VLAN and DHCP
The use of VLAN has implications on DHCP if DHCP is being used for support of IP phones and or PC’s. The table below details the available options when using a single port for PC and IP Phones on a VLAN enabled network.
DHCP Option | Description | |
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None (Static addressing) | Manually configuration of each IP Phone. | |
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Separate DHCP Servers | Two PCs, one for each VLAN. | |
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Multihomed DHCP Server | A single PC with two NIC Cards; one for each VLAN. | |
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DHCP Relay | The option must be supported by the Ethernet switch. | |
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If using DHCP, when the IP Phone starts it will first perform a DHCP discovery without a VLAN tag. If the DHCP reply contains a new VLAN setting (scope option 176), the Phones will release all existing IP parameters and then perform a new DHCP discovery using the supplied VLAN ID. If the IP Phone does not get a new VLAN ID, the phone will continue with the settings provided in the original DHCP reply.
The VLAN ID can also be passed to the phones through the 46xxsettings file. Again if this method is used the IP phone will release all its existing IP parameters and perform a new DHCP discovery on the supplied VLAN ID.
A potential error loop condition can occur if the DHCP server and 46setting files have conflicting VLAN values. This is because the IP Phones release all their IP parameters and restart if their VLAN ID is changed. Another way this error loop can occur is if two DHCP servers are used; The Avaya 4600 IP Phone would repost this condition if it occurs.
As stated, when an IP phone is given a new VLAN ID, via TFTP or DHCP, it will immediately releases its current IP parameters and issue a new DHCP request using the new VLAN ID. In this way, when the IP Phones are first a DHCP reply from the DHCP server on the data VLAN, it contains the VLAN ID of the voice VLAN. The phone will release the data VLAN settings and send a new DHCP request tagged for the voice VLAN.
Data VLAN DHCP Settings
Option | Data VLAN DHCP Settings | Voice VLAN DHCP Settings | |
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IP Address | 192.168.43.x | 192.168.202.x | |
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Mask | 255.255.255.0 | 255.255.255.0 | |
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3: Router | 192.168.43.1 | 192.168.202.1 | |
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176 | L2Q=1, L2QVLAN=202, | MCIPADD=192.168.202.1, MCPORT=1719, TFTPSRVR=192.168.202.X | |
| VLANTEST=0 | VLANTEST=0 | |
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The VLANTEST parameter is the length of time the IP Phone is to continue DHCP requests in a VLAN (0 means unlimited
time).
H323 Install | Page 44 |
IP Office 4.2 |