dhcp relay

[Alan Batie]

I'm trying to setup dhcp relay and not sure how the server determines
the network to assign an address from?  The agent device (a Calix E7)
requires that the uplink vlan be different from the client vlan, so the
agent address is the uplink address of the device (which doesn't have an
address on the client vlan anyhow as it's mostly a switch):

Bootstrap Protocol
    Message type: Boot Request (1)
    Hardware type: Ethernet (0x01)
    Hardware address length: 6
    Hops: 0
    Transaction ID: 0xa217414b
    Seconds elapsed: 21935
    Bootp flags: 0x0000 (Unicast)
        0... .... .... .... = Broadcast flag: Unicast
        .000 0000 0000 0000 = Reserved flags: 0x0000
    Client IP address: 0.0.0.0 (0.0.0.0)
    Your (client) IP address: 0.0.0.0 (0.0.0.0)
    Next server IP address: 0.0.0.0 (0.0.0.0)
    Relay agent IP address: 172.20.44.15 (172.20.44.15)
    Client MAC address: 48:77:46:f9:5b:b0 (48:77:46:f9:5b:b0)
    Client hardware address padding: 00000000000000000000
    Server host name not given
    Boot file name not given
    Magic cookie: DHCP
    Option: (53) DHCP Message Type
        Length: 1
        DHCP: Discover (1)
    Option: (57) Maximum DHCP Message Size
        Length: 2
        Maximum DHCP Message Size: 576
    Option: (55) Parameter Request List
        Length: 11
        Parameter Request List Item: (1) Subnet Mask
        Parameter Request List Item: (3) Router
        Parameter Request List Item: (6) Domain Name Server
        Parameter Request List Item: (12) Host Name
        Parameter Request List Item: (15) Domain Name
        Parameter Request List Item: (28) Broadcast Address
        Parameter Request List Item: (42) Network Time Protocol Servers
        Parameter Request List Item: (43) Vendor-Specific Information
        Parameter Request List Item: (120) SIP Servers
        Parameter Request List Item: (121) Classless Static Route
        Parameter Request List Item: (125) V-I Vendor-specific Information
    Option: (60) Vendor class identifier
        Length: 24
        Vendor class identifier: GS4220E.ONT.dslforum.org
    Option: (82) Agent Information Option
        Length: 96
        Option 82 Suboption: (1) Agent Circuit ID
            Length: 58
            Agent Circuit ID:
73706172652d65372d312e696e7472616e65742e7065616b...
        Option 82 Suboption: (2) Agent Remote ID
            Length: 34
            Agent Remote ID:
73706172652d65372d312e696e7472616e65742e7065616b...
    Option: (255) End
        Option End: 255
    Padding