HELLO & WELCOME

[Dreampro]

I have Cisco phone 7841 when I connected to the wall jak power on for few seconds and then Power down again.

[WelshPaul]

Hi @Dreampro

Are you using an official Cisco PSU (CP-PWR-CUBE-3=) or a third party one?

[Dreampro]

it is POE we don't use Cisco CP-PWR-CUBE-3
just plug in the ethernet cable to the wall jack then it suppose to turn on.

[WelshPaul]

Apologies, the Cisco 7821 is 802.3af PoE certified. Does your POE offer 802.3af PoE and if so, you sure that your network isn't overloaded? Ask with your network administrator how many POE devices the network can handle...

[Dreampro]

Does your POE offer 802.3af PoE and if so, you sure that your network isn't overloaded?

How to check of it is offer 802.3af POE ? how to check the network is overloaded (I think this is the issue)
note : the phone is worked fine when I plugged in other switch interface

[WelshPaul]

You need to ask your network administrator, only they know what hardware they have installed and what it's rated for.

To give an example, a NETGEAR GS116LP 16-Port Gigabit Ethernet PoE Network Switch comes with a 76W PSU. A Cisco 7821 draws 3.84 watts and so if you took 16 phones and connected them all to the NETGEAR GS116LP used in this example the total current drawn would be 61.44 watts, this is acceptable. If you used the same PSU on a 24-port switch running 24 Cisco 7821's then the total current drawn is now 92.16 watts, this is bad and you will encounter issues because the POE switch doesn't have enough power to run all 24 phones.

Because each POE device that you connect to your POE switch has a "watt" ratting and the total shouldn't exceed the POE switches max rating, most manufacturers such as NETGEAR offer higher rated PSU's (sold separately) for use with their switches for this very reason.