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New Member

Catalyst 3550 and 7960 IP Phones with PC(s) Connected

Should both the PC and IP Phone be on a separate subnet even though they are connected to the same port on the Catalyst? If so, would you have to enable trunking on the port to support such a configuration? How would DHCP allocate addresses if both were in a differenct VLAN but on the same switch port? I presume by using the IP Helper command.

Cisco Employee

Re: Catalyst 3550 and 7960 IP Phones with PC(s) Connected

It's not mandatory that the voice and data VLANs have to be separate. If the VLANs are different, you can either configure the link to be a trunk or you could use voice VLANs. Since IOS versiosn 12.1(9)EA1 there is a new approach to connecting Cisco IP phones implemented in Catalyst2950/3550 switches. This is closely resembling the Auxiliary VLAN on CatOS switches, it's called Voice Vlan and it does not require the port to be trunking. A previous thread on a similar issue should help as well

New Member

Re: Catalyst 3550 and 7960 IP Phones with PC(s) Connected

Thank you. If I do not elect to use the voice VLAN option, how would IP addresses be allocated? Hypothetically, if both VLAN 10 and 20 are both on the trunk the 7960 is attached to, how does the system discern which network it is on when requesting an IP Address? Secondly, since the 3550 is a L3 switch, does the IP Helper command reside on the interface?

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