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CDP compatibility enables your device to receive and recognize CDP packets from the neighboring
CDP device and send CDP packets to the neighboring device. The CDP packets sent to the
neighboring CDP device carry the following information:
•
Device ID.
•
ID of the port connecting to the neighboring device.
•
Port IP address.
•
TTL.
The port IP address is the primary IP address of a VLAN interface in up state. The VLAN ID of the
VLAN interface must be the lowest among the VLANs permitted on the port. If no VLAN interfaces of
the permitted VLANs are assigned an IP address or all VLAN interfaces are down, no port IP address
will be advertised.
You can view the neighboring CDP device information that can be recognized by the device in the
output of the
display lldp neighbor-information
command. For more information about
the
display lldp neighbor-information
command, see LLDP commands in
Layer 2—LAN
Switching Command Reference
.
To make your device work with Cisco IP phones, you must enable CDP compatibility.
If your LLDP-enabled device cannot recognize CDP packets, it does not respond to the requests of
Cisco IP phones for the voice VLAN ID configured on the device. As a result, a requesting Cisco IP
phone sends voice traffic without any tag to your device. Your device cannot differentiate the voice
traffic from other types of traffic.
CDP compatibility enables your device to receive and recognize CDP packets from a Cisco IP phone
and respond with CDP packets carrying TLVs with the configured voice VLAN. If no voice VLAN is
configured for CDP packets, CDP packets carry the voice VLAN of the port or the voice VLAN
assigned by the RADIUS server. The assigned voice VLAN has a higher priority. According to TLVs
with the voice VLAN configuration, the IP phone automatically configures the voice VLAN. As a result,
the voice traffic is confined in the configured voice VLAN and is differentiated from other types of
traffic.
For more information about voice VLANs, see "Configuring voice VLANs."
When the device is connected to a Cisco IP phone that has a host attached to its data port, the host
must access the network through the Cisco IP phone. If the data port goes down, the IP phone will
send a CDP packet to the device so the device can log out the user.
CDP-compatible LLDP operates in one of the following modes:
•
TxRx
—CDP packets can be transmitted and received.
•
Rx
—CDP packets can be received but cannot be transmitted.
•
Disable
—CDP packets cannot be transmitted or received.
Restrictions and guidelines
When you configure CDP compatibility for LLDP, follow these restrictions and guidelines:
•
To make CDP-compatible LLDP take effect on a port, follow these steps:
a.
Enable CDP-compatible LLDP globally.
b.
Configure CDP-compatible LLDP to operate in TxRx mode on the port.
•
The maximum TTL value that CDP allows is 255 seconds. To make CDP-compatible LLDP
work correctly with Cisco IP phones, configure the LLDP frame transmission interval to be no
more than 1/3 of the TTL value.
Prerequisites
Before you configure CDP compatibility, complete the following tasks:
•
Globally enable LLDP.
•
Enable LLDP on the port connecting to a CDP device.
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