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Posts Tagged 'CDP'

Understanding Cisco Discovery Protocol: Part 2

Published May 14, 2009 Jason Wyatte , Routing & Switching Leave a Comment
Tags: CDP, VLAN

In Understanding Cisco Discovery Protocol Part 1, we discussed how CDP was a useful tool for finding directly connected devices. We saw how to enable CDP per device or for the entire interface. We learned how to modify the update interval for how often CDP is sent and when the neighbor information may flush out expired cached information. However there are other things to just be aware of when CDP is in use.

First, Cisco Discovery Protocol has a given vulnerability. CDP is sent out periodically as a layer 2 multicast. As long as a protocol analyzer such as Wireshark is running, anyone would inertly see these frames. The example below, displays an interface with these default timers. We can also see with this command again that it is running on that interface, gigabitethernet 4/1. If this was a public facing interface then every minute the service provider will see some this information.
example 1

Continue reading ‘Understanding Cisco Discovery Protocol: Part 2′

Understanding Cisco Discovery Protocol: Part 1

Published May 8, 2009 Jason Wyatte , Routing & Switching Leave a Comment
Tags: CDP

In our last blog discussion, I’ve been building on an important point involving Mac Address Tables, Arp Caches and Routing tables. This is to show traffic flow and forwarding decisions that intermediate network will have to make. However, to it may also be necessary to discover what devices can be found in a given transit path. In the best scenario, Network Engineers would have made a network diagram and would have also maintained it when new devices are added.

Also networking tools such as Cisco Works or HP Openview can use Simple Network Management Protocol (SNMP) to find and discover devices and their current layout. Cisco Discover Protocol (CDP) is another protocol that is useful for making a layout of the network devices and it is an essential tool for a CCNA technician.

To begin, CDP is a Layer 2 Multicast protocol that is used primarily by Cisco devices. Routers, switches, Cisco Unified Call Managers and IP phones are examples of devices that uses CDP to learn which devices are directly connect to it. These packets are transmitted in the multicast group of 0100.CCCC.CCCC. The example below displays the Multicast Group’s address.

example 1

If a device such as a switch that isn’t a Cisco switch receives this, then it will flood the frame out of all its interfaces. Otherwise it will be terminated on that switch. There are several commands show commands for CDP and a few commands that can be used for configuration. This next example shows the basic options that can be used to learn about CDP information (we will cover all these option in this discussion).

Continue reading ‘Understanding Cisco Discovery Protocol: Part 1′