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nicolae
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Joined: Sat Jul 02, 2016 9:22 pm

RB960PGS PoE short-circuit

Sun Feb 14, 2021 1:50 pm

Hi.

I recently have been experiencing a short-circuit on one of my ports from a RB960PGS.

ether1 is the internet on which i do PPPoE.

I have ether2 connected to a "cAP ac" with PoE set to "auto-on" and ether3 connected to a "wAP ac" with PoE set to "forced-on" (because it does not work with "auto-on", i am guessing because of the long cable, about 15m).

ether4 goes to a dumb gigabit 5 port switch and had PoE set to "auto-on" also.

I'm not sure if this was an issue before, to be honest i don't remember checking, but now i see the status of the PoE changing from "waiting for load" to "short-circuit" intermitently. This goes away when i force PoE to "off" but i'm not sure if i should live with this issue in my lan. Another issue is that the link on ether2 goes away and i am guessing this "short-circuit" is the culprit, "cAP ac" gets powered via PoE, but the eth1 link on it (which goes to ether2 of the router) is off. So it just broadcasts the SSID but has no uplink to the network, causing all unlucky connected devices to lose internet and lan connectivity. "wAP ac" on ether3 however continues to have no issues. Also all devices connected to the dumb switch and "wAP ac" seem to work.

I did some tests and replaced ports, ether4 with ether5, to see if it was something wrong with ether4 port, but the problem seems to follow the cable going to the dumb switch.

Then i tested the cable going to the switch with a RJ45 cable tester, all 8 links seem to be working.

Then i plugged the uplink to my laptop instead of the switch and the "short-circuit" went away. After that i plugged it in the switch again, and disconnected everybody else from the switch, and the "short-circuit" came back.

I use a 48V adapter on my RB960PGS, not sure about the implications, there is just this dumb switch and network cables in my lan (and 2 windows computers + 2 network printers, but those are endpoints not network gear so i am not suspecting them).

Any ideas on what could be wrong? Does the switch need to support PoE coming from the uplink to it? Is the current too much?
 
mkx
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Re: RB960PGS PoE short-circuit

Sun Feb 14, 2021 5:52 pm

I suggest to have PoE set to off on all ports where peer does not expect to be PoE powered. Checks done to verify that remote device is PoE client are pretty basic for passive PoE and can fail quite easily. And if RB960PGS wrongly decides to power remote end with 48 volts, it can cause damage on both remote end and (less likely) on local PoE out port. Due to checks being simple it can also fail to detect PoE-enabled client over a long ethernet wire (in this case it is very important to use high quality cables).

Keep in mind that PoE has nothing to do with data transmissions over same port.
BR,
Metod
 
nicolae
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Joined: Sat Jul 02, 2016 9:22 pm

Re: RB960PGS PoE short-circuit

Sun Feb 14, 2021 11:34 pm

Hi.

Thanks for the suggestion, i will keep PoE"off" for every port that doesn't require it.

As for the passive PoE concern, i almost never use it, both the router and both my APs support 802.3af and 802.3at, so i guess they will all negotiate 802.3at in my case. I specifically chose that when i was buying them.

I will try to test more cables and replace them if necessary.

Thank you!
 
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krafg
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Re: RB960PGS PoE short-circuit

Mon Feb 15, 2021 10:49 pm

Most probably is caused by cable/connector issue.

Regards.
If you want to know my network, you can find it here.

Telegram: @krafgrgi

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