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• Sub-option 2—Specifies the IP address of the backup network calling processor. DHCP clients
contact the backup processor when the primary one is unreachable.
• Sub-option 3—Specifies the voice VLAN ID and the result whether the DHCP client takes this
VLAN as the voice VLAN.
• Sub-option 4—Specifies the failover route that includes the IP address and the number of the
target user. A SIP VoIP user uses this IP address and number to directly establish a connection
to the target SIP user when both the primary and backup calling processors are unreachable.
Protocols and standards
• RFC 2131, Dynamic Host Configuration Protocol
• RFC 2132, DHCP Options and BOOTP Vendor Extensions
• RFC 1542, Clarifications and Extensions for the Bootstrap Protocol
• RFC 3046, DHCP Relay Agent Information Option
• RFC 3442, The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP)
version 4