Obtaining Information with CDP The primary use of CDP is to discover all Cisco devices that are directly connected to a local device.. Example 13-1 show cdp entry Command Output routerA
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Trang 2Upon completion of this chapter, you will be able to
■ Describe how to implement, monitor, and maintain CDP
■ Understand how to create a network map of the environment
■ Disable and troubleshoot CDP
■ Telnet remotely to other routers
■ Verify, disconnect, and suspend a Telnet connection
■ Perform alternative connectivity tests
Trang 3Chapter 13
Learning About Neighboring and Remote Devices
In this chapter, you learn how to implement, monitor, and maintain Cisco Discovery Protocol by using the correct router commands In addition, this chapter explains the commands most often used to perform address-related troubleshooting
Please be sure to look at the e-Labs, Videos, and PhotoZooms associated with this chapter that you will find on the CD-ROM accompanying this book These CD elements are designed to supplement the material and reinforce the concents introduced in this chapter
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 13-1 CDP is used to obtain
infor-mation about neighboring devices This inforinfor-mation shows 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 is a propri-etary protocol native to Cisco networking devices and will run only on Cisco networking devices
CDP Version 2 (CDPv2) is the most recent release of the protocol Cisco IOS Software Release 12.0(3)T version or later supports CDPv2 CDP Version 1 (CDPv1) is enabled globally by default with Cisco IOS Software Release 10.3 or later
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Figure 13-1 Where CDP Fits In
When a Cisco device boots up, CDP starts up automatically and allows the device to detect neighboring devices that are also running CDP CDP 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 (TTL) or holdtime information This holdtime information indi-cates the length of time that receiving devices hold CDP information before discarding
it Additionally, each device listens to periodic CDP messages sent by others to learn about neighboring devices
Obtaining Information with CDP
The primary use of CDP is to discover all Cisco devices that are directly connected to a local device A CDP frame can be small, yet can retrieve many useful information
about neighboring routers and switches Use the command show cdp entry [device
name], as demonstrated in Example 13-1, to display a single cached CDP entry.
Example 13-1 show cdp entry Command Output
routerA# show cdp entry routerB -Device ID: routerB
Entry address(es):
Upper Layer Entry Addresses
Cisco Proprietary Data Link Protocol
Media Supporting SNAP
TCP/IP NovellIPX Apple Talk Others
CDP Discovers and Shows Information About Directly Connected Cisco Devices
¥ Media and Protocol Interactions
Trang 5Introduction to CDP 623
Notice that the output from this command includes all the Layer 3 addresses present
in the neighbor router, Router B An administrator can view the IP addresses of the
targeted CDP neighbor (Router B) with the single command entry on Router A The
holdtime value indicates the amount of elapsed time since the CDP frame arrived with
this information The show cdp entry [device name] command includes abbreviated
version information about Router B Knowing the version of neighboring devices
provides you with an idea of the exact physical topology of the network to properly
configure the devices
Showing CDP Neighbors
Figure 13-2 displays an example of how CDP delivers its collection of information to a
network administrator Each router that is running CDP exchanges protocol
informa-tion with its neighbors The network administrator displays the results of this CDP
information exchange on a console that is connected to a local router
Figure 13-2 CDP Neighbors
IP address: 198.92.68.18
Platform: 2501 Capabilities: Router
Interface: Ethernet), Port ID (outgoing port): Ethernet0
Holdtime: 155 sec
output omitted
-Example 13-1 show cdp entry Command Output (Continued)
CDP
CDP
Router
IP, CLNS
Router
IP, CLNS, DECnet
Router
IP, AppleTalk
IP, IPX
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You use 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) TLVs are blocks of information embedded in CDP advertisements
Device TLVs displayed by the show cdp neighbors command, and as shown in
Example 13-2, include the following:
■ Device ID
■ Local Interface
■ Capability
■ Platform
■ Port ID
■ Full/Half-Duplex (CDPv2 only)
To display all the information output generated by show cdp neighbors, as well as information like that from show cdp entry, you use the optional show cdp neighbors
detail command, as demonstrated in Example 13-3.
Example 13-2 show cdp neighbors Command Output
routerA# show cdp neighbors Capability Codes:
R – Router, T – Trans Bridge,
B – Source Route Bridge,
S – Switch, H – Host, I – IGMP
Device ID Local Interface Holdtime Capability Platform Port ID routerB Eth 0 151 R 2501 Eth 0 routerB Ser 0 165 R 2501 Ser 0
Example 13-3 show cdp neighbors detail Command Output
routerA# show cdp neighbors detail Device ID: routerB
Entry address(es):
Trang 7Introduction to CDP 625
Notice that the lowest router in Figure 13-2 is not directly connected to the
adminis-trator’s console router To obtain CDP information about this device, the administrator
needs to Telnet to a router that is directly connected to this device As previously
stated, you will find this information helps you gain full knowledge about the devices
that are on the network, which provides you a better understanding of the physical
topology
Implementating, Monitoring, and Maintaining CDP Information
The commands in Table 13-1 are used to implement, monitor, and maintain CDP
information
IP address: 198.92.68.18
Platform: 2501, Capabilities: Router
Interface: Ethernet0, Port ID (outgoing port): Ethernet0
Holdtime: 143 sec
Table 13-1 CDP Commands
cdp run Global configuration
mode
Enables CDP globally on the router
cdp enable Interface configuration
mode
Enables CDP on an interface
clear cdp counters Privileged EXEC mode Resets the traffic counters to
zero
show cdp Privileged EXEC mode Displays the interval between
transmissions of CDP adver-tisements, the number of seconds the CDP advertise-ment is valid for a given port, and the version of the advertisement
show cdp entry
{entry-name [protocol |
version]}
specific neighbor Display can
be limited to protocol or ver-sion information
continues
Example 13-3 show cdp neighbors detail Command Output (Continued)
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Use the cdp run command to enable CDP globally on the router By default, CDP is globally enabled Use the cdp enable command to enable CDP on a particular
inter-face On Cisco IOS Software Release 10.3 and later, CDP could be enabled on each
interface of the device by using the cdp enable command Even though CDP is enabled
by default on most Cisco devices, you might encounter instances in which you need to manually enable CDP on a per interface basis Examples of when this manual enabling
might need to be done occur on 1900 series switches that do not support using the cdp
run command On 1900 series switches, you need to enable or disable CDP on a per
interface basis Other examples include particular interfaces thatmight not have CDP enabled for security purposes
Use the show cdp interface command to gather information that CDP uses for its
advertisement and discovery frame transmission Example 13-4 demonstrates some sample output from this command This information helps provide detailed informa-tion about such things as the holdtime, the frequency at which CDP packets are sent, the encapsulation time for the interface, and the administrative and protocol condi-tions of the interface
show cdp interface
[type number]
interfaces on which CDP is enabled
show cdp neighbors
[type number] [detail]
that has been discovered, the name of the device, the num-ber and type of the local interface (port), the number
of seconds the CDP advertise-ment is valid for the port, the device type, the device prod-uct number, and the port ID
Issuing the detail keyword
displays information on the native VLAN ID, the duplex mode, and the VLAN Trunk-ing Protocol (VTP) domain name associated with neigh-bor devices
Table 13-1 CDP Commands (Continued)
Trang 9Introduction to CDP 627
Creating a Network Map of the Environment
CDP was designed and implemented as a simple, low-overhead protocol Though a
CDP frame can be very small, CDP can retrieve a great deal of useful information
about connected neighboring routers
The show cdp neighbors command can be used to obtain the following information:
■ Device ID
■ Port ID
■ Capabilities
■ Version
■ Platform
■ Full/Half-Duplex (CDPv2 only)
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 Use the show cdp neighbors detail command to discover what devices are
connected to those neighbors
Disabling and Troubleshooting CDP
As previously mentioned, CDP is globally enabled on the router by default To disable
CDP at the global level, use the no cdp run command in global config mode If CDP
is disabled globally, the interface cannot be enabled for CDP For example, you might
disable CDP for security purposes when you do not want information made available
about a particular device
Example 13-4 show cdp interface Command Output
routerA# show cdp interface
Serial 0 is up, line protocol is up, encapsulation is Frame Relay
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
Ethernet0 is up, line protocol is up, encapsulation is ARPA
Sending CDP packets every 60 seconds
Holdtime is 180 seconds
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On Cisco IOS Software Release 10.3 and later, CDP is disabled by default on each of
interface of the device You must use the cdp enable command before CDP is enabled
and CDP messages can be sent and received To disable CDP on a specific interface
after it has been enabled, use the no cdp enable command in interface config mode.
Table 13-2 lists and describes troubleshooting commands that you can use to show the version, update information, tables, and traffic
Table 13-2 CDP Troubleshooting Commands
clear cdp table Deletes the CDP table of information about neighbors
clear cdp counters Resets the traffic counters to zero
show cdp traffic Displays CDP counters, including the number of packets
sent and received and checksum errors
show debugging Displays information about the types of debugging that are
enabled for your router
debug cdp adjacency Displays information about CDP neighbors
debug cdp events Displays information about CDP events
debug cdp ip Displays information about CDP IP information
debug cdp packets Displays information about CDP packet-related information
cdp timers Specifies how often Cisco IOS Software sends CDP updates
cdp holdtime Specifies the holdtime the receiving device should hold a
CDP packet from your router before discarding
show cdp Displays information about CDP update packets
Lab Activity Creating a Network Map Using CDP
In this activity, you use the CDP commands to get information about neighbor-ing networks and routers
Lab Activity CDP Neighbors
In this activity, you use the CDP commands to get information about neighbor-ing networks and routers You display information on how CDP is configured for its advertisement and discovery frame transmission