Friday, April 2, 2010


The most common problems in a switched network is the speed and duplex. By default every port on a Cisco switch has speed and duplex set to auto. The auto-detect mechanisms are old and does not work correctly a lot of times. The speed is correct most of the time, but the duplex is commonly mismatched.
The only time we see duplex mismatch messages is when we connect through the console port or use terminal monitor on a VTY. During a mismatch, data will get dropped a lot and network experience will be terribly slow.

To hardcode the speed, use:
speed 100

To hardcode duplex, use:
duplex full/half

In older switches, the commands may be:

Sometimes status messages may appear when you are typing, and cut off your text. One way to fix this is to press the TAB key. To automatically shift your text to the next line after status messages, use:
line vty 0 4
logging synchronous
line con 0 4
logging synchronous

By default if you're idle for 5 minutes in a VTY session, you'll be kicked out. To change the time, use:
exec-timeout 30

To disable exec-timeout, either set it to 0 minutes or use:
no exec-timeout

Note that the above commands are typed in the line modes. At times when you make typographical errors, the switch or router may try to resolve what you typed. An example is:
Translating "hello"...domain server (

This may cause the router to hang for 30 seconds while it tries to resolve it. To prevent this lookup, move to Global Configuration and use:
no ip domain-lookup

The Privileged Mode is also known as the EXEC. If you type "show ip int br" a lot, you can use aliases to make short cuts. To do this, you can type:
alias exec s show ip int br

The next time you type "s", it would be replaced with "show ip int br". You can even type "do s" from anywhere.

Spanning tree is a protocol designed to limit broadcast storms. Switches forward broadcast packets out all ports by default. But if you have redundant connections forwarding, a broadcast would be forwarded back to the origin switch and the origin switch would then broadcast it back. Spanning tree blocks redundant connections until it is required and figures out which one is the best to block.

The three best show commands to troubleshoot connectivity problems are:
show ip int br
show int
show run

Status downs are typically caused by bad cables, while protocol downs are typically caused by duplex mismatches, speed mismatches and encapsulation mismatches. In the show interface command, you can see the reliability of a link. The reliability is a number between 1-255 which will drop as it begins encountering problems. txload and rxload describes how loaded is the interface.

A high level of broadcast in a production network is about 20%. A runt is a packet that is too small. A giant is a packet that exceeds the MTU. A late collision typically happens when a cable is too long or it is connected through too many repeaters. A late collision is said to occur when an acknowledgment is received while a packet is thought to have been dropped and is being retransmitted.

In certification examinations, the "show run" command is typically disabled.

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