From: Darby Weaver (ccie.weaver@gmail.com)
Date: Sat Sep 20 2008 - 15:40:46 ART
This should answer the question:
When a router first comes up, it does not have a complete routing table. If
it is configured to preempt, it will become the active router, yet it is
unable to provide adequate routing services. Solve this problem by
configuring a delay before the preempting router actually preempts the
currently active router.
Remember routing protocols need time to reconverge. So the delay can be a
handy feature in terms of increasing the stability of a given network.
Otherwise we may see flapping and we may also lose traffic while the network
stabilizes from the preemption.
I found some more stuff below.
Also if you have a chance to take a look at Diane Teare's CCDA Book from
Cisco Press you will note she goes into ample detail explaining just how
long this reconvergence can take and why under different failure scenarios.
HTH
Darby Weaver
Later
According to the Cisco Documentation for these commands they appear quite
clear on the matter:
1. Defaults
The default group number is 0.
The default delay is 0 seconds; if the router wants to preempt, it will do
so immediately.
By default, the router that comes up later becomes the standby.
2. (Optional) Specifies the preemption delay, in seconds, after a reload
only. This delay period applies only to the first interface-up event after
the router has reloaded.
Here's some verbage:
When this command is configured, the router is configured to preempt, which
means that when the local router has a Hot Standby priority higher than the
current active router, the local router should attempt to assume control as
the active router. If preemption* *is not configured, the local router
assumes control as the active router only if it receives information
indicating no router is in the active state (acting as the designated
router).
When a router first comes up, it does not have a complete routing table. If
it is configured to preempt, it will become the active router, yet it is
unable to provide adequate routing services. Solve this problem by
configuring a delay before the preempting router actually preempts the
currently active router.
When group number 0 is used, no group number is written to NVRAM, providing
backward compatibility.
IP redundancy clients can prevent preemption from taking place. The *standby
preempt* *delay* *sync* *seconds* command specifies a maximum number of
seconds to allow IP redundancy clients to prevent preemption. When this
expires, then preemption takes place regardless of the state of the IP
redundancy clients.
The* standby preempt* *delay* *reload* *seconds** *command allows preemption
to occur only after a router reloads. This provides stabilization of the
router at startup. After this initial delay at startup, the operation
returns to the default behavior.
The *no standby preempt delay* command will disable the preemption delay but
preemption will remain enabled. The *no standby preempt delay minimum* *
seconds* command will disable the minimum delay but leave any
synchronization delay if it was configured. So this should answer the
question:
On Sat, Sep 20, 2008 at 1:20 PM, Hobbs <deadheadblues@gmail.com> wrote:
> Hello,
>
> The HSRP preemption configuration options are as follows:
>
> standby [group-number] preempt [delay {minimum delay | reload delay | sync
> delay}]
>
> I always thought that the "minimum" keyword was used for preemption when
> the
> router was already loaded and it's priority was just increased (through
> tracking or other means). However the documentation explains it like this:
>
> *minimum *seconds
> ...causes the local router to postpone taking over the active role for a
> minimum number of seconds since that router was last restarted.
>
> *reload *seconds
> Specifies the preemption delay, in seconds, after a reload only.
>
> What is the difference between reload and restarted in these explanations?
>
> thanks
>
>
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>
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