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