Medianet Reference Guide - Cisco

Chapter 3
Medianet Availability Design Considerations
Operational Availability Technologies
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 use of system
processing resources by other devices in the network and improves overall network stability.
IP Event Dampening uses a series of administratively-defined thresholds to identify flapping interfaces,
to assign penalties, to suppress state changes (if necessary), and to make stabilized interfaces available
to the network. These thresholds are as follows:
• Suppress threshold—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. The default
suppress threshold value is 2000.
• 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 remains dampened. If the interface stabilizes and stops
flapping, the penalty is reduced by half after each half-life period expires. The accumulated penalty
is reduced until the penalty drops to the reuse threshold. The default half-life period timer is five
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
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 default maximum
penalty timer is 20 seconds.
IP Event Dampening is configured on a per-interface basis (where default values are used for each
threshold) as follows:
interface FastEthernet0/0
dampening
IP Event Dampening can be complemented with the use of route summarization, on a per-routing
protocol basis, to further compartmentalize the effects of flapping interfaces and associated routes.
Operational Availability Technologies
As has been shown, the predominant way that availability of a network can be improved is to improve
its MTBF by using devices that have redundant components and by engineering the network itself to be
as redundant as possible, leveraging many of the technologies discussed in the previous sections.
However, glancing back to the general availability formula from Figure 3-1, another approach to
improving availability is to reduce MTTR. Reducing MTTR is primarily a factor of operational
resiliency.
MTTR operations can be significantly improved in conjunction with device and network redundant
design. Specifically, the ability to make changes, upgrade software, and replace or upgrade hardware in
a production network is extensively improved because of the implementation of device and network
redundancy. The ability to upgrade individual devices without taking them out of service is based on
having internal component redundancy complemented with the system software capabilities. Similarly,
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Medianet Reference Guide
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