Configuring Tracking And Failure Detection For A Host Or Gateway - Cisco 4700M Administration Manual

Configuring Redundant ACEs
The ACE supports the tracking and failure detection of several network items. You can configure an ACE
to track and detect failures in the following items in the Admin context and any user context:
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If one of the items that you configure for tracking and failure detection becomes unresponsive and is
associated with the active member of an FT group, by default, the ACE subtracts a value of 0 from the
configured priority of the active member. If you configure a nonzero value for the tracking priority and
the resulting priority value of the active member is less than that of the standby member, the active
member switches over and the standby member becomes the new active member. All active flows that
exist at the time of the switchover continue uninterrupted on the new active member of the FT group.
When the failed item comes back up, the ACE increments the priority of the associated group member
by a value of 0 by default. If you configure a non-zero value for the tracking priority and the resulting
priority of the standby member is greater than the priority of the active member, a switchover occurs
back to the original active group member.
You can configure the unit priority associated with tracked items to be greater than 0. This option allows
you to fine tune the switchover scenario so that a switchover occurs when either all or any of the tracked
objects fails.
To prevent an unexpected switchover from occurring, we strongly recommend that you disable
Note
preemption while you are configuring tracking. After you configure tracking and before you reenable
preemption, ensure that the tracked network objects are up and operating properly. A switchover may
occur immediately when you reenable preemption. Preemption must be enabled for a tracking
switchover to work. For details about preemption, see the
For example, suppose that on ACE 1 you configure the active FT group member with a priority of 100
and on ACE 2 you configure the standby FT group member with a priority of 70. Assume that you
configure the FT group to track three critical interfaces, each with a unit priority of 15. To trigger a
switchover, all three interfaces must fail so that the priority of the active member is less than the priority
of the standby member (100 – 45 = 55).
To illustrate the "any" scenario, assume that the active and the standby FT group members have the same
individual priorities as in the previous example (100 and 70, respectively). However, this time you
configure the three tracked interfaces, each with a unit priority of 40. If any one of the interfaces
associated with the active member goes down, then the priority of the active member falls below the
priority of the standby member and a switchover occurs. If that failed interface later returns to service,
the ACE increments the associated group member priority by 40, and a switchover would occur back to
the original active member. To guarantee a switchover if any tracked item goes down, configure the unit
priority on each tracked item equal to the group member's priority. In this case, you could configure the
unit priority to be 100.
This section contains the following topics:
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Configuring Tracking and Failure Detection for a Host or Gateway

This section describes how to configure tracking and failure detection for a gateway or a host.
Cisco 4700 Series Application Control Engine Appliance Administration Guide
6-22
Gateways or hosts
Interfaces
Configuring Tracking and Failure Detection for a Host or Gateway
Configuring Tracking and Failure Detection for an Interface
Chapter 6 Configuring Redundant ACEs
"Configuring an FT Group" section.
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