Learning about other devices
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
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[edit] 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
[edit] 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:
- addressing
- naming
- access permission problems
[edit] Disconnecting and Suspending Telnet sessions
A router can have simultaneous incoming Telnet sessions. The numbers zero through four are used to specify five vty or Telnet lines. Having several telnet sessions open at the same time could be a problem if not properly managed. The telnet command have two features, arguments, that help an administrator managed serveral telnet session and tehy are; a) disconnect, b)suspend.
A Telnet connection by default will terminate after ten minutes of inactivity or when the exit command is entered at the EXEC prompt. (ie Router# Exit) What are the choices an administrator besides waiting 10 minutes or typing exit?
[edit] Disconnecting
The procedure that is used to disconnect a Telnet session is as follows: Router# disconnect <router_name> | <router_ip> | <router_session_number>
Denver>disconnect paris
[edit] Suspending
Another option available instead of disconnecting is to suspend the session. Remember that by using this command the session is not ended but just suspended; to suspend a telnet session used: a) CTRL+SHIFT+6+x b)<router_name>|<router_ip>
You want to go back to that session, no problem the session can be resumed with the Enter key. When the Enter key is used, the Cisco IOS will resume the connection to the most recently suspended Telnet connection.
You want to go back but not to the most recently suspended session used the resume command with a connection ID.
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
The ping command has 3 characteristics:
- Evaluate the path-to-host reliability.
- Evaluates delays over the path.
- 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.
