Module Overview
15.1 Discovering
and Connecting to Neighbors
15.1.1
Introduction to CDP
15.1.2 Information
obtained with CDP
15.1.3
Implementation, monitoring, and maintenance of CDP
15.1.4 Creating a
network map of the environment
15.1.5 Disabling
CDP
15.1.6
Troubleshooting CDP
15.2 Getting
Information about Remote Devices
15.2.1 Telnet
15.2.2
Establishing and verifying a Telnet connection
15.2.3
Disconnecting and suspending Telnet sessions
15.2.4 Advanced
Telnet operation
15.2.5 Alternative
connectivity tests
15.2.6
Troubleshooting IP addressing issues
Module: Summary
Overview
Sometimes network
documentation is incomplete or inaccurate. Cisco Discovery Protocol (CDP) is a
useful tool in these situations because it can build a basic picture of a
network. CDP is a media and protocol independent, Cisco proprietary protocol
used for neighbor discovery. CDP will only show information about directly
connected neighbors but it is still a powerful tool.
After a router is
initially configured it is often difficult to connect directly to the router
for configuration changes or other activities. Telnet is a TCP/IP-based
application that allows remote connection to the router command-line interface
(CLI) for configuration, monitoring, and troubleshooting purposes. Telnet is an
essential tool for network professionals.
This module
covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND
640-811 exams. -
Students who complete
this module should be able to perform the following tasks:
- Enable and disable CDP
- Use the show cdp neighbors
command
- Determine which neighbor
devices are connected to each local interface
- Gather network address
information about neighbor devices that use CDP
- Establish a Telnet connection
- Verify a Telnet connection
- Disconnect from a Telnet
session
- Suspend a Telnet session
- Perform alternative
connectivity tests
- Troubleshoot remote terminal
connections
15.1
Discovering and Connecting to Neighbors
15.1.1
Introduction to CDP
This page will
introduce Cisco Discovery Protocol (CDP). CDP is a Layer 2 protocol that
connects lower physical media and upper network layer protocols, as shown in
Figure . CDP is used to obtain information about neighboring Cisco devices,
such as the types of devices connected, the router interfaces they are
connected to, the interfaces used to make the connections, and the model
numbers of the devices. CDP is media and protocol independent, and runs on all
Cisco equipment over the Subnetwork Access Protocol (SNAP).
CDP Version 2
(CDPv2) is the most recent release of the protocol. Cisco IOS Release 12.0(3)T
or later supports CDPv2. CDP Version 1 (CDPv1) is enabled by default with Cisco
IOS Release 10.3 to 12.0(3)T.
When a Cisco
device boots up, CDP starts up automatically and allows the device to detect
neighbor devices that use CDP. CDP operates at the data link layer and allows
two systems 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 advertisements also contain time-to-live or holdtime
information, which indicates the length of time that receiving devices should
hold CDP information before they discard it. Each device also listens to
periodic CDP messages that are sent by others to learn about neighbor devices.
The next page
will explain how CDP collects and delivers information.
15.1
Discovering and Connecting to Neighbors
15.1.2
Information obtained with CDP
This page will
explain how CDP is used to obtain information about network devices.
The primary use
of CDP is to discover all Cisco devices that are directly connected to a local
device. Use the show cdp neighbors command to display CDP updates on the local
device.
Figure displays an example of how CDP delivers its
collection of information to a network administrator. Each router that uses CDP
exchanges protocol information with its neighbors. The network administrator
can display the results of this CDP information exchange on a console that is connected
to a local router.
An administrator
can use the show cdp neighbors command to display information about the
networks that are directly connected to a router. CDP transmits type length
values (TLVs) to provide information about each CDP neighbor device. TLVs are
blocks of information embedded in CDP advertisements.
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
Notice that the
router at the bottom of Figure is not
directly connected to the console router that is used by the administrator. To
obtain CDP information about this device, the administrator would need to
Telnet to a router that is directly connected to this device.
The next page
will introduce the commands that are used to monitor CDP information.
15.1
Discovering and Connecting to Neighbors
15.1.3
Implementation, monitoring, and maintenance of CDP
This page will
introduce the commands that are used to implement, monitor, and maintain CDP
information:
- cdp run
- cdp enable
- show cdp traffic
- clear cdp counters
- show cdp
- show cdp entry {*|device-name
[*][protocol | version]}
- show cdp interface [type
number]
- show cdp neighbors [type
number] [detail]
The cdp run
command is used to enable CDP globally on a router. By default, CDP is globally
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. CDP can be
enabled on all device interfaces with the cdp enable command.
The Lab Activity
on this page will teach students about some basic CDP commands.
The next page
will explain how CDP can be used to create a network map.
15.1
Discovering and Connecting to Neighbors
15.1.4
Creating a network map of the environment
This page will
explain how the information that is obtained by CDP can be used to create a
network map.
CDP was designed
and implemented as a simple, low-overhead protocol. Though a CDP frame can be
small, it can retrieve a lot of useful information about directly connected
Cisco devices.
This information
can be used to create a network map of the connected devices. To discover
devices that are connected to neighbor devices, use Telnet to connect to the
neighbors. Then use the show cdp neighbors command.
The Lab Activity
will show students how to use CDP commands to learn about neighboring network
devices.
The next page
will describe how CDP can be disabled.
15.1
Discovering and Connecting to Neighbors
15.1.5
Disabling CDP
This page will
show students how to disable CDP.
To disable CDP at
the global level, use the no cdp run command in global configuration mode. If CDP is disabled globally, individual
interfaces cannot be enabled for CDP.
On Cisco IOS
Release 10.3 or higher, CDP is enabled by default on all supported interfaces
to send and receive CDP information. However, on some interfaces, such as
asynchronous interfaces, CDP is disabled by default. If CDP is disabled use the
cdp enable command in interface configuration mode. To disable CDP on a
specific interface after it has been enabled, use the no cdp enable command in
interface configuration mode.
The next page
will introduce some commands that can be used to troubleshoot CDP.
15.1
Discovering and Connecting to Neighbors
15.1.6
Troubleshooting CDP
This page will
introduce some commands that can be used to show the version and update CDP
information, tables, and traffic:
- clear cdp table
- clear cdp counters
- show cdp traffic
- show debugging
- debug cdp adjacency
- debug cdp events
- debug cdp ip
- debug cdp packets
- cdp timer
- cdp holdtime
- show cdp
The Lab
Activities on this page will require students to use CDP commands to learn
about other devices.
This page
concludes this lesson. The next lesson will explain how to test and obtain
information from remote devices.
15.2
Getting Information about Remote Devices
15.2.1
Telnet
This page will
introduce Telnet.
Telnet is a
virtual terminal protocol that is part of the TCP/IP protocol suite. It allows
connections to be made to remote hosts. Telnet provides a network terminal or
remote login capability. 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.
Telnet functions
at the application layer of the OSI model.
Telnet depends on TCP to guarantee the correct and orderly delivery of
data between the client and server.
A router can have
simultaneous incoming Telnet sessions. The numbers zero through four are used
to specify five vty or Telnet lines.
The verification
of application layer connectivity is a by-product of Telnet. Telnet is mainly
used to establish remote connections to network devices. Telnet is a simple and
universal application program.
The next page
will explain how a Telnet connection is initiated and verified.
15.2
Getting Information about Remote Devices
15.2.2
Establishing and verifying a Telnet connection
This page will
show students how to establish and test a Telnet connection.
The Telnet IOS
EXEC command allows a user to Telnet from one Cisco device to another. 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 end a Telnet session, use the
EXEC commands exit or logout.
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
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 can be
used to determine if a remote router can be accessed. As shown in Figure , if
Telnet is used successfully to connect the York router to the Paris router,
then a basic test of the network connection is successful. This operation can
be performed at either the user or privileged EXEC levels.
If remote access
can be obtained through another router, then at least one TCP/IP application
can reach the remote router. 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 by addressing,
naming, or access permission problems. The problem may exist on the original
router or on the router that failed as a Telnet target. The next step is to use
the ping command, which is covered later in this lesson. The ping command can
be used to test end-to-end connections at the network layer.
Once the Telnet
is completed, log off the host. The Telnet connection will terminate after ten
minutes of inactivity by default or when the exit command is entered at the
EXEC prompt.
Students can use
the Lab Activity on this page to establish and verify a Telnet connection.
The next page
will explain how to disconnect or suspend a Telnet connection.
15.2
Getting Information about Remote Devices
15.2.3
Disconnecting and suspending Telnet sessions
This page will
introduce two important features of the telnet command. These are the
disconnect feature and the suspend feature.
A potential
problem exists when a Telnet session is suspended and the Enter key is pressed.
Cisco IOS software resumes the connection to the most recently suspended Telnet
connection. The Enter key is used frequently. With a suspended Telnet session,
it is possible to reconnect to another router. This is dangerous when changes
are made to the configuration or EXEC commands are used. Always check which router
is connected when the suspended Telnet feature is used.
The show sessions
command will show which Telnet sessions are active.
The procedure
that is used to disconnect a Telnet session is as follows:
- Enter the disconnect command.
- Follow the command with the
name or IP address of the router or the session number. An example is as
follows:
Denver>disconnect
paris
The procedure
that is used to suspend a Telnet session is as follows:
- Press Ctrl-Shift-6, then x.
- Enter the name of the router or
IP address.
The Lab Activity
will instruct students on how to suspend a Telnet session.
The next page
will introduce some advanced features that can be used for multiple Telnet
sessions.
15.2
Getting Information about Remote Devices
15.2.4
Advanced Telnet operation
This page will
describe some features that can be used when several Telnet sessions are open
at the same time. A user may switch back and forth between these sessions. The
number of open sessions that are allowed at one time is defined by the session
limit command.
Use the commands
shown in Figure to escape from one
session and resume a previously opened session.
A new connection
can be made from the EXEC prompt.
Multiple Telnet
sessions can be used and suspended with the Ctrl-Shift-6, then x sequence. 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. If the resume command is used it requires a connection ID. Use the
show sessions command to view the connection ID, as shown in Figure .
The Lab Activity
on this page will show students how to use Telnet to remotely access routers.
The next page
will describe some tests that can be used to verify connectivity.
15.2
Getting Information about Remote Devices
15.2.5
Alternative connectivity tests
This page will
introduce some tools that are used to verify basic network connectivity.
Many network
protocols support an echo protocol. Echo protocols are used to test if protocol
packets are routed. The ping command sends a packet to the destination host and
then waits for a reply packet from that host. Results from this echo protocol
can help evaluate the path-to-host reliability, delays over the path, and
whether the host can be reached or is functional. This is a basic test
mechanism. This operation can be performed at either the user or privileged
EXEC modes.
In Figure , the
ping target 172.16.1.5 responded to all five datagrams that were sent. Each
exclamation point (!) indicates a successful echo. Each period (.) on the
display indicates that the application on the router timed out while it waited
for a packet echo from a target. The ping user EXEC command can be used to
diagnose basic network connectivity. The ping command uses Internet Control
Message Protocol (ICMP).
The traceroute
command, which is often referred to as the trace command in reference
materials, can be used to find where data is sent in a network. The traceroute
command is similar to the ping command. The main difference is that ping tests
end-to-end connectivity and traceroute tests each step along the way. This
operation can be performed at either the user or privileged EXEC levels.
In Figure , the
path from York to Rome is traced. Along the way the path must go through London
and Paris. If one of these routers is unreachable, three asterisks (*) will be
returned instead of the name of the router. The traceroute command will attempt
to reach the next step until the Ctrl-Shift-6 escape sequence is used.
A basic
verification test also focuses on the network layer. Use the show ip route
command to see if a routing table entry exists for the target network. This
command will be discussed in more detail in a later module of this course.
The procedure to
use the ping command is as follows:
- Enter the ping [IP address or
name of destination ] command.
- Press the Enter key.
The procedure to
use the traceroute command is as follows:
- Enter the traceroute [IP
address or name of destination ] command.
- Press the Enter key.
The Lab
Activities on this page will allow students to practice three network
connectivity tests.
The next page
will introduce some tools that are used to troubleshoot IP address-related
problems.
15.2
Getting Information about Remote Devices
15.2.6
Troubleshooting IP addressing issues
IP
address-related problems are the most common problems that occur on IP
networks. This page will describe three commands that are used to perform
address-related troubleshooting:
- ping uses
the ICMP protocol to verify the hardware connection and the IP address of
the network layer. This is a basic test mechanism.
- telnet
verifies the application layer software between a source and a
destination. This is the most complete test mechanism available.
- traceroute
locates failures in a path from a source to a destination. This command
uses Time to Live values to generate messages from each router along a
path.
The Lab Activity
will require students to configure devices in a WAN and then troubleshoot IP
address issues.
This page
concludes this lesson. The next page will summarize the main points from this
module.
Summary
This page
summarizes the topics discussed in this module.
CDP is used to
obtain information about directly connected Cisco devices. This includes the
router interfaces the devices are connected to, the interfaces used to make the
connections, and the model numbers of the devices. CDP is media and protocol
independent, and runs on all Cisco equipment over SNAP. It is a Layer 2
protocol that connects lower physical media and upper network layer protocols.
When a Cisco
device boots up, CDP starts up automatically and allows the device to detect
directly connected Cisco devices that also use CDP. It operates at the data
link layer and allows two systems to learn about each other, even if they use
different network layer protocols. The show cdp neighbors command is used to
display information about the networks that are directly connected to a router.
The cdp run
command is used to enable CDP globally on a router. The cdp enable command is
used to enable CDP on a particular interface. To disable CDP at the global
level, use the no cdp run command in global configuration mode.
The telnet command may be run from the user or privileged
EXEC mode. It allows a user to remotely access another device. It is not
necessary to enter the command connect or telnet to establish a Telnet
connection. To end a Telnet session, use the exit or logout commands. Once the
Telnet session is completed, log off the host. The Telnet connection will
terminate after ten minutes of inactivity by default or when the exit command
is entered at the user or privileged EXEC prompt.
Other
connectivity tests include ping and traceroute. The ping command sends a packet
to the destination host and then waits for a reply packet from that host.
Results from this echo protocol can help determine the path-to-host
reliability, delays over the path, and if the host can be reached or is
functional. The traceroute command is similar to the ping command, except that
instead of testing end-to-end connectivity, traceroute tests each step along
the way. This operation can be performed at either the user or privileged EXEC
levels.
No comments:
Post a Comment