CDP
The Cisco Discovery Protocol (CDP) is a proprietary protocol that all Cisco devices can be configured to use. CDP discovers other Cisco devices that are directly connected, which allows the devices to auto-configure their connection in some cases, simplifying configuration and connectivity. CDP messages are not encrypted.
By default, most Cisco routers and switches have CDP enabled. CDP information is sent in periodic broadcasts that are updated locally in each device's CDP database. Because CDP is a Layer 2 protocol, it is not propagated by routers.
CDP contains information about the device, such as the IP address, software version, platform, capabilities, and the native VLAN.
When this information is available to an attacker, they can use it to find exploits to attack your network, typically in the form of a Denial of Service (DoS) attack.
http://icon.clnchina.com.cn/attachment/2/6/7/17762_RouterVulnerabilities.pdf
