LANCOM Support Knowledgebase Document No. 1804.2511.2748.RHOO - V1.90

LANCOM router as DHCP forwarder: IP address of a client is stored in DHCP table with 255.255.255.255



Description:

If a LANCOM router is configured as a DHCP forwarder, it forwards all DHCP requests to a central DHCP server (master DHCP server).

By executing a DHCP trace, it can be seen that in the DHCP table of the LANCOM router (initially) not the assigned local IP address of the client is entered, but the address 255.255.255.255.

    Example:

    [Table] 2018/01/10 12:33:08,254
    Content of table: /Status/TCP-IP/DHCP/DHCP-Table

    IP-Address      MAC-Address  Timeout Hostname                 Type       LAN-Ifc Ethernet-Port VLAN-ID Network-name Assignment        
    ---------------------------------------------------------------------------------------------------------------------------------------
    172.23.56.2     0021cc6a55fc 490     Test-NB                 dyn.,Relay LAN-1   ETH-1         0       INTRANET     01/10/2018 12:39:40
    255.255.255.255 68c44d460653 2       android-3d90a2845f028649 Relay      WLAN-1  unknown       0       INTRANET     01/10/2018 12:49:35



Explanation:

The address 255.255.255.255 is a wildcard value that will be used until the DHCP server to which the request is forwarded has given the client an IP address. Then the actual IP address will be stored in the DHCP table.

If several DHCP negotiations run in parallel, the placeholder IP address is decremented by one. Thus, the LANCOM router is able to remember ongoing DHCP negotiations without occupying already valid IP addresses, since the LANCOM router at this time does not yet know which IP address of the master DHCP server the client assigns.

Catchwords: dhcp; forwarding; forwarder
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