55
•
Add options for which the vendor defines the contents, for example, Option 43. DHCP servers
and clients can use vendor-specific options to exchange vendor-specific configuration
information.
•
Add options for which the Web interface does not provide a dedicated configuration page. For
example, you can use Option 4 to specify the time server address
1.1.1.1 for DHCP clients.
•
Add all option values if the actual requirement exceeds the limit for a dedicated option
configuration page. For example, on the DNS server configuration page, you can specify up to
eight DNS servers. To specify more than eight DNS servers, you can use Option 6 to specify all
DNS servers.
The following table shows the most commonly used DHCP options.
Option number
Option name
Recommended padding format
3 Router
IP
address
6
Domain Name Server
IP address
15
Domain Name
ASCII string
44
NetBIOS over TCP/IP Name Server
IP address
46
NetBIOS over TCP/IP Node Type
Hexadecimal string
66
TFTP server name
ASCII string
67
Bootfile name
ASCII string
43
Vendor Specific Information
Hexadecimal string
IP address conflict detection
Before assigning an IP address, the DHCP server pings the IP address.
•
If the server receives a response within the specified period, it selects and pings another IP
address.
•
If it does not receive a response, the server continues to ping the IP address until a specific
number of ping packets are sent. If it still does not receive a response, the server assigns the IP
address to the requesting client.
DHCP relay agent
The DHCP relay agent enables clients to get IP addresses from a DHCP server on another subnet.
This feature centralizes management and reduces investment by not deploying a DHCP server for
each subnet.
DHCP relay entry recording
This feature enables the DHCP relay agent to automatically record clients' IP-to-MAC bindings (relay
entries) after they obtain IP addresses through DHCP.
Some security functions use the relay entries to check incoming packets and block packets that do
not match any entry. In this way, illegal hosts are not able to access external networks through the
relay agent. Examples of the security functions are ARP address check, authorized ARP, and IP
source guard.
Periodic refreshing of dynamic DHCP relay entries
A DHCP client unicasts a DHCP-RELEASE message to the DHCP server to release its IP address.
The DHCP relay agent conveys the message to the DHCP server and does not remove the
IP-to-MAC entry of the client.
