From Mycomputer Notes

Cisco Didcovery Protocol (CDP) is a layer 2 protocol that connects lower physical media and upper network layer protocols. CDP operates at the data link layer and allows two systems, Cisco Devices, to learn about each other, even if they use different network layer protocols.

Each device that is configured for CDP sends periodic messages, which are known as advertisements, to directly connected Cisco devices. Each device advertises at least one address at which it can receive Simple Network Management Protocol (SNMP) messages.

The primary used of CDP is to discover all CISCO devices, neighboring devices, that are direclty connected to a local device. Since each device exchange protocol information with its neighbors an administrator can display the results of this information exchange thorough a console that is directly connected to the router. CDP transmits type length values (TLVs) to provide information about each CDP neighbor device

Device TLVs displayed by the show cdp neighbors command include the following:

• Device ID
• Local Interface
• Holdtime
• Capability
• Platform
• Port ID

The following TLVs are only included in CDPv2:

• VTP management domain name
• Native VLAN
• Full or half-duplex

Implementation, monitoring and maintenance of CDP

This section will introduce the commands to implement, monitored and maintain CDP. The cdp run command is used to enable CDP Globally on a router. By default CDP is enabled. The cdp enable command is used to enable cdp on a particular interface On Cisco IOS Release 10.3 or higher, CDP is enabled by default on all supported interfaces to send and receive CDP information. Below you will have the commands used to implement, monitor, and maintain CDP

Getting Information about Remote Devices

Telnet is a virtual terminal protocol that is part of the TCP/IP protocol suite. It allows an administrator to make connections to remote hosts. Telnet provides a network terminal or remote login capability (SEE MY NOTES FOR MORE DETAILS)

Telnet is an IOS EXEC command used to verify the application layer software between source and destination. This is the most complete test mechanism available.

In the Cisco implementation of TCP/IP, it is not necessary to enter the connect or telnet commands to establish a Telnet connection. The hostname or the IP address of the remote router may be entered. To initiate a Telnet session any of the following alternatives can be used:

Denver>connect paris 
Denver>paris 
Denver>131.108.100.152 
Denver>telnet paris

In you want to be able to used the name of the router instead of the IP address A hostname table or access to DNS for Telnet must be present for a name to work. Otherwise, the IP address of the remote router must be entered.

Telnet could be used to:

• Determined if a remote router can be accessed.
• A successful Telnet connection indicates that the upper-layer application functions properly
• If Telnet to one router is successful, failure to another router is likely caused:

1. addressing
2. naming
3. access permission problems

Disconnecting and Suspending Telnet sessions

To dispaly all the Telenet sesssions used the show session command:

Router#show sessions
Con	Host	Address		Idle	Conn Name
1	Paris	131.108.100.152 0	Paris
2	Tokyo	126.102.57.63	0	Tokyo

• To End a session used command Router# Exit

But instead of end the session, it could be suspended by using the a series of keys on the keybord

• To suspend a session : CTRL+SHIFT+6+x

Remember that by using this command the session is not ended but just suspended, if you want to go back you used ENTER. You do need to go back to either keep working on that telenet sessionor to end the session. Please remember that when you used the keyword ENTER, you are going back, resuming, to the last suspended session.

The ping command has 3 characteristics:

1. Evaluate the path-to-host reliability.
2. Evaluates delays over the path.
3. Evaluates whether the host can be reached or is functional.

Ping and Tracerout are commands that workat the network layer. Telnet is a command that works at the Application Layer.