69
Table of Contents
Linksys
69
Table of Contents
Linksys
DHCP Trusted Packet Handling
The actions are as follows:
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.
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 Interface
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.
DHCPLEASEQUERY Filtered. Forward.
DHCP Snooping
This section describes how the DHCP Snooping feature is implemented via
the Web-based interface.
In Layer 2, DHCP Snooping can only be enabled on VLANs with IP addresses.
DHCP Default Configuration
The following describes DHCP Snooping and default options.
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