1Version 3.1

Module 4Learning About Other Devices

2Version 3.1

Cisco Discovery Protocol

• A Layer 2 protocol

• Obtains information about neighboring Cisco devices (interfaces, model numbers)

• CDP will discover directly connected Cisco devices regardless of which Layer 3 and 4 protocol suite they run

• Runs over the Subnetwork Access Protocol (SNAP)

• CDPv2 is most recent release

• Starts automatically when a device boots up – enabled by default

• Each Cisco device sends periodic advertisements to multiple routers (and listens for other messages)

• Enables network administrators to access a summary of what the configurations look like on other directly connected routers.

3Version 3.1

show cdp neighbors command· Device identifiers - the configured host name of the router

and domain name (if any).

· Address list - At least one protocol for SNMP, up to one address for each protocol supported.

· Port identifier - Ethernet 0, Ethernet 1, Serial 0, etc.

· Capabilities list - does the device acts as a bridge as well as a router?

· Version - Information such as that provided by the local command show version.

· Platform - The device hardware platform: for example, Cisco 7000.

Router#show cdp neighbors (displays CDP updates)

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Type Length Value (TLV)TLV is simply a piece of information in a CDP advertisement

Device ID Identifies the device name

Local interface Contains a list of network addresses of both receiving and transmitting devices

Port ID Identifies the port on which the CDP packet is sent

Capabilities Describes the device’s capability – i.e. – a switch

Version Software release version on which the device is running

Platform Hardware platform – i.e. – Cisco 2600

IP Network Prefix A list of network prefixes to which the sending device can forward IP packets

Holdtime The amount of time, in seconds, the device directs the neighbor to hold a CDP advertisement before discarding it

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CDP

If an administrator needs CDP information from a Cisco router that is not directly, then a Telnet session can be established to connect to another Cisco device that is connected to the device he needs the information on.

Needs CDP info on this router, he could telnet to router above to run show CDP neighbors command

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CDP Commands

7Version 3.1

Enabling CDP on a Router

To enable CDP, use the cdp run global configuration command. CDP is on by default. Use the no form of this command to disable CDP.

Router(config)#cdp run

To enable Cisco Discovery Protocol (CDP) on an interface, use the cdp enable interface configuration command. Use the no form of this command to disable CDP on an interface. CDP is enabled by default on all supported interfaces.

Router(config)#interface serial 0/0 Router(config-if)#cdp enable

8Version 3.1

clear cdp counters

9Version 3.1

show cdp

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show cdp entry

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show cdp interface

Status of the carrier detect signal

Keepalive messages

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show cdp neighbors

Router#show cdp neighbors detail will also give the IP address of the neighboring Cisco device

Know this output!!!!

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Other cdp commands

show cdp traffic – shows number of packets sent and received

cdp timer - specifies how often the IOS sends CDP updates

debug cdp - for debugging all aspects of cdp

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Telnet

•Part of TCP/IP suite

•Routers have 5 virtual terminal sessions

•Main use is remote connection to network devices

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Enabling CDP on a Router

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 131.108.100.152

Denver>telnet paris

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Reopening a suspended Telnet session

• Multiple Telnet sessions can be used and suspended by using

the Ctrl-Shift-6, then x sequence.

• Then the session can be resumed by using the Enter key.

• If the Enter key is used to resume the session, then Cisco IOS

resumes the most recently suspended session.

• You can also use the resume command to start up a previous

session, but to do that you must know the connection ID.

• To get the connection ID, use the show sessions command

first, so you can resume the correct session.

17Version 3.1

Reopening a suspended Telnet session

18Version 3.1

The ping Command

• Ping is a basic Internet program that lets you verify that a particular Internet address exists and can accept requests

• If, for example, a user cannot ping a host, then the user will be unable to use the File Transfer Protocol (FTP) to send files to that host

• Ping can also be used with a host that is operating to see how long it takes to get a response back

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What Ping can tell you

· Ping places a unique sequence number on each packet it

transmits, and reports which sequence numbers it receives

back. Thus, it can be determine if packets have been dropped,

duplicated, or reordered.

· Ping places a timestamp in each packet, which is echoed back

and can easily be used to compute how long each packet

exchange took - the Round Trip Time (RTT).

· Ping reports other ICMP messages that might otherwise get

buried in the system software. It reports, for example, if a router

is declaring the target host unreachable.

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What Ping can not tell you

· Some routers may silently discard undeliverable packets. Others may believe a packet has been transmitted successfully when it has not been. (This is especially common over Ethernet, which does not provide link-layer acknowledgments) Therefore, ping may not always provide reasons why packets go unanswered.

· Ping can not tell you why a packet was damaged, delayed, or duplicated. It can not tell you where this happened either, although you may be able to deduce it.

· Ping can not give you a blow-by-blow description of every host that handled the packet and everything that happened at every step of the way. It is an unfortunate fact that no software can reliably provide this information for a TCP/IP network.

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traceroute

• traceroute is a utility that records the route through the Internet between your computer and a specified destination computer

• It also calculates and displays the amount of time each hop took

• traceroute tests each step (router) along the way

• If one router in the path is unreachable, three asterisks (***) will be returned instead of the name of the router

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Module 4Learning About Other Devices

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