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Posted by HamidAliKhan

ICMP is a layer 3 (network) protocol designed to carry status messages. ICMP explains via two programs, Ping and Traceroute. The two messages used by the Ping program, Echo Request and Echo Reply, test both connectivity and integrity; the responding station's job is to reply and repeat the payload, thus testing the quality of the connection. The ping command followed by an IP address or name uses a default payload and primarily tests connectivity.

If a host is unreachable, you will get an ICMP Type 3 Destination Unreachable message. If a firewall or access-list is blocking ICMP, you will get an ICMP Type 3/Code 13 Destination Unreachable:Administratively Prohibited Message. Unreachables will show a "U" in the output on your screen, whereas a successful ping will show exclamation points (!). Timeouts will show a "." (period) in the output. The extended ping has options to test integrity, such as the capability to change the size and content of the payload to be echoed back. Cisco also supports an extended ping feature that is accessible from privileged EXEC. To access the extended ping feature, enter privileged EXEC and type ping. Do not enter an IP address, however; instead, press Enter, and you will be presented with a number of questions. With extended ping, you have the ability to set the size of your ping messages, source interface, number of pings, and timeout settings. Following is the output of the extended ping command. Note that the exclamation mark is an indication of a successful ping:

Router#ping
Protocol [ip]:
Target IP address: 10.0.0.1
Repeat count [5]: 1000
Datagram size [100]: 1024
Timeout in seconds [2]:
Extended commands [n]: y
Source address or interface: 172.16.0.1
Type of service [0]:
Set DF bit in IP header? [no]:
Validate reply data? [no]:
Data pattern [0xABCD]:
Loose, Strict, Record, Timestamp, Verbose[none]:
Sweep range of sizes [n]:
Type escape sequence to abort.
Sending 1000, 1024-byte ICMP Echos to 10.0.0.1, timeout is 2 seconds:
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Traceroute is a technique used when you suspect that a router on the path to an unreachable network is at fault. Traceroute sends out a packet to a destination with a Time To Live (TTL) of 1. If the first hop is not the destination, an ICMP type 11/Code 0 (ICMP Time Exceeded) message is sent back and the response time in milliseconds is recorded. Routers decrement TTL so that a packet will not circulate forever if there is a problem such as a routing loop. When a TTL gets to 0, the router drops the packet and returns the unreachable message.

A second packet is then sent out with a TTL value of 2, and if it is not the destination, an unreachable message is sent back and the response time in milliseconds is recorded. This continues until the destination is reached or until the maximum TTL as defined by the vendor is reached. (Cisco uses 30 as its maximum TTL with traceroute, but this is configurable.)

Many devices support traceroute. On Windows machines, the command is tracert. On Cisco devices, the command is traceroute, but this can be abbreviated as trace.

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Posted by steve10

Please Visit:

www.faculty.kfupm.edu.sa/COE/sadiq/richfiles/rich/ppt/MON.PPT

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Posted by sagitraz