IP Configuration
IPv4 Management and Interfaces
Cisco Sx350, SG350X, SG350XG, Sx550X & SG550XG Series Managed Switches, Firmware Release 2.2.5.x 272
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DHCP Trusted Ports
Ports can be either DHCP trusted or untrusted. By default, all ports are untrusted. To create a
port as trusted, use the Interface Settings page. Packets from these ports are automatically
forwarded. Packets from trusted ports are used to create the Binding database and are handled
as described below.
If DHCP Snooping is not enabled, all ports are trusted by default.
How the DHCP Snooping Binding Database is Built
The following describes how the device handles DHCP packets when both the DHCP client
and DHCP server are trusted. The DHCP Snooping Binding database is built in this process.
DHCP Trusted Packet Handling
The actions are:
STEP 1 Device sends DHCPDISCOVER to request an IP address or DHCPREQUEST to accept an IP
address and lease.
STEP 2 Device snoops packet and adds the IP-MAC information to the DHCP Snooping Binding
database.
STEP 3 Device forwards DHCPDISCOVER or DHCPREQUEST packets.
STEP 4 DHCP server sends DHCPOFFER packet to offer an IP address, DHCPACK to assign one, or
DHCPNAK to deny the address request.
STEP 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.
STEP 6 Device forwards DHCPOFFER, DHCPACK, or DHCPNAK.
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