The way ISSU works with routing protocols is by just not responding and
coming back online before dead time expires. The warning is saying that
ISSU isn't guaranteed to come back within the deadtime you've configured.
With a smaller deadtime ISSU may not have finished and BGP will timeout and
the peer with flap.
Also worth mentioning that since the time it takes to execute ISSU isn't
fixed, it's really a "this is a safe timer to use" recommendation, so maybe
it works, maybe it doesn't.
On Sat, Apr 2, 2016 at 11:37 AM, Yham wrote:
> Hi All,
>
> As Nexus 7K platform doesn't support multihop BFD so i have to
> use faster the BGP hello and hold timers. When i try to configure BGP
> timers to "timers 2 6" (hold time = 6), i get a warning message as below
>
> "% ISSU will be affected if hold time is less than system switchover time
> (8 seconds)"
>
> Does it means if hold time is higher than 8 seconds (e.g. timers 3 10 or
> timers 5 15), ISSU can be performed in future. I was in impression that if
> i make any change in default timers (keepalive 60sec & hold 180sec), ISSU
> is not longer an option. Moreover, below highlighted in red seem contrary
> statements. Can someone please clarify?
>
>
> ISSU
>
> Cisco NX-OS supports in-service software upgrades (ISSU). ISSU allows you
> to upgrade software without impacting forwarding.
>
> The following conditions are required to support ISSU:
>
>- Graceful restart must be enabled (default)
>- *Keepalive and hold timers must not be smaller than their default
>values*
>
> If either of these requirements is not met, Cisco NX-OS issues a warning.
> You can proceed with the upgrade or downgrade, but service might be
> disrupted.
>
> Note
> --
>
> *Cisco NX-OS cannot guarantee ISSU for non-default timer values if the
> negotiated hold time between BGP peers is less than the system switchover
> time.*
>
> Source:
>
> http://www.cisco.com/c/en/us/td/docs/switches/datacenter/sw/6_x/nx-os/unicast/configuration/guide/b-7k-Cisco-Nexus-7000-Series-NX-OS-Unicast-Routing-Configuration-Guide-Release-6x/n7k_unicast_config_adv_bgp.html#concept_3126984786C2406680150E527351AA2D
>
> Thanks
>
> YH
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