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Configuring for Network Management Applications
LLDP (Link-Layer Discovery Protocol)
CDP Operation and Commands
By default the switches covered in this guide 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.
Protocol State Packet
Generation
Inbound Data Management
Inbound Packet Forwarding
CDP Enabled
1
n/a
Store inbound CDP data.
No forwarding of inbound
CDP packets.
CDP Disabled
n/a
No storage of CDP data from
neighbor devices.
Floods inbound CDP packets
from connected devices to
outbound ports.
LLDP Enabled
1
Generates and
transmits
LLDP packets
out all ports on
the switch.
Store inbound LLDP data.
No forwarding of inbound
LLDP packets.
LLDP Disabled No packet
generation.
No storage of LLDP data
from neighbor devices.
No forwarding of inbound
LLDP packets.
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.
3500-5400-6200-8200-MCG-Jan08-K_13_01.book Page 81 Monday, January 28, 2008 10:04 AM