Content Overview Sometimes network
administrators are faced with situations where network
documentation is incomplete or inaccurate. Cisco Discovery
Protocol (CDP) can be a useful tool in these situations because
it can help to build a basic picture of the 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 nevertheless a powerful
tool. In many cases after a router is initially configured it
is difficult or inconvenient for a network administrator 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. It is an essential tool for the network professional.
Students completing this module should be able to:
- Enable and disable CDP
- Use the show cdp
neighbors command
- Determine which neighboring
devices are connected to which local interfaces
- Gather network address information about neighboring
devices using 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
Content 4.1 Discovering and Connecting to
Neighbors 4.1.1 Introduction to CDP Cisco
Discovery Protocol (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 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 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. 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. Web Links Tech Notes
http://www.cisco.com/en/US/ tech/tk389/tk214/
tech_tech_notes_list.html
Content 4.1
Discovering and Connecting to Neighbors 4.1.2
Information obtained with CDP 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 is running 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. The
administrator uses the show cdp neighbors command to
display information about the networks directly connected to
the router. 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: - Device ID
- Local
Interface
- Holdtime
- Capability
- Platform
- Port ID
- VTP Management Domain
Name (CDPv2 only)
- Native VLAN (CDPv2 only)
- Full/Half-Duplex (CDPv2 only)
Notice that the
lowest router in the figure is not directly connected to the
administrator’s console router. To obtain CDP information about
this device, the administrator would need to Telnet to a router
that is directly connected to this device. Web Links CDP
Commands http://www.cisco.com/en/US/ products/sw/
iosswrel/ps1831/ products_command_ reference_
chapter09186a00800d983f.html
Content 4.1
Discovering and Connecting to Neighbors 4.1.3
Implementation, monitoring, and maintenance of CDP The
following commands are used to implement, monitor, and maintain
CDP information: - 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]
The cdp run command is used to enable CDP
globally on the 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 could be enabled on each of the
devices interfaces by using the cdp enable command.
Lab Activity e-Lab Activity: Implementation, Monitoring,
and Maintenance of CDP In this lab, the students will learn
some commands associated with the Cisco Discovery Protocol
(CDP) and learn to view some CDP information. Web Links
CDP Commands http://www.cisco.com/en/US/ products/sw/
iosswrel/ps1831/ products_command_ reference_
chapter09186a00800d983f.html
Content 4.1
Discovering and Connecting to Neighbors 4.1.4
Creating a network map of the environment CDP was designed
and implemented as a simple, low-overhead protocol. 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. Lab Activity
Lab Exercise: Creating a Network Map using CDP In this lab, the
students will use CDP commands to get information about
neighboring network devices.
Content 4.1
Discovering and Connecting to Neighbors 4.1.5
Disabling 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. Web Links CDP
Commands http://www.cisco.com/en/US/products/sw/
iosswrel/ps1831/products_command_reference_
chapter09186a00800d983f.html#xtocid2
Content
4.1 Discovering and Connecting to
Neighbors 4.1.6 Troubleshooting CDP The
following commands can be used to show the version, update
information, tables, and traffic: - clear cdp
table
- clear cdp counters
- show