Cisco Systems CCNA 2 manual Alternative connectivity tests, StanlyLab#show sessions

The show sessions command output is as follows:

Stanly_Lab#show sessions			Byte Idle		Conn Name
Conn Host		Address
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-6escape 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.

47 - 238 CCNA 2: Routers and Routing Basics v3.1 Instructor Guide – Module 4 Copyright ♥ 2004, Cisco Systems, Inc.