3Com Switch 8800 Advanced Software V5 Configuration Guide

Configuring DHCP Relay Agent 739
Configure dynamic binding update interval
Via the DHCP relay agent, a DHCP client sends a DHCP-RELEASE unicast message
to the DHCP server to relinquish its IP address. In this case the DHCP relay agent
simply conveys the message to the DHCP server, thus it does not remove the IP
address from its bindings. To solve this, the DHCP relay agent can update dynamic
bindings at a specified interval.
The DHCP relay agent use its own MAC address and the IP address to be assigned
to a client to regularly send a DHCP-REQUEST message to the DHCP server. If the
server returns a DHCP-ACK message, which means IP address to be assigned to
the client is assignable now, the DHCP relay agent will update its bindings by
aging out the binding entry of the client’s IP address. If the server returns a
DHCP-NAK message, which means the IP address is still in use, the relay agent will
not age it out.
To configure dynamic binding update interval, use the following commands:
n
A large number of binding entries may result in a slow refreshing speed, so you
are recommended to use the default refreshing interval.
Enable unauthorized DHCP servers detection
There are invalid DHCP servers on networks, which reply DHCP clients with wrong
IP addresses. These invalid DHCP servers are unauthorized DHCP servers.
With this feature enabled, upon receiving a DHCP message with the siaddr field (IP
address of the server assigning IP addresses to clients) not being 0 from a client,
the DHCP relay agent will record the value of the siaddr field and the information
on the interface receiving the DHCP message. The administrator can use this
information to check out any DHCP unauthorized servers.
To enable unauthorized DHCP server detection, use the following commands:
n
With the unauthorized DHCP server detection enabled, the device puts a record
once for each DHCP server. The administrator needs to find unauthorized DHCP
servers from the log information. After the recorded information of a DHCP server
is cleared, a new record will be put for the DHCP server.
To do... Use the command... Remarks
Enter system view system-view -
Configure binding update
interval
dhcp relay security tracker {
interval | auto }
Optional
auto by default (auto interval
is calculated by the relay
agent according to the
number of bindings)
To do... Use the command... Remarks
Enter system view system-view -
Enable unauthorized DHCP
server detection
dhcp relay server-detect Required
Disabled by default