[c-nsp] WS-C2970G-24TS as access switches

Jay Hennigan jay at west.net
Thu Dec 29 03:28:54 EST 2011

On 12/28/11 11:02 AM, Mike wrote:

> I was using these for exactly the same reasons stated above. This year,
> I have had three seperate instances where the switch had to lose power
> (move, re-work pwr arrangements, etc), and all three times the PSU
> apparently gave up the ghost and refused to power back up. Nothing
> 'happened' funny power wise, not zapped or otherwise mistreated in any
> way. I think these units were of a vintage vulnerable to the bad
> chineese capacitor problem and I think whatever cap in the psu just went
> fizzle while it was operating, which would let the units continue
> running but once it lost power, would prevent a successful full power on
> start up.

This is a very common failure mode with some types of switching power
supplies.  It is typically a resistor and not a capacitor.  We saw a lot
of it with the power bricks supplied with Fujitsu DSL modems a few years
ago.  It's real fun when there's a widespread power outage and customers
all over town are down once power is restored.

There's a high value resistor, typically in the hundreds of kilo-ohms
used to "kick-start" the switcher.  Once it's going, the resistor isn't
needed until power is removed and restored.  These typically fail open.
 If the gear is worth salvaging or if it's crucial to get it back online
while waiting for a replacement, I typically replace these with a
resistor of substantially higher power rating than the original.

> I was able to find and deploy the rps-675 (redundant power) after being
> burned this way three times, and it came in damm handy because there was
> a 4th event (another burned up 2970 psu) and this time the 675 kept it
> running till I was able to have an orderly replacement and maintinence
> window (with a 3560). I would reccomend deploying the rps units if you
> are going to use any cisco products with single power supply, but
> especially if you're going to be using the 2970's which have proven (in
> my shop) to be a (literally) dying breed.

These power supplies are commodity items from Chinese manufacturers that
are used in a variety of gear, not just Cisco switches.  You can often
Google the part number on the power supply brick itself and find

Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV

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