CREATING A REMOTE ACCESS POLICY FOR DOMAIN ADMINISTRATORS IN THIS PROC...
6.
Ping several computers on another network that are accessible through the default
gateway.
If this test fails (and the previous test succeeded), then you know that although the
default gateway is up and running, it might not be routing packets properly. A fail
ure to ping a single computer on another network could indicate that the destina
tion system is not running, but if you cannot ping several systems on another
network, it is likely that there is a routing problem.
Tip
For best results, you should try to ping systems on a network to which the default gate-
way is directly connected. This way, you know that if the test fails, the default gateway is the
problem. If the packets are passing through two or more routers to get to their destinations,
any one of the routers could be at fault, and you must use another tool (such as Tracert.exe
or Pathping.exe) to determine which router is malfunctioning.
Using Tracert.exe
Tracert.exe is the Windows operating system’s implementation of the UNIX traceroute
program. TRACERT enables you to view the path that packets take from a computer to
a specific destination. When you type tracert and an IP address at the Windows com
mand prompt, the program displays a list of the hops to the destination, including the
IP address and DNS name (where available) of each router along the way, as follows:
Tracing route to https://traloihay.net [10.146.1.1]
over a maximum of 30 hops:
1
<10 ms
1 ms
<10 ms
192.168.2.99
2
105 ms
92 ms
98 ms
qrvl-67terminal01.epoch.net [172.24.67.3]
3
101 ms
110 ms
98 ms
qrvl.epoch.net [172.24.67.1]
4
123 ms
109 ms
118 ms
svcr03-7b.epoch.net [172.24.103.125]
5
123 ms
112 ms
114 ms
clsm02-2.epoch.net [172.24.88.26]
6
136 ms
130 ms
133 ms
sl-0-T3.sprintlink.net [10.228.116.5]
7
143 ms
126 ms
138 ms
sl-3.sprintlink.net [192.168.5.117]
8
146 ms
129 ms
133 ms
sl-12-0.sprintlink.net [192.168.5.1]
9
131 ms
128 ms
139 ms
sl-13-0.sprintlink.net [192.168.18.38]
10
130 ms
134 ms
134 ms
sl-8-0.sprintlink.net [192.168.7.94]
11
147 ms
149 ms
152 ms
sl-0.sprintlink.net [192.168.173.10]
12
154 ms
146 ms
145 ms
ny2-ge021.router.demon.net [172.21.173.121]
13
230 ms
225 ms
226 ms
tele-ge023.router.demon.net [172.21.173.12]
14
233 ms
220 ms
226 ms
tele-fxp1.router.demon.net [10.159.252.56]
15
223 ms
224 ms
224 ms
tele-14.router.demon.net [10.159.254.245]
16
236 ms
221 ms
226 ms
tele-165.router.demon.net [10.159.36.149]
17
220 ms
224 ms
210 ms
https://traloihay.net [10.146.1.1]
Trace complete.
Tracert.exe is an excellent tool for locating a malfunctioning router, because it is able
to inform you how far packets have gotten on the way to their destination. When one
of the routers on the path is not forwarding packets properly, the TRACERT output
stops at the last functioning router. You know then that the next router on the path is
the one experiencing the problem.
Lesson 4 Troubleshooting TCP/IP Routing
5
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43
How Tracert.exe Works
Tracert.exe works by sending ICMP Echo Request messages to the destination,
much as PING does, but with a special difference. For the first group of three Echo
Request messages, TRACERT assigns a value of 1 to the IP header’s Time to Live
(TTL) field. The TTL field is a safety measure designed to prevent packets from cir
culating endlessly around an internetwork. Normally, computers running Windows
operating systems assign a value of 128 to the TTL field. When a router processes
a packet, it reduces the TTL value by 1; if the TTL value reaches 0, the router dis
cards the packet and returns an error message to the system that transmitted it.
Because the first three TRACERT packets have a TTL value of 1, when they reach
the first router on their path, the router reduces their TTL values to 0 and discards
them, sending error messages back to the sender. Then, for each successive
group of three Echo Request messages, TRACERT increments the initial TTL value
by 1, causing each group of packets to travel one more hop on the way to the
destination before the router discards them. The TRACERT program uses the error
messages generated by the routers (which contain the routers’ IP addresses) to
create the output display.
Tip
It is important to understand that routes through a large internetwork can change fre-
quently, for a variety of reasons, and packets can take different paths to the same destina-
tion. Therefore, when you use TRACERT, it is possible (although not probable) for the path
through the internetwork taken by successive sets of Echo Request messages to be different.
When you are using TRACERT to locate a malfunctioning router, you should run the program at
least twice, using the same destination, to ensure that you are seeing an accurate path
through the network.
Using Pathping.exe
Pathping.exe is another tool available from the Windows command prompt that is sim
ilar to Tracert.exe in that it traces a path through the network to a particular destination
and displays the names and addresses of the routers along the path. PATHPING is dif
ferent, however, because it reports packet loss rates at each of the routers on the path.
TRACERT is the preferred tool for locating a router failure that completely interrupts
communications, while PATHPING is more useful when you can connect to a destina
tion, but you are experiencing data loss or transmission delays.
After displaying the path to the destination, PATHPING sends 100 packets (by default)
to each of the routers on the path and computes the packet loss rate in the form of a
percentage. A typical PATHPING output display appears as follows:
Computing statistics for 125 seconds...
Source to Here
This Node/Link
Hop
RTT
Lost/Sent=Pct
Lost/Sent
=
Pct
Address
0
172.16.87.35
0/ 100
=
0%
|
1
41ms
0/ 100 = 0%
0/ 100
=
0%
172.16.87.218