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4.
DHCP server sends DHCPOFFER packet to offer an IP address, DHCPACK to assign one,
or DHCPNAK to deny the address request.
5.
Device snoops packet. If an entry exists in the DHCP Snooping Binding table that matches
the packet, the device replaces it with IP-MAC binding on receipt of DHCPACK.
6.
Device forwards DHCPOFFER, DHCPACK, or DHCPNAK.
The following summarizes how DHCP packets are handled from both trusted and untrusted ports.
The DHCP Snooping Binding database is stored in non- volatile memory.
DHCP Snooping Packet Handling
Packet Type
Arriving from Untrusted Ingress
Interface
Arriving from Trusted Ingress
DHCPDISCOVER
Forward to trusted interfaces
only.
Forwarded to trusted interfaces
only.
DHCPOFFER
Filter.
Forward the packet according to
DHCP information. If the
destination address is unknown
the packet is filtered.
DHCPREQUEST
Forward to trusted interfaces
only.
Forward to trusted interfaces
only.
DHCPACK
Filter.
Same as DHCPOFFER and an
entry is added to the DHCP
Snooping Binding database.
DHCPNAK
Filter.
Same as DHCPOFFER. Remove
entry if exists.
DHCPDECLINE
Check if there is information in
the database. If the information
exists and does not match the
interface on which the message
was received, the packet is
filtered. Otherwise, the packet is
forwarded to trusted interfaces
only, and the entry is removed
from database.
Forward to trusted interfaces
only
Packet Type
Arriving from Untrusted Ingress
Interface
Arriving from Trusted Ingress
Interface
DHCPRELEASE
Same as DHCPDECLINE .
Same as DHCPDECLINE .
DHCPINFORM
Forward to trusted interfaces
only.
Forward to trusted interfaces
only.
DHCPLEASEQUE RY
Filtered.
Forward.