The ping command
is irreplaceable when it comes to troubleshooting. At some point, you will
undoubtedly use this command to solve a networking problem. But how do you
properly use this command in the Cisco IOS?
The Basics of Ping
The ping command
works just like on those old submarine movies. You are on one network device
and you “ping” another. When you do this, in your head think of the sound that
you heard on those old submarine movies- “PPiiiiiiiiiiiing”. The sound would go
out and, on the sonar operator’s-screen, he would or would not see the other
submarine. This is exactly how the ping-command in networking, works. Your
sonar screen is your Cisco router’s command prompt. Usage of the ping command
can be as simple as this:
As you can see in
this example, I simply typed ping, and the IP address of the host I wanted to
ping. In response, I got five exclamation points that told me that I sent 5
ping packets out, and they were all returned (a complete success).
In other words, a
ping request is sent out to the remote device, and a ping response is received
back, acknowledging the request. As ping uses the ICMP protocol, these packets
are technically called ICMP echo request, and ICMP echo reply. ICMP is
considered the management protocol for IP. ICMP uses the IP protocol but ICMP
is not TCP, or UDP. ICMP does work at Layer 3.
Note that if the ping was not successful, you would have received one of the
following (instead of an exclamation point): - “.” = network server timed out -
“U” = destination unreachable - “Q” = source quench (destination too busy) -
“M” = could not fragment - “?” = unknown packet type - “&” = packet
lifetime exceeded Besides the five exclamation points, I was also told that I
was sending “5, 100-byte ICMP echoes”. This means that I actually sent five
“ping packets” of 100 bytes each. I was told that the timeout was 2 seconds.
That means that if a response was not received within 2 seconds, ping would
decide that the packet was not going to return at all. This is a safe
assumption considering 2 seconds is 2000ms and I am getting pings back in about
36ms. Notice on the last line that the “Success rate is 100 percent”. That is
because it says that I sent 5 pings and received 5 ping replies back (that is
the “(5/5)”). I was told that the round-trip minimum time for a ping reply to
return was 36ms, the average time (of all 5 pings) for a reply to return is
36ms, and the maximum time for a ping reply to return was 40ms. If you have DNS
or a local hostname configured, you can use ping with names, like this:
You should know
that there are many more types of ICMP traffic other than that used for
“pinging” (echo and echo-reply). ICMP is used to redirect hosts to the proper
router, to inform hosts that they need to resize their packets, and many types
of IP management communications. Each of these types of ICMP packets has a type
number (and optionally, a code number). For example, an ICMP echo is type 8. An
echo-reply is a type 0. A redirect to another router for an entire network is a
type 5, code 0 (with there being possible codes of 0-3). Finally, you can
abbreviate ping by only typing “p”. For example: Router# p 1.1.1.1
What Else Can I
Do With Ping?
Now that you
understand the basics, let’s look at an advanced version of ping. Cisco calls
this “extended ping”. Extended ping will ask you many questions and
“interactively” configure the options for ping. If you have never seen this
before, you may be surprised at how many options the ping command can have.
Here is an example:
In typing ping,
by itself, I was asked a list of questions. I have put a red arrow by each of
the questions for which I typed a response. On other lines, I simply
pressed Enter to take the default. In this example, I still
ping-ed “Router3”. I stuck with the default of 5 ping packets (but could have
changed it). I kept the default of a 100 byte ping packet but could have
changed this to a ping packet as large as 18,024 bytes. Next, I chose to use
the extended options, where I was able to choose the source interface of my ping
packets. I also chose verbose output. With verbose output, I was able to see
each reply to each ICMP echo that I sent, and the time it took for that reply
to return to my router. One thing you may be surprised by, is the first
question that asked what protocol you want to ping with. Yes, you can ping with
protocols other than IP (such as Appletalk, DECnet, and IPX), but rarely are
those protocols used anymore.
How Do I Allow Ping Through An IOS Access-list?
Because ICMP is
not TCP or UDP, you must specify ICMP specifically when you create an
access-list (ACL). Here is an example: access-list 101 permit icmp any
any echo-reply In this ACL, we are permitting ICMP traffic from any
source, and any destination, as long as it is a reply to an echo request. Many administrators
enter the following ACL and expect ICMP to flow through it: access-list
101 permit ip any any This ACL does NOT allow ICMP traffic. To allow
ICMP and IP, you need the following two entries in your ACL: access-list
101 permit ip any any access-list 101 permiticmp any any
In summary, the
ping utility is invaluable when it comes to troubleshooting network issues.
While just about everyone has “pinged” something at one point or another, most
people don’t know that there is more to ping than the simple ping command.
Extended ping on Cisco routers and switches is a very powerful troubleshooting
utility. While the ping command does use the ICMP protocol, there is much more
to ICMP than just “ping”. Finally, don’t forget to save yourself three
keystrokes by abbreviating the ping command with “p”. Over the years, those
keystrokes add up!
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