Cisco Systems CCNA 2 4.2.5 Alternative connectivity tests

The show sessions command output is as follows:

Stanly_Lab#show sessions

Conn Host Address Byte Idle Conn Name

1 lab-b 192.168.10.1 4 5 lab-b

2 lab-d 192.168.10.1 0 0 lab-d

* 3 lab-e 192.168.10.1 0 0 lab-e

4.2.5 Alternative connectivity tests

Connectivity can be tested with several other commands such as ping, traceroute, and

show ip route. The ping command uses ICMP to send an echo request to a destination

and then awaits an echo reply from that destination. This is a good test for basic connectivity,

reliability, and delay. This test can be performed from the user or privileged EXEC mode. A

successful ping is indicated by exclamation points (!). A period (.) indicates a ping that has

timed out.

The traceroute command is used to view the path that packets use to reach a particular

destination. This is an excellent test to identify where packets are dropped in the network. An

asterisk (*) indicates that the probe timed out. Traceroute will continue to reach the next router

in a path until the process times out or it is interrupted by the Ctrl-Shift-6 escape sequence.

The purpose behind the traceroute command is to record the source of each ICMP "time

exceeded" message to provide a trace of the path the packet took to reach the destination.

The device executing the traceroute command sends out a sequence of User Datagram

Protocol (UDP) datagrams, each with incrementing Time-To-Live (TTL) values, to an invalid

port address (Default 33434) at the remote host.

First, three datagrams are sent, each with a TTL field value set to 1. The TTL value of 1

causes the datagram to "timeout" as soon as it hits the first router in the path. This router then

responds with an ICMP "time exceeded" message indicating that the datagram has expired.

Next, three more UDP messages are sent, each with the TTL value set to 2. This causes the

second router in the path to the destination to return ICMP "time exceeded" messages.

This process continues until the packets reach the destination and until the system originating

the traceroute has received ICMP "time exceeded" messages from every router in the path

to the destination. Since these datagrams are trying to access an invalid port (Default 33434)

at the destination host, the host responds with ICMP "port unreachable" messages indicating

an unreachable port. This event signals the traceroute program to finish.

The show ip route command is used to identify the routes that are shown in the routing

table. These are routes to directly connected networks, networks with static routes, or

networks that have been learned through a routing protocol.

Due to many security configurations throughout the Internet, ping and trace may not always

work to test connectivity through networking equipment outside of your control. Many firewalls

and access-lists today do not allow ICMP traffic.