Module 4
Learning About
Other Devices
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Cisco Discovery Protocol
• A Layer 2 protocol
• Obtains information about neighboring Cisco devices
(interfaces, model numbers)
• CDP will discover directly connected Cisco devices regardless
of which Layer 3 and 4 protocol suite they run
• Runs over the Subnetwork Access Protocol (SNAP)
• CDPv2 is most recent release
• Starts automatically when a device boots up – enabled by
default
• Each Cisco device sends periodic advertisements to multiple
routers (and listens for other messages)
• Enables network administrators to access a summary of what
the configurations look like on other directly connected routers.
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show cdp neighbors command
 Device identifiers - the configured host name of the router
and domain name (if any).
 Address list - At least one protocol for SNMP, up to one
address for each protocol supported.
 Port identifier - Ethernet 0, Ethernet 1, Serial 0, etc.
 Capabilities list - does the device acts as a bridge as well as
a router?
 Version - Information such as that provided by the local
command show version.
 Platform - The device hardware platform: for example, Cisco
7000.
Router#show cdp neighbors (displays CDP updates)
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Type Length Value (TLV)
TLV is simply a piece of information in a CDP advertisement
Device ID
Identifies the device name
Local interface
Contains a list of network addresses of both receiving
and transmitting devices
Port ID
Identifies the port on which the CDP packet is sent
Capabilities
Describes the device’s capability – i.e. – a switch
Version
Software release version on which the device is running
Platform
Hardware platform – i.e. – Cisco 2600
IP Network Prefix
A list of network prefixes to which the sending device
can forward IP packets
Holdtime
The amount of time, in seconds, the device directs the
neighbor to hold a CDP advertisement before discarding
it
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CDP
If an administrator needs CDP information from a Cisco
router that is not directly, then a Telnet session can be
established to connect to another Cisco device that is
connected to the device he needs the information on.
Needs CDP info on this
router, he could telnet to
router above to run show
CDP neighbors command
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CDP Commands
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Enabling CDP on a Router
To enable CDP, use the cdp run global configuration
command. CDP is on by default. Use the no form of this
command to disable CDP.
Router(config)#cdp run
To enable Cisco Discovery Protocol (CDP) on an interface,
use the cdp enable interface configuration command. Use
the no form of this command to disable CDP on an interface.
CDP is enabled by default on all supported interfaces.
Router(config)#interface serial 0/0
Router(config-if)#cdp enable
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clear cdp counters
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show cdp
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show cdp entry
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show cdp interface
Status of the
carrier detect
signal
Keepalive
messages
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Know this
output!!!!
show cdp neighbors
Router#show cdp neighbors detail will
also give the IP address of the neighboring Cisco
device
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Other cdp commands
show cdp traffic – shows number of packets sent and
received
cdp timer - specifies how often the IOS sends CDP
updates
debug cdp - for debugging all aspects of cdp
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Telnet
•Part of TCP/IP
suite
•Routers have 5
virtual terminal
sessions
•Main use is
remote
connection to
network
devices
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Enabling CDP on a Router
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 131.108.100.152
Denver>telnet paris
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Reopening a suspended Telnet session
• Multiple Telnet sessions can be used and suspended by using
the Ctrl-Shift-6, then x sequence.
• Then the session can be resumed by using the Enter key.
• If the Enter key is used to resume the session, then Cisco IOS
resumes the most recently suspended session.
• You can also use the resume command to start up a previous
session, but to do that you must know the connection ID.
• To get the connection ID, use the show sessions command
first, so you can resume the correct session.
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Reopening a suspended Telnet session
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The ping Command
• Ping is a basic Internet program that lets you verify that a
particular Internet address exists and can accept requests
• If, for example, a user cannot ping a host, then the user will be
unable to use the File Transfer Protocol (FTP) to send files to that
host
• Ping can also be used with a host that is operating to see how
long it takes to get a response back
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What Ping can tell you
 Ping places a unique sequence number on each packet it
transmits, and reports which sequence numbers it receives
back. Thus, it can be determine if packets have been dropped,
duplicated, or reordered.
 Ping places a timestamp in each packet, which is echoed back
and can easily be used to compute how long each packet
exchange took - the Round Trip Time (RTT).
 Ping reports other ICMP messages that might otherwise get
buried in the system software. It reports, for example, if a router
is declaring the target host unreachable.
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What Ping can not tell you
 Some routers may silently discard undeliverable packets.
Others may believe a packet has been transmitted successfully
when it has not been. (This is especially common over
Ethernet, which does not provide link-layer acknowledgments)
Therefore, ping may not always provide reasons why packets
go unanswered.
 Ping can not tell you why a packet was damaged, delayed, or
duplicated. It can not tell you where this happened either,
although you may be able to deduce it.
 Ping can not give you a blow-by-blow description of every host
that handled the packet and everything that happened at every
step of the way. It is an unfortunate fact that no software can
reliably provide this information for a TCP/IP network.
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traceroute
• traceroute is a utility that records the route through the
Internet between your computer and a specified
destination computer
• It also calculates and displays the amount of time each hop
took
• traceroute tests each step (router) along the way
• If one router in the path is unreachable, three asterisks (***)
will be returned instead of the name of the router
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Module 4
Learning About
Other Devices
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