Hi,
I have bought a RB5009UPr+S+IN that has POE on the Ethernet ports. I also have a couple of CRS310-8G+2S+IN switches that from what I gather does not support to be powered with POE, I have seen these injector devices, can something like that be used to power my switches from the router, or is it just not possible ?
Thanks
Yes and no (mostly no).
Your RB5009 can provide PoE (PoE 802.3af and - almost - PoE+ 802.3at) and comes with a 48V2A96W power adapter, BUT the output on a single port is current limited, so you can have on a single port 440 mA (@48V).
The CRS310 has no provision for PoE in, but accepts 18-57 V on the jack (so the 48v will be fine), the 440 mA mean 48*0.44=21,12W.
With a PoE splitter (not an injector, even if often the same device can be used as both), you can thus in theory get 21 W from the ethernet cable and feed it to the CRS-310, but 21W is exactly what is declared as “Max power consumption without attachments” for the CRS-310, so you are very near the border, the “Max power consumption” is stated at 34 W, so as soon as you will add one or two SFP’s the RB5009 won’t simply be able to provide enough current.
Personally I wouldn’t risk to go so near the limit even without SFP’s, as you may well have a sllght voltage drop over the ethernet cable (it depends on quality of cable and distance), but if you need to use one or two SFP’s you are definitely over the capabilities of the RB5009.
You could use a higher voltage power supply (max 57V), at 57V you would have 57*0.44=25,08 W, that would probably be OK for the “naked” CRS-310 and maybe also with a single optical SFP or a DAC, but it would be still on the border line.
If you want to PoE power the CRS-310(s) with some comfortable margins, you will need to go for a separate 802.3bt (or PoE++) power supply/injector and compatible splitter(s), which are not common and - last time I looked for those - cost a lot of money.