Cisco Discovery Protocol (CDP)

In this post, we will examine the Cisco Discovery Protocol in some detail. In the next post, we will tackle LLDP.

CDP is a Layer 2, media-independent, and network-independent protocol that runs on Cisco devices and enables networking applications to learn about directly connected devices nearby. This protocol facilitates the management of Cisco devices by discovering these devices, determining how they are configured, and allowing systems using different network-layer protocols to learn about each other.

NX-OS

While it is a Cisco invention, this protocol is so important to so many companies that many of them have adopted it for those devices as well. If the devices do not actively send the protocol, they can at least understand it when receiving it from true Cisco devices.

One reason you might have heard of companies disabling CDP on devices would be a result of security concerns. It is not necessarily a great idea to have network devices describing their properties and even their capabilities to other devices on the network.

Cisco Discovery Protocol is enabled by default. Each device configured for Cisco Discovery Protocol advertises at least one address at which the device can receive messages and sends periodic advertisements to the well-known multicast address 01:00:0C:CC:CC:CC. Devices discover each other by listening at that address. They also listen to messages to learn when interfaces on other devices are up or go down.

Advertisements contain time-to-live information, which indicates the length of time a receiving device should hold Cisco Discovery Protocol information before discarding it. Advertisements supported and configured in Cisco software are sent, by default, every 60 seconds on interfaces that support Subnetwork Access Protocol (SNAP) headers. Cisco devices never forward Cisco Discovery Protocol packets. Cisco devices that support Cisco Discovery Protocol store the information received in a table. Information in this table is refreshed every time an advertisement is received, and information about a device is discarded after three advertisements from that device are missed.

Cisco Discovery Protocol Version 2 provides more intelligent, device-tracking features than those available in Version 1. One of the features available is an enhanced reporting mechanism for more rapid error tracking, which helps to reduce network downtime. Errors reported include mismatched native VLAN IDs (IEEE 802.1Q) on connected ports and mismatched port-duplex states between connected devices. Messages about reported errors can be sent to the console or to a logging server.

Remember, to disable CDP on the entire device, use no cdp run. To disable CDP on a select interface, use no cdp enable under that interface.

CCNA Data Center – Overlay Transport Virtualization (OTV) Terms

Overlay Transport Virtualization

Remember, we love OTV because it has the ability to connect Data Centers and make it appear as if they are connected Layer 2 domains. While there are other technologies that can do this, OTV is appealing for many reasons including its flexibility and simplicity of configuration and operation.

In order to understand the further study of OTV, you really need to be able to speak its language, and that means learning some terms that are commonly used to describe it. Here they are:

  • OTV Edge Device – this device takes the Layer 2 frames and encapsulates them in Layer 3 packets; in a “classic” implementation, the OTV device is a VDC of a Nexus 7K
  • OTV Internal Interface – a layer 2 interface on an edge device that connects to the VLANs that are to be encapsulated
  • OTV Join Interface – a Layer 3 interface that is used to join the two domains and discover the remote OTV device
  • Transport Network – the network connecting the OTV sites
  • Overlay Network – the logical network that connects the two OTV devices
  • Site VLAN – a VLAN that carries hellos between edge devices that might exist at the same site; it is best to use a dedicated VLAN for this role; this VLAN is not extended across the overlay
  • AED – the Authoritative Edge Device is elected for a site and is the designated forwarding edge device; devices maintain adjacency with each edge device in a site (site adjacency); they use the Site VLAN for this purpose; they also maintain the overlay adjacency using the join interface to a remote site