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SG500-28P voice vlan configuration issues

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Oct 10th, 2013
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Hello,


I am currently in the process of configuring an SG500-28P Small Biz switch and am having difficulties obtaining a DHCP address for a Cisco ATA SPA 122 device that's plugged directly into the switch. The ATA uses CDP to communicate with the switch, and shows up in the CDP neighbor details section, but it never sends a DHCPDISCOVER to the router that is acting as the DHCP server. I have "debug ip dhcp server packet detail" running on the router and can see when requests are hitting it.


The SG300 uses the following VLANS: 1 for DATA and 2 for VOICE. The ports are set to "untagged" for VLAN 1 and "Tagged" for VLAN 2 (with the exception of the Uplink port which is set to access in the data VLAN only. I also have IP helper configured on VLAN 2 to forward requests to the DHCP server on the router. If I plug an AVAYA phone set into one of the ports, it works as expected, the ATA receives an IP in the voice VLAN. The ATA, however doesn't. I've tried with both LLDP and CDP (both of which the ATA supports) and the same behavior is experienced on both.


The "auto voice vlan" feature is also enabled, and I have tried disabling it without any success as well for troubleshooting. Any suggestions on why the ATA wouldn't be getting an IP, but the AVAYA sets would with the same port configuration?

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Tom Watts Thu, 10/10/2013 - 14:31
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Hi Jonathan, the ATA is not an IP phone, unless the ATA supports a tag packet on it's link it wouldn't get a DHCP address. I'd imagine if you configured a port as 2 untagged the ATA would get an IP.


Also being mindful the ATA likely has an adminstrative VLAN and likely by default, that is VLAN 1.


-Tom
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jonathan.macphe... Mon, 10/14/2013 - 14:42
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Tom,


Thanks for the reply. The ATA performs in a similar function to an IP phone on the traditional catalyst series. Meaning, if I have the following configuration:


int g0/1

switchport mode access

switchport access vlan 1

switchport voice vlan 2


, and have cdp enabled on the switch/interface, the device will boot into the voice VLAN and function as expected. Additionally, the ethernet port on the ATA becomes bridged at that point, so if a pc is plugged behind the ATA, it will be sending frames in the untagged (data) VLAN on the port, which is the desired method from  a security standpoint.You are correct that the ATA gets an IP when the port is just configured as access in the Voice VLAN, but that also means any device plugged in behind it will be in the same VLAN (not ideal for security).


Also, the smartports macro on the SG300 identifies it as an IP phone, in the same fashion as an AVAYA set -- it just doesn't receive an IP, let alone send a DHCPOFFER.

Tom Watts Tue, 10/15/2013 - 08:06
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Hi Jonathan, on the small business switch the port should be a trunk and with that terminology the configuration would like something like this


config t

int gi0/1

switchport mode trunk

switchport trunk native vlan 1

switchport trunk allowed vlan add 2.


This will make the port 1 untag, 2 tag.


-Tom
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