by the IP Event Dampening feature. However, IPX variants of these protocols will still receive up and down state event information when this feature is enabled. This should not create any problems or routing issues.
Information About IP Event Dampening
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IP Event Dampening Overview
Interface state changes occur when interfaces are administratively brought up or down or if an interface changes state. When an interface changes state or flaps, routing protocols are notified of the status of the routes that are affected by the change in state. Every interface state change requires all affected devices in the network to recalculate best paths, install or remove routes from the routing tables, and then advertise valid routes to peer routers. An unstable interface that flaps excessively can cause other devices in the network to consume substantial amounts of system processing resources and cause routing protocols to lose synchronization with the state of the flapping interface.
The IP Event Dampening feature introduces a configurable exponential decay mechanism to suppress the effects of excessive interface flapping events on routing protocols and routing tables in the network. This feature allows the network operator to configure a router to automatically identify and selectively dampen a local interface that is flapping. Dampening an interface removes the interface from the network until the interface stops flapping and becomes stable. Configuring the IP Event Dampening feature improves convergence times and stability throughout the network by isolating failures so that disturbances are not propagated, which reduces the utilization of system processing resources by other devices in the network and improves overall network stability.
Interface State Change Events
This section describes the interface state change events of the IP Event Dampening features. This feature employs a configurable exponential decay mechanism that is used to suppress the effects of excessive interface flapping or state changes. When the IP Event Dampening feature is enabled, flapping interfaces are dampened from the perspective of the routing protocol by filtering excessive route updates. Flapping interfaces are identified, assigned penalties, suppressed if the necessary, and made available to the network when the interface stabilizes. Figure 1 is a chart that displays interface state events as they are perceived by routing protocols.
Suppress Threshold
The suppress threshold is the value of the accumulated penalty that triggers the router to dampen a flapping interface. The flapping interface is identified by the router and assigned a penalty for each up and down state change, but the interface is not automatically dampened. The router tracks the penalties that a flapping interface accumulates. When the accumulated penalty reaches the default or preconfigured suppress threshold, the interface is placed in a dampened state.
Half-Life Period
The half-life period determines how fast the accumulated penalty can decay exponentially. When an interface is placed in a dampened state, the router monitors the interface for additional up and down state changes. If the interface continues to accumulate penalties and the interface remains in the suppress threshold range, the interface will remain dampened. If the interface stabilizes and stops flapping, the penalty is reduced by half after each half-life period expires. The accumulated penalty will be reduced until the penalty drops to the reuse threshold. The configurable range of the half-life period timer is from 1 to 30 seconds. The default half-life period timer is 5 seconds.
Reuse Threshold
When the accumulated penalty decreases until the penalty drops to the reuse threshold, the route is unsuppressed and made available to the other devices on the network. The range of the reuse value is from 1 to 20000 penalties. The default value is 1000 penalties.
Maximum Suppress Time
The maximum suppress time represents the maximum amount of time an interface can remain dampened when a penalty is assigned to an interface. The maximum suppress time can be configured from 1 to 20000 seconds. The default of the maximum penalty timer is 20 seconds or four times the default half-life period (5 seconds). The maximum value of the accumulated penalty is calculated, based on the maximum suppress time, reuse threshold, and half-life period.
Figure 1 Interface State Change Events Perceived by the Routing Protocols
Affected Components
When an interface is not configured with dampening, or when an interface is configured with dampening but is not suppressed, the routing protocol behavior as a result of interface state transitions is not changed by the IP Event Dampening feature. However, if an interface is suppressed, the routing protocols and routing tables are immune to any further state transitions of the interface until it is unsuppressed.
Route Types
The following interfaces are affected by the configuration of this feature:
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Supported Protocols
The IP Event Dampening feature supports Routing Information Protocol (RIP), Open Shortest Path First (OSPF), Enhanced Interior Gateway Routing Protocol (EIGRP), Intermediate System-to-Intermediate System (IS-IS), Border Gateway Protocol (BGP), Connectionless Network Services (CLNS), and Hot Standby Routing Protocol (HSRP). The following list provides some general information about the operation of this feature with these protocols.
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Network Deployments
In real network deployments, some routers would not be configured with interface dampening, and not all of the routers would even support this feature. No major routing issues are expected, even if the router at the other end of a point-to-point interface or if routers of the same multicast LAN do not have interface dampening turned on or do not have this feature implemented at all. On the router where the interface is dampened, any routes associated with this interface will not be used. No packets will be sent out of this interface, and no routing protocol activity will be initiated with the routers on the other side of the interface. However, those other routers could still install some routes in their routing tables that are associated with this subnet because those routers recognize that their own interfaces are up and could start forwarding packets to the dampened interface. In this situation, the router with the dampened interface will start forwarding these packets, depending on the routes in its routing table.
The IP Event Dampening feature does not introduce new information into the network. In fact, the effect of dampening is to subtract a subset of all such routing information from the network. Therefore, looping should not occur as a result of dampening.
Benefits of IP Event Dampening
Reduced Processing Load
The IP Event Dampening Feature employs a configurable exponential decay mechanism to suppress the effects of excessive interface flapping events on the routing protocols. Excessive interface up and down state changes that are received in a short period of time are not processed and do not consume system resources. Other routers in the network need not waste system resources because of a flapping route.
Faster Convergence
The IP Event Dampening feature improves convergence times and stability throughout the network by isolating failures so that disturbances are not propagated. Routers that are not experiencing link flap reach convergence sooner, because routing tables are not rebuilt each time the offending router leaves and enters service
Improved Network Stability
The IP Event Dampening feature provides increased network stability. A router with a flapping interface removes the flapping interface from the network until the interface stabilizes, so other routers simply redirect traffic around the affected router until the interface becomes stable, which ensures that the router loses no data packets.
How to Configure IP Event Dampening
This section contains the following tasks:
Enabling IP Event Dampening
The dampening command is entered in interface configuration mode to enable the IP Event Dampening feature. If this command is applied to an interface that already has dampening configured, all dampening states are reset and the accumulated penalty will be set to 0. If the interface has been dampened, the accumulated penalty will fall into the reuse threshold range, and the dampened interface will be made available to the network. The flap counts, however, are retained.
SUMMARY STEPS
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DETAILED STEPS
Verifying IP Event Dampening
Use the show dampening interface or show interface dampening EXEC commands to verify the configuration of the IP Event Dampening feature.
The clear counters Privileged EXEC command be used to clear the flap count and reset it to zero. All other parameters and status, including dampening states and accumulated penalties, are not affected by this command.
SUMMARY STEPS
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DETAILED STEPS
Configuration Examples for IP Event Dampening
The following examples show how to configure this feature:
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Configuring IP Event Dampening: Example
The following example configures interface dampening on Ethernet interface 0/0 and sets the half life to 30 seconds, the reuse threshold to 1500, the suppress threshold to 10000; and the maximum suppress time to 120 seconds:
interface Ethernet 0/0dampening 30 1500 10000 120The following example configures interface dampening on ATM interface 6/0 and uses the default interface dampening values:
interface atm 6/0dampeningThe following example configures the router to apply a penalty of 500 on Ethernet interface 0/0 when the interface comes up for the first time after the router is reloaded:
interface Ethernet 0/0dampening 5 500 1000 20 500Verifying IP Event Dampening: Example
The output of the show dampening interface command displays a summary of interface dampening. The following is sample output from the show dampening interface command:
Router# show dampening interface3 interfaces are configured with dampening.No interface is being suppressed.Features that are using interface dampening:IP RoutingCLNS RoutingThe output of the show interface dampening command displays the a summary of the dampening parameters and status. The following is sample output from the show interface dampening command.
Router# show interface dampeningFastEthernet0/0Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart0 0 FALSE 0 5 1000 2000 20 16000 0ATM2/0Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart0 0 FALSE 0 5 1000 2000 20 16000 0POS6/0Flaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart0 0 FALSE 0 5 1000 2000 20 16000 0Additional References
The following sections provide references related to IP Event Dampening feature.
Related Documents
Routing Protocol Commands
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Routing Protocol Configuration Tasks
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Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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MIBs
RFCs
No new or modified RFCs are supported by this feature, and support for existing standards has not been modified by this feature.
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Technical Assistance
Command Reference
This section documents new commands.
dampening
To configure a router to automatically dampen a flapping interface, use the dampening command in interface configuration mode. To disable automatic route dampening, use the no form of this command.
dampening[half-life-period reuse-threshold] [suppress-threshold max-suppress [restart-penalty]]
no dampening
Syntax Description
Defaults
This command is disabled by default. To manually configure the timer for the restart keyword, the value for all arguments must be manually entered.
Command Modes
Interface configuration
Command History
Usage Guidelines
The IP Event Dampening feature will function on a subinterface but cannot be configured on only the subinterface. Only primary interfaces can be configured with this feature. Primary interface configuration is applied to all subinterfaces by default.
When an interface is dampened, the interface is dampened to both IP and Connectionless Network Services (CLNS) routing equally. The interface is dampened to both IP and CLNS because integrated routing protocols like Intermediate System-to-Intermediate System (IS-IS), IP, and CLNS routing protocols are closely interconnected, so it is impossible to apply dampening separately.
This occurs because for integrated protocols like Intermediate System-to-Intermediate System (IS-IS), IP, and CLNS routing are closely interconnected, so it is impossible to apply dampening separately.
Copying a dampening configuration from virtual templates to virtual access interfaces is not supported because dampening has limited usefulness to existing applications using virtual templates. Virtual access interfaces are released when an interface flaps, and new connections and virtual access interfaces are acquired when the interface comes up and is made available to the network. Because dampening states are attached to the interface, the dampening states would not survive an interface flap.
If this command is applied to an interface that already has dampening configured, all dampening states are reset and the accumulated penalty will be set to 0. If the interface has been dampened, the accumulated penalty will fall into the reuse threshold range, and the dampened interface will be made available to the network. The flap counts, however, are retained.
Examples
The following example sets the half life to 30 seconds, the reuse threshold to 1500, the suppress threshold to 10000, and the maximum suppress time to 120 seconds:
interface Ethernet 0/0dampening 30 1500 10000 120The following example configures the router to apply a penalty of 500 on Ethernet interface 0/0 when the interface comes up for the first time after the router is reloaded:
interface Ethernet 0/0dampening 5 500 1000 20 500
Related Commands
Clears the interface counters.
Displays a summary of interface dampening.
Displays a summary of the dampening parameters and status.
debug dampening
To display debug trace information messages for interface dampening, use the debug dampening command in privileged EXEC mode. To disable the debug messages, use the no form of this command.
debug dampening [all | interface]
no debug dampening [all | interface]
Syntax Description
all
(optional) Enables trace debugging for all dampening features.
interface
(optional) Enables trace debugging for IP event dampening.
Defaults
No default behavior or values.
Command Modes
Privileged EXEC
Command History
Examples
The following sample output is similar to the output that will be displayed when the debug dampening command is entered with the interface keyword. The sample output shows the following information:
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router# debug dampening interface00:07:17:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from UP to DOWN00:07:17:EvD(Ethernet1/1):charge penalty 1000, new accum. penalty 1000, flap count 100:07:17:EvD(Ethernet1/1):accum. penalty 1000, now suppressed with a reuse intervals of 3000:07:17:IF-EvD(Ethernet1/1):update CLNS Routing state to DOWN, interface is suppressed00:07:17:IF-EvD(Ethernet1/1):IP Routing reports state transition from DOWN to DOWN00:07:17:IF-EvD(Ethernet1/1):update IP Routing state to DOWN, interface is suppressed00:07:17:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from DOWN to UP00:07:17:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from UP to DOWN00:07:17:EvD(Ethernet1/1):accum. penalty decayed to 1000 after 0 second(s)00:07:17:EvD(Ethernet1/1):charge penalty 1000, new accum. penalty 2000, flap count 200:07:17:EvD(Ethernet1/1):accum. penalty 2000, now suppressed with a reuse intervals of 6000:07:17:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from DOWN to UP00:07:17:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from UP to DOWN00:07:17:EvD(Ethernet1/1):accum. penalty decayed to 2000 after 0 second(s)00:07:17:EvD(Ethernet1/1):charge penalty 1000, new accum. penalty 3000, flap count 300:07:17:EvD(Ethernet1/1):accum. penalty 3000, now suppressed with a reuse intervals of 7800:07:17:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from DOWN to UP00:07:17:IF-EvD(Ethernet1/1):IP Routing reports state transition from UP to UP00:07:17:IF-EvD(Ethernet1/1):IP Routing reports state transition from UP to UP00:07:17:IF-EvD(Ethernet1/1):IP Routing reports state transition from UP to UP00:07:17:IF-EvD(Ethernet1/1):IP Routing reports state transition from UP to UP00:07:17:IF-EvD(Ethernet1/1):IP Routing reports state transition from UP to UP00:07:20:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from UP to UP00:07:20:IF-EvD(Ethernet1/1):IP Routing reports state transition from UP to UP00:07:47:IF-EvD:unsuppress interfaces00:08:36:IF-EvD:unsuppress interfaces00:08:36:EvD(Ethernet1/1):accum. penalty decayed to 483 after 79 second(s)00:08:36:EvD(Ethernet1/1):accum. penalty 483, now unsuppressed00:08:36:IF-EvD(Ethernet1/1):update IP Routing state to UP, interface is not suppressed00:08:36:IF-EvD(Ethernet1/1):update CLNS Routing state to UP, interface is not suppressed00:08:36:IF-EvD(Ethernet1/1):CLNS Routing reports state transition from UP to UPTable 1 describes the significant fields shown in the sample output of the debug dampening command.
show dampening interface
To display a summary of dampened interfaces, use the show damping interface command in EXEC mode.
show dampening interface
Syntax Description
This command has no arguments or keywords.
Command Modes
EXEC
Command History
Examples
The following is sample output from the show damping interface command in EXEC mode:
Router# show dampening interface3 interfaces are configured with dampening.No interface is being suppressed.Features that are using interface dampening:IP RoutingCLNS RoutingTable 2 describes the significant fields shown in the sample output of the show dampening interface command.
Related Commands
Clears the interface counters.
Enables IP event dampening at the interface level.
Displays a summary of the dampening parameters and status.
show interface dampening
To display dampened interfaces on the local router, use the show interface dampening command in EXEC mode.
show interface dampening
Syntax Description
This command has no keywords or arguments.
Command Modes
EXEC
Command History
Examples
The following is sample output from the show interface dampening command:
Router# show interface dampeningFlaps Penalty Supp ReuseTm HalfL ReuseV SuppV MaxSTm MaxP Restart0 0 FALSE 0 5 1000 2000 20 16000 0Table 3 describes the significant fields shown in the display.
Related Commands
Clears the interface counters.
Enables IP event dampening at the interface level.
Displays a summary of interface dampening.
Glossary
event dampening—The process in which a router dampens a flapping interface from the perspective of the routing tables and routing protocols of IP and CLNS by filtering the excessive route adjust message because of the interface state change.
Flap—Rapid interface state changes from up to down and down to up within a short period of time.
half life—The rate of the exponential decay of the accumulated penalty is determined by this value.
maximum penalty—The maximum value beyond which the penalty assigned does not increase. It is derived from the maximum suppress time.
maximum suppress time—The maximum amount of time the interface can stay suppressed at the time a penalty is assigned.
penalty—A value assigned to an interface when it flaps. This value increases with each flap and decreases over time. The rate at which it decreases depends on the half life.
reuse threshold—The threshold value after which the interface will be unsuppressed and can be used again.
suppress threshold—Value of the accumulated penalty that triggers the router to dampen a flapping interface. When the accumulated penalty exceeds this value, the interface state is considered to be down from the perspective of the routing protocol.
suppressed—Suppressing an interface removes an interface from the network from the perspective of the routing protocol. An interface enters the suppressed state when it has flapped frequently enough for the penalty assigned to it to cross a threshold limit.
Note
Copyright © 2004 Cisco Systems, Inc. All rights reserved.
