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Page 4 | AlliedWare™ OS How To Note: DHCP Snooping on Rapier-style switches
DHCP snooping
The database
The switch watches the DHCP packets that it is passing back-and-forth. It also maintains a
database that lists the DHCP leases it knows are being held by devices downstream of its
ports.
Each lease in the database holds the following information:
z the MAC address of the client device
z the IP address that was allocated to that client
z time until expiry
z VLAN to which the client is attached
z the port to which the client is attached
When inserting Option 82 information into the DHCP packets, the switch uses the
information it has stored in the database for filtering and for filling in the fields.
DHCP snooping database time-out
The CPU will time-out database entries if the lease, also stored in the database, expires.
Database survival across reboots
The database is periodically saved as a .dsn file into non-volatile storage. Therefore the
database will survive a reboot.
Verifying the status of snooped users
To verify the status of snooped users, use the command show dhcpsnooping database.
Manager > show dhcpsnooping database
DHCP Snooping Binding Database
-------------------------------------------------------------------
Full Leases/Max Leases ... 1/52
Check Interval ........... 60 seconds
Database Listeners ....... CLASSIFR
Current valid entries
MAC Address IP Address Expires(s) VLAN Port ID Source
-----------------------------------------------------------------------------
00-03-47-6b-a5-7a 10.11.67.50 56 48 16 3 Dynamic
-----------------------------------------------------------------------------
Entries with client lease but no listeners
MAC Address IP Address Expires(s) VLAN Port ID Source
-----------------------------------------------------------------------------
None...
-----------------------------------------------------------------------------
Entries with no client lease and no listeners
MAC Address IP Address Expires(s) VLAN Port ID Source
-----------------------------------------------------------------------------
None...