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Chapter 4

Cisco's Diagnostic Commands

Datagram size [100]:

Timeout in seconds [2]:

Verbose [n]: y

Sweep range of sizes [n]:

Type escape sequence to abort.

Sending 5, 100-byte AppleTalk Echoes to 1.112, timeout is
2 seconds:

0 in 12 ms from 1.112 via 2 hops

1 in 12 ms from 1.112 via 2 hops

2 in 12 ms from 1.112 via 2 hops

3 in 12 ms from 1.112 via 2 hops

4 in 12 ms from 1.112 via 2 hops

Success rate is 100 percent (5/5), round-trip min/avg/max
= 12/12/12 ms

Router_C#

Because the verbose option was selected, the RTT along with the hop

count are listed.

traceroute Command

The traceroute command is used for displaying the packet's path toward
its destination. The functionality of the traceroute utility works on error
messages that are generated by expired TTL values in the IP packet header.
When the TTL value in an IP header reaches zero, the entire packet is
discarded. At the same time, the IP host responsible for discarding the packet
sends an error message to the source IP address in the header, informing the
source that the packet was dropped. The TTL value is decremented by one
every time the packet transits a router or IP host.

Traceroute capitalizes on this message exchange. When the traceroute

function is used, the TTL in the IP header is set to a value of one. It then
sends the packet to the specified destination. Because the next-hop decre-
ments the TTL counter to zero, the packet is discarded and a message is
sent back to the source address. The traceroute utility records the IP
address from the error message and echoes it to the screen. An nslookup
is performed on the IP address; if a result is received, the DNS name is dis-
played in addition to the IP address.

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