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Chapter 4

Cisco Discovery Protocol (CDP)

CDP is a layer 2 protocol.

CDP is used to:

• obtain information about neighboring devices

• types of devices (switch, type of router)

• router interfaces they are connected to

• interfaces used to make the connection ( E0, S0, etc.)

• model number of the devices

CDP is protocol independent, and runs on Cisco equipment over the Subnetwork Access Protocol (SNAP).

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Cisco Discovery Protocol (CDP)

Primary use of CDP is to discover all Cisco devices that are directly connected to a local device.

This is done by transmitting type length values (TLVs) which are blocks of information embedded in CDP advertisements.

To display the CDP information, use the command:

show cdp neighbors

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Cisco Discovery Protocol (CDP)

Device TLVs displayed by the show cdp neighbors include:

• device ID

• local interface

• holdtime

• capability

• platform

• port ID

• VTP Management Domain Name (CDPv2* only)

• Native VLAN (CDPv2 only)

• Full/Half-Duplex (CDPv2 only)

* Supported by Cisco IOS release 12.0(3)T or later

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Cisco Discovery Protocol (CDP)

To enable CDP globally:

cdp run

CDP is globally enabled by default.

To enable CDP on a particular interface, do:

cdp enable

On Cisco IOS Release 10.2 or higher, CDP is enabled by default

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Cisco Discovery Protocol (CDP)

NOTE:

1. Disable CDP at the global level, use no cdp run

2. If CDP is disabled globally, individual interfaces cannot be enabled for CDP

3. IOS Release 10.3 or higher, CDP is enabled by default. However, on asynchronous interfaces, CDP is disabled by default.

4. To disable CDP on a specific interface after it has been enable, do no cdp enable

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Chapter 4

Telnet

Telnet is a virtual terminal protocol that is part of the TCP/IP protocol suite.

Provides a remote login capability.

Telnet function at the application layer. If a telnet command is successful from one device to another, then all 7 layers of the OSI model is ‘working’.

It is the most complete testing mechanism available, say Cisco.

Telnet uses the five (5) vty sessions. However, there has to be a ‘login’ and password associated with the vty session in order for telnet to work.

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Telnet

If one has a ‘host table’ on the router ( ip host <name> <ip add> ), then you can do telnet <host name>.

As an example:

telnet routerB

DUE THE LABS FOR resuming and suspending SESSION.

NOTE:

1. Enter command, you go to the most recent session

2. To resume a session it needs a connection ID.

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Chapter 4

Ping

Ping command is used to determine if a particular ip address is reachable.

Traceroute command is used to trance the route to an ip address.

Ping sends ICMP echo request messages and receive an echo reply.

Ping Command Code Explanation

! ICMP echo reply received

. Nothing received

U ICMP unreachable (destination) received

N ICMP unreachable (network) received

P ICMP unreachable (port) received

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Interface Protocol

Serial is up, line protocol is up operational

Serial is up, line protocol is down Connection problems (L2)

Serial is down, line protocol is down Interface problems (L1)

Serial is administratively down, line protocol is down disabled

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Interface Protocol

For serial port:

Serial is up, line protocol is down maybe caused by:

• network mismatched

• clock rate not set on DCE

• mismatch of encapsulation (HDLC vs. PPP)