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CDP compatibility
CDP compatibility enables your device to receive and recognize CDP packets from a Cisco IP phone
and respond with CDP packets.
DHCP snooping
DHCP snooping works between the DHCP client and server, or between the DHCP client and DHCP
relay agent. DHCP snooping provides the following functions:
•
Ensures that DHCP obtain IP addresses only from authorized DHCP servers.
DHCP snooping defines trusted and untrusted ports to make sure clients obtain IP addresses
only from authorized DHCP servers.
{
Trusted
—A trusted port can forward DHCP messages correctly to make sure the clients get
IP addresses from authorized DHCP servers.
{
Untrusted
—An untrusted port discards received DHCP-ACK and DHCP-OFFER
messages to prevent unauthorized servers from assigning IP addresses.
Configure ports facing the DHCP server as trusted ports, and configure other ports as untrusted
ports.
•
Records DHCP snooping entries.
DHCP snooping reads DHCP-ACK messages received from trusted ports and
DHCP-REQUEST messages to create DHCP snooping entries. A DHCP snooping entry
includes the MAC and IP addresses of a client, the port that connects to the DHCP client, and
the VLAN. ARP detection uses DHCP snooping entries to filter ARP packets from unauthorized
clients.
•
Backs up DHCP snooping entries automatically.
The auto backup function saves DHCP snooping entries to a backup file, and allows the DHCP
snooping device to download the entries from the backup file at device reboot. The entries on
the DHCP snooping device cannot survive a reboot. The auto backup helps some other
features provide services if these features must use DHCP snooping entries for user
authentication.
•
Supports Option 82.
Option 82 records the location information about the DHCP client so the administrator can
locate the DHCP client for security and accounting purposes. Option 82 contains two
sub-options: Circuit ID and Remote ID.
If the DHCP relay agent supports Option 82, it handles DHCP requests by the strategies
described in the following table.
If a response returned by the DHCP server contains Option 82, DHCP snooping removes
Option 82 before forwarding the response to the client. If the response does not contain Option
82, DHCP snooping forwards it immediately.
The following table shows the Option 82 handling strategies for DHCP requests:
If a DHCP request
has…
Handling
strategy
DHCP snooping…
Option 82
Drop
Drops the message.
Keep
Forwards the message without changing Option 82.
Replace
Forwards the message after replacing the original Option 82
with the Option 82 padded according to the configured
padding format, padding content, and code type.