[c-nsp] Redundancy vs. Paranoia

[Volodymyr Yakovenko]

On Fri, May 13, 2005 at 07:21:34AM +0300, Hank Nussbacher wrote:
>On Thu, 12 May 2005, John Neiberger wrote:
>
>My motto is always KISS.  That goes here as well.  I would go for 6500s
>with dual power supplies only for the LAN and 720XVXRs for the WAN in your
>case.  Always best to seperate LAN and WAN function if possible.
>
>I have been now buying Cisco products for well over 15 years now and for
>the life of me I can't remember the time a power supply went or the
>processing card went.  Therefore, I buy all my routers and switches with
>single processors and usually single power supplies (a dual power supply
>is really only needed if you have the ability to connect each power supply
>to totally seperate power sources/utilities) and have never had failures
>from those aspects.  With the extra budget one should buy more remote
>management boxes like remote power on boxes and remote out of band console
>boxes and anything else that makes life easier.

You are a lucky man. With all >300 our Cisco boxes I have seen about 5 power
supplies outages during last two years, including one 6513's WS-CAC-2500W.  

If you are running 6500/7600 as your core router and can afford two PS - buy
two, even if you have only one power line input.

One more thing - having two 6500/7600 with two SUPs in each can cause 
convergency problems. 6500/7600 uses IOS for routing, only recent IOS for 
this platform supports SSO/NSF. Even in SSO/NSF mode there is sort of 
convergency during switchover from one SUP to another, during this time 
the box is not fully adequate from the external hosts point of view and 
second 6500/7600 could decide to take over the first one, which also cause 
sort of convergency. This double convergency case in some circumstances 
could cause problems. 

I do not know is it Cisco's official point of view or not, but one of the
Cisco's Engineers told us that it is good idea to avoid two SUPs in two boxes
configuration.

And last but not the least - there is no ISSU (In Service Software Upgrade) for
6500/7600. So single box with two SUPs in case of software upgrade should be 
thrown away for approx 60 seconds from business (RPR+ switchover). In case of
two boxes with one SUP depending on your environment you could have up to
submillisecond switchover. 

My personal recommendation - two boxes with one SUP in each plus one spare SUP
for quick hardware and sofware (RPR+) replacement (three SUPs for two boxes).

-- 
Regards,
Volodymyr.