Management and Configuration Guide K/KA/KB.15.15

devices running either CDP or LLDP can retrieve neighbor information from the switch regardless
of whether LLDP or CDP is used to collect the device-specific information.
Inbound packet forwardingInbound data
management
Packet generationProtocol state
No forwarding of inbound CDP
packets.
Store inbound CDP
data.
N/ACDP Enabled
1
Floods inbound CDP packets
from connected devices to
outbound ports.
No storage of CDP
data from neighbor
devices.
N/ACDP Disabled
No forwarding of inbound
LLDP packets.
Store inbound LLDP
data.
Generates and
transmits LLDP
LLDP Enabled1
packets out all ports
on the switch.
No forwarding of inbound
LLDP packets.
No storage of LLDP
data from neighbor
devices.
No packet
generation.
LLDP Disabled
1
Both CDP data collection and LLDP transmit/receive are enabled in the default configuration. If a switch receives CDP
packets and LLDP packets from the same neighbor device on the same port, it stores and displays the two types of
information separately if the chassis and port ID information in the two types of advertisements is different. In this case,
if you want to use only one type of data from a neighbor sending both types, disable the unwanted protocol on either
the neighbor device or on the switch. However, if the chassis and port ID information in the two types of advertisements
is the same, the LLDP information overwrites the CDP data for the same neighbor device on the same port.
CDP operation and commands
By default the switches have CDP enabled on each port. This is a read-only capability, meaning
that the switch can receive and store information about adjacent CDP devices but does not generate
CDP packets.
When a CDP-enabled switch receives a CDP packet from another CDP device, it enters that device's
data in the CDP Neighbors table, along with the port number where the data was received—and
does not forward the packet. The switch also periodically purges the table of any entries that have
expired. (The hold time for any data entry in the switch's CDP Neighbors table is configured in
the device transmitting the CDP packet and cannot be controlled in the switch receiving the packet.)
A switch reviews the list of CDP neighbor entries every three seconds and purges any expired
entries.
NOTE: For details on how to use an SNMP utility to retrieve information from the switch's CDP
Neighbors table maintained in the switch's MIB, see the documentation provided with the particular
SNMP utility.
266 Configuring for Network Management Applications