Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 1425)

DHCP Snooping Introduction and Configuration
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 55.3
DHCP Snooping Database
When you enable DHCP snooping, the switch intercepts all DHCP packets it receives, and
sends them to the Central Processing Unit (CPU), where they are verified. The DHCP
snooping database stores and maintains this information. The database contains entries
for:
■ current IP address leases dynamically allocated by a DHCP server
■ static or dynamic entries added from the command line—typically used to add a
DHCP snooping entry for a client that has a preconfigured IP address on an untrusted
port
Database backup If such a database file exists, it is loaded when the switch starts up with DHCP snooping
enabled, or when DHCP snooping is subsequently enabled.
Lease entries Each entry in the database corresponds to a DHCP IP address lease.
For dynamic entries added automatically by DHCP snooping, each entry contains the
following information:
■ the IP address that was allocated to that client
■ the MAC address of the client device
■ the time until expiry
■ the VLAN to which the client is attached
■ the port to which the client is attached
■ the IP address of the DHCP server
For static entries added from the command line, each entry contains the following subset
of information:
■ the IP address allocated to the client
■ the MAC address of the client device (optional)
■ the VLAN to which the client is attached
■ the port to which the client is attached
Each entry also shows its source: Dynamic or Static.
On this switch, the maximum number of lease entries that can be stored in the DHCP
snooping database for each port can be configured—the default is 1.
Expired entries For dynamic entries, the switch receives expiry information with the client lease
information in DHCP packets. Entries expire when the time left to expiry is 0 seconds.
Expired entries are automatically deleted from the database. Static entries have no expiry
information, and are not checked. All dynamic entries in the database are written to the
backup file. Whenever DHCP snooping is enabled, the DHCP snooping database is
repopulated from the backup file and any static entries in the start-up configuration file.
Any entries present in the backup file that have expired are ignored.