Transcript Chapter 4

Semester 2 Module 4
Learning about Other Devices
Andres, Wen-Yuan Liao
Department of Computer Science and Engineering
De Lin Institute of Technology
[email protected]
http://www.cse.dlit.edu.tw/~andres
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Outline
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Discovering and Connecting to
Neighbors
Getting Information about Remote
Devices
Introduction to CDP
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Cisco Discovery Protocol (CDP) is a Layer 2
protocol that connects lower physical media and
upper network layer protocols.
CDP is used to obtain information about
neighboring 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.
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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).
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When a Cisco device boots up, CDP starts
up automatically and allows the device to
detect neighboring devices that are also
running CDP.
It runs over the data link layer and allows
two systems to learn about each other,
even if they are using different network
layer protocols.
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Each device configured for CDP sends periodic
messages, known as advertisements, to multiple
routers.
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, indicating the length of
time that receiving devices should hold CDP
information before discarding it.
Additionally, each device listens to periodic CDP
messages sent by others in order to learn about
neighboring devices.
Information obtained with CDP
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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.
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CDP provides information about each CDP neighbor
device by transmitting type length values (TLVs), which
are blocks of information embedded in CDP
advertisements.
Device TLVs displayed by the show cdp neighbors
command include the following:
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Device ID
Local Interface
Holdtime
Capability
Platform
Port ID
VTP Management Domain Name (CDPv2 only)
Native VLAN (CDPv2 only)
Full/Half-Duplex (CDPv2 only)
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Implementation, monitoring, and
maintenance of CDP
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The following commands are used to implement,
monitor, and maintain CDP information:
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cdp run
cdp enable
clear cdp counters
show cdp
show cdp entry {*|device-name[*][protocol |
version]}
show cdp interface [type number]
show cdp neighbors [type number] [detail]
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Creating a network map of the
environment
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Though a CDP frame can be small, it can
retrieve a great deal of useful information about
connected neighboring Cisco devices.
This information can be used to create a
network map of the connected devices.
Devices connected to neighboring devices can
be discovered by using Telnet to connect to the
neighbors, and using the show cdp neighbors
command to discover what devices are
connected to those neighbors.
Disabling CDP
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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.
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Troubleshooting CDP
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The following commands can be used to show the
version, update information, tables, and traffic:
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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
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Outline
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Discovering and Connecting to
Neighbors
Getting Information about Remote
Devices
Telnet
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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 functions at the application layer of
the OSI model.
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A router can have multiple simultaneous
incoming Telnet sessions.
The range zero through four is used to
specify five VTY or Telnet lines.
These five incoming Telnet sessions could
take place at one time.
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Establishing and verifying a Telnet
connection
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The Telnet IOS EXEC command allows a user to
Telnet from one Cisco device to another.
With Cisco's implementation of TCP/IP, it is not
necessary to enter the command connect or
telnet 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.
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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
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The Telnet connection will terminate after
ten minutes of inactivity by default or when
the exit command at the EXEC prompt is
entered.
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Disconnecting and suspending
Telnet sessions
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One important feature of the Telnet
command is the suspend feature.
However, one 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.
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The command show sessions will show what
Telnet sessions are taking place.
The procedure for disconnecting a Telnet
session is as follows:
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The procedure for suspending a Telnet session
is as follows:
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Enter the command disconnect
Follow the command with the name or IP address of
the router. Example: Denver>disconnect paris
Press Ctrl-Shift-6, then x
Enter the name of the router or IP address
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Advanced Telnet operation
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A user may switch back and forth between concurrent
Telnet sessions.
The number of open sessions that are allowed at one
time is defined by the session limit command.
Multiple Telnet sessions can be used and suspended by
using the Ctrl-Shift-6, then x sequence.
The session can be resumed by using the Enter key.
If the resume command is used it requires a connection
ID.
The connection ID is shown by using the show
sessions command.
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Alternative connectivity tests
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Echo protocols are used to test whether protocol
packets are being 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 functioning.
This operation can be performed at either the
user or privileged EXEC modes.
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The ping target responded successfully to all
five datagrams sent.
The exclamation points (!) indicate each
successful echo.
If one or more periods (.) are received instead of
exclamations on the display, the application on
the router timed out waiting for a given packet
echo from the ping target.
The command ping uses ICMP (Internet Control
Message Protocol).
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The traceroute command is the ideal tool for
finding where data is being sent in a network.
The traceroute command is similar to the ping
command, except that instead of testing end-toend connectivity, traceroute tests each step
along the way.
This operation can be performed at either the
user or privileged EXEC levels.
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If one of these routers is unreachable,
three asterisks (*) will be returned instead
of the name of the router.
The traceroute command will continue
attempting to reach the next step until the
Ctrl-Shift-6 escape sequence is used.
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A basic verification test also focuses on
the network layer.
Use the show ip route command to
determine whether a routing table entry
exists for the target network.
This command will be discussed in more
detail in a later module of this course.
Troubleshooting IP addressing
issues
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The following three commands are used to
perform address-related troubleshooting:
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ping uses the ICMP protocol to verify the hardware
connection and the IP address of the network layer.
This is a basic testing mechanism.
telnet verifies the application layer software between
source and destination. This is the most complete
test mechanism available.
traceroute allows the location of failures in the path
from the source to the destination. Trace uses Time
to Live values to generate messages from each
router along the path.