(OT) vlans and cisco switches and dhcp to my system

[Guillaume]

Bart Silverstrim a écrit :
> Off topic question but network related, and I think there's people here 
> that have expertise in this field that could clarify this for me.
> 
> I'm a tech that normally works on OS and system related problems, so 
> Cisco switches aren't my forte.  We have a Cisco inline powered switch 
> that is set up, according to my boss, "for VLANs and QOS, etc..."
> 
> That switch is running to offices for Cisco IP phones, which get 
> addresses on a 192.168.10.x range via DHCP to the Callmanager system 
> from Cisco.  Some of the phones have a switchport on the back into which 
> a computer can be plugged in (so one drop to the desk can support 2 
> network devices).  The computers were getting an address in the 
> 192.168.0.x range, again via DHCP.
> 
> We had a call about a computer that wasn't going through the gateway to 
> the Internet.  The computer was plugged into a wall drop that in turn 
> ran into the inline-power switch meant for the phones rather than a 
> second non-powered switch that is usually used for drops that aren't 
> using the phones, and the computer was getting an address in the 
> 192.168.10.x range as if it were a phone.
> 
> I asked, if you can plug a computer into a phone that then goes to the 
> powered switch, how is the computer getting the "proper" address, while 
> if the computer were plugged directly into the drop, it was getting the 
> wrong address?  Wouldn't the computer getting an address from the 
> phone's port in turn have to pass through that same inline-powered switch?
> 
> The response I got was that the phones are on a separate VLAN.  I never 
> had an answer about how the switch would differentiate the computer 
> passing through the phone rather than directly into the drop.
> 
> Anyone willing to try giving a summary of what had happened, what I'm 
> missing here?  I was just repeatedly told that the phones are on a 
> separate VLAN despite the proper address being "passed through" to the 
> computer when the computer was connected to the phone.  I thought VLANs 
> were port-specific on the switch.
> 

Hi,

What I can tell you is:

 From example, let say the vlan for the phone is 10 abd the vlan for the 
computer is 5

On a port used for a phone, you set both vlans on one port so you have 
vlan 5 and 10. you setup the switch to use the vlan of the computer (5) 
as the default vlan.

On the phone, you setup to tell it the phone vlan is 10.

On the computer, you don't change anything so it will use the default vlan.

So if you connect a phone to this port, it will use its setup vlan (10) 
and forward on its second port both vlan and the vlan 5 as default so 
the computer will use vlan 5.

If you connect a computer to this port, it will directly use the default 
vlan (5) which the vlan for the computer.

I think you have a setup something like that.

Regards
Guillaume