This information may include the operating system version, hostname, every address (i.e. The information contained in CDP announcements varies by the type of device and the version of the operating system running on it. The holdtime specifies the lifetime of an entry in the table - if no announcements are received from a device for a period in excess of the holdtime, the device information is discarded (default 180 seconds).
The CDP table information is refreshed each time an announcement is received, and the holdtime for that entry is reinitialized.
This table is also accessible via Simple Network Management Protocol (SNMP). Each Cisco device that supports CDP stores the information received from other devices in a table that can be viewed using the show cdp neighbors command. By default, CDP announcements are sent every 60 seconds on interfaces that support Subnetwork Access Protocol (SNAP) headers, including Ethernet, Frame Relay and Asynchronous Transfer Mode (ATM). This multicast destination is also used in other Cisco protocols such as Virtual Local Area Network (VLAN) Trunking Protocol (VTP). These multicast frames may be received by Cisco switches and other networking devices that support CDP into their connected network interface. Cisco devices send CDP announcements to the destination MAC address 01:00:0c:cc:cc:cc, out each connected network interface.
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