The extended ping command is a very important part of any network engineer’s Cisco IOS commands knowledge, but you don’t find this being mentioned in many CCNA books.  Many beginners do not know how to use this command effectively, but it is definitely a must-know and can isolate many network issues.  In this post, I’ll go through some of the most important and useful parameters of this command:

Target IP address

The target IP address is the IP you want to send the ping/ICMP packet to.  This is fairly straightforward, but one important note that if this IP is not routable from this router, you won’t be able to reach it, and will get no replies.  The next step would be to do a traceroute to further isolate the issue.

Repeat count

Some physical line problems might only show up upon repeated ping tests.  If it’s an intermittent issue, it’s best to put a larger number to simulate a stress test in order to ensure that there are no issues.  For example, 1,000 or 10,000 packets might be a suitable number.

Datagram size

For stress test purposes, you can put a larger number in the datagram size to test for larger packets.  It is not recommended to put a datagram size larger than the maximum MTU size of all the routers along the routing path as this would mean the packets would be fragmented (unless you are testing whether fragmentation is working properly).  For an TCP/IP network running on ethernet, a good datagram size to use is 1500.

Some issues might also only show up with larger datagram sizes.  These are usually related to some fragmentation issue along the way.  It makes sense to test with larger datagram sizes as well as small ones in order to be sure.

Source address or interface

This part of the extended ping command is very useful to test whether it’s a routing issue or a switching issue.  If you reach the directly connected router for a problematic host, and pings are successful when sourced from a directly connected interface (but not from another interface which is not directly connected), it usually indicates a routing configuration or process issue on that router, since no routing is needed for directly connected interfaces.

I hope you’ve found this post on Cisco IOS commands useful.  Do subscribe to my RSS feed or follow me on twitter (@savvynetcert) for the latest updates.

Cheers,
Francis (SavvyNetCert.Com – @savvynetcert on Twitter)

Technorati Tags: Cisco IOS Commands, ios commands, router ios commands

Mastery of Cisco IOS commands is a very important part of the CCNA test and is also essential for anybody who is serious about a career in computer networks.  Of all the commands, the most crucial are those you should have at the tips of your fingers when something goes wrong and you are suddenly called upon to troubleshoot.  Every so often, the issue is related to a switchport and it is important to know how to use the show interface Cisco IOS command properly.  If there is a network issue regarding slowness or disconnects, here are a 6 things you should be looking at when you check on the output of the “show interface” Cisco IOS command:

1. Is the physical port and line protocol up? If you don’t have layer 1 and layer 2 connectivity, there is no way any IP packet is going through this port.

2. Any duplex mismatch issues? Duplex mismatch could lead to slowness and errors in the network.  If the speed does not match, the port won’t even come up.

3. Any QoS drops? If you have a QoS policy in place, it might be shaping and policing the traffic and dropping the packets.  In this case, you might need to either rework your QoS policies, or advise the user about the high traffic levels.

4. What are the traffic levels, is this normal? You can check the input and output rates to see if this is normal for this time of day.

5. Any CRC or other input errors? Input and CRC errors could signify a physical problem with the port or cabling.

6. Any output errors or collisions? Output errors could also indicate a physical issue with the port or cabling.  A full-duplex connection should have no collisions while it is quite normal for a half-duplex port to have collisions.

Cheers,
Francis (SavvyNetCert.Com)

P.S. Still preparing for your CCNA? Do get the right CCNA books – have you seen our CCNA book comparison chart?

Technorati Tags: ccna network engineer, cisco ios command, Cisco IOS Commands, ios commands