PoE and fiber transciever power redundancy

Answered Question
Jul 24th, 2008

We are attempting to use copper to fiber Black Box transceivers to connect to a remote site (3560). The transceiver can be powered by PoE or an AC adapter. We are attempting to use both, with the AC as a back-up. Is this the best option? or is PoE extremely reliable? If we should have redundant power, are there any commands where the switch would err-disable the port, but then immediately re-establish the port?

Correct Answer by andrew.burns about 8 years 7 months ago

Hi,


The commands you need are:


errdisable detect cause


and


errdisable recovery


If you're not running the latest IOS you may need to upgrade, but a quick check of the feature navigator should help with that. (The inline-power cause appeared in 12.2(37)SE for example).


HTH

Andrew.


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andrew.burns Thu, 07/24/2008 - 07:14

Hi,


PoE is as reliable as the switch that provides it - so from a risk point of view there are a bunch of things to look at, such as UPS, dual-power supplies, dual supervisors (if applicable), etc. The line card (or port) itself which supplies PoE is always going to be a single point-of-failure. You have to weigh up the cost of providing resilience (in this case the local AC supply to the convertor) with the cost of downtime.


If it's easy to hook it up to an external supply then it's probably a no-brainer but if you need to rewire a whole building to get a power socket then it's a much harder decision.


Regarding the last point about err-disabling, I'm not quite sure what you're after there, could you elaborate?


HTH

Andrew.





jimmyc_2 Thu, 07/24/2008 - 07:19

PoE can put the port into err-disable, just like port security. With port security, you have several options after the rouge MAC address is discovered, but there doesn't seem to be as many options for err-disable caused by PoE issues.


I guess the bottom line is: is it worth the effort to resolve this problem, or should we just go single-power-source from the switch?

andrew.burns Thu, 07/24/2008 - 07:58

Is the concern that a glitch in the inline power causes the port to err-disable? I wouldn't rule this out but I'd say it's unlikely. However, most Cisco switches (apart from low end ones) have some kind of auto-recovery facility that will auto-enable the port after a configured amount of time (the 3560 can do it on a cause basis).


If the link is so important that you can't afford it to be unavailable then you'll have a backup link - adding dual power to a single link just won't buy much.


HTH

Andrew.


jimmyc_2 Thu, 07/24/2008 - 08:37

What are the commands to auto-enable the 3560 on a cause basis? I haven't found them in the cisco documentation. Thanks.

Correct Answer
andrew.burns Fri, 07/25/2008 - 01:50

Hi,


The commands you need are:


errdisable detect cause


and


errdisable recovery


If you're not running the latest IOS you may need to upgrade, but a quick check of the feature navigator should help with that. (The inline-power cause appeared in 12.2(37)SE for example).


HTH

Andrew.


jimmyc_2 Mon, 07/28/2008 - 11:23

Hi Andrew,

Thanks for the help. I got a call from Black Box and they said I shouldn't configure PoE and external power, despite what the brochure says.


On the other hand, Black Box was extremely quick, helpful, and honest; probably the best help-desk system I've seen.


Thanks again.

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