LLDP and CDP
LLDP (Link Layer Discovery Protocol) and CDP (Cisco Discovery Protocol) are link layer protocols for directly connected LLDP and CDP capable neighbors to advertise themselves and their capabilities to each other. By default, the switch sends an LLDP/CDP advertisement periodically to all its interfaces and terminates and processes incoming LLDP and CDP packets as required by the protocols. In LLDP and CDP, advertisements are encoded as TLV (Type, Length, Value) in the packet.
The following are additional points about CDP/LLDP configuration:
- CDP/LLDP can be globally enabled or disabled and enabled/disabled per port. The CDP/LLDP capability of a port is relevant only if CDP/LLDP is globally enabled.
- If CDP/LLDP is globally enabled, the switch filters out incoming CDP/LLDP packets from ports that are CDP/LLDP-disabled.
- If CDP/LLDP is globally disabled, the switch can be configured to discard, VLAN-aware flooding, or VLAN-unaware flooding of all incoming CDP/LLDP packets. VLAN-aware flooding floods an incoming CDP/LLDP packet to the VLAN where the packet is received excluding the ingress port. VLAN- unaware flooding floods an incoming CDP/LLDP packet to all the ports excluding the ingress port. The default is to discard CDP/LLDP packets when CDP/LLDP is globally disabled. You can configure the discard/ flooding of incoming CDP and LLDP packets from the CDP Properties page and the LLDP Properties page respectively.
- Auto Smartport requires CDP and/or LLDP to be enabled. Auto Smartport automatically configures an interface based on the CDP/LLDP advertisement received from the interface.
- CDP and LLDP end devices, such as IP phones, learn the voice VLAN configuration from CDP and LLDP advertisements. By default, the switch is enabled to send out CDP and LLDP advertisement based on the voice VLAN configured at the switch. Refer to the Voice VLAN and Auto Voice VLAN sections for details.
NOTE CDP/LLDP does not distinguish if a port is in a LAG. If there are multiple ports in a LAG, CDP/LLDP transmit packets on each port without taking into account the fact that the ports are in a LAG.
The operation of CDP/LLDP is independent of the STP status of an interface.
If 802.1x port access control is enabled at an interface, the switch will transmit and receive CDP/LLDP packets to and from the interface only if the interface is authenticated and authorized.
If a port is the target of mirroring, then for CDP/LLDP it is considered down.
NOTE CDP and LLDP are link layer protocols for directly connected CDP/LLDP capable devices to advertise themselves and their capabilities. In deployments where the CDP/LLDP capable devices are not directly connected and are separated with CDP/LLDP incapable devices, the CDP/LLDP capable devices may be able to receive the advertisement from other device(s) only if the CDP/LLDP incapable devices flood the CDP/LLDP packets they receives. If the CDP/LLDP incapable devices perform VLAN-aware flooding, then CDP/LLDP capable devices can hear each other only if they are in the same VLAN. It should be noted that a CDP/LLDP capable device may receive advertisement from more than one device if the CDP/ LLDP incapable devices flood the CDP/LLDP packets.