Cisco 500 series Administration Manual page 391

Configuring DHCP
DHCP Snooping Binding Database
STEP 1
STEP 2
STEP 3
STEP 4
STEP 5
STEP 6
Cisco 500 Series Stackable Managed Switch Administration Guide
DHCP Trusted Packet Handling
The actions are:
Switch sends DHCPDISCOVER to request an IP address or DHCPREQUEST to
accept an IP address and lease.
Switch snoops packet and adds the IP-MAC information to the DHCP Snooping
Binding database.
Switch forwards DHCPDISCOVER or DHCPREQUEST packets.
DHCP server sends DHCPOFFER packet to offer an IP address, DHCPACK to
assign one, or DHCPNAK to deny the address request.
Switch snoops packet. If an entry exists in the DHCP Snooping Binding table that
matches the packet, the switch replaces it with IP-MAC binding on receipt of
DHCPACK.
Switch 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 Packet Handling
Packet Type
DHCPDISCOVER
Arriving from
Untrusted Ingress
Interface
Forward to trusted
interfaces only.
Arriving from Trusted Ingress
Interface
Forwarded to trusted interfaces
only.
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