Ethernet frame and CRC

[sarahr202]

Hi everybody

R1--f0/0----f0/1Sw1f0/1-----f0/1Sw2--F0/2--------host

R1: f0/1 199.199.199.1

Host : 199.199.199.2

When R1 sends Ethernet frame to Host, it calculates CRC and put the result in Ethernet frame.

This is my understanding , please correct me if i am wrong:

Sw1 and Sw2 simply looks up destination mac in mac table and switch the frame without checking  CRC.  Only Host will perform CRC and accepts or discards frame based on that 

Am I correct ?

( I understand the CRC can be caused by faulty NIC at sender /receiver, physical medium, duplex mismatch)

If i am correct, that also means CRC not always shows the issue with the local segment  i,.e the link between  sw2 f0//2--Host,  to put in another way if we have bad cabling sw1 f0/1-- f0/1 sw2 that is causing some bits to change, it will cause CRC errors when the frame arrives at HOST.

thanks

[Leo Laohoo · Accepted solution]

The receiver will always see CRC.  This is why CRC counters are found in the Receive column.  

If you've got a Catalyst 2K or 3K, use the command "sh controller e <INTERFACE>".  If you have a 6500, then use the command "sh counters interface <INTERFACE>".

If you look at the result of the command in Catalyst 2K and/or 3K, you'll notice two columns:  Transmit and Receive.  The rest is self-explanatory.

The counters in this command is, by far, superior to the output you get with "sh interface <INTERFACE>" because of two reasons:  

• Granularity of the counters; and 
• Not everyone knows how to clear the counters to this command.  

[acampbell · Accepted solution]

Hi Sarah,

Along with the info Leo (+5 L) has provided the type of
switching method should also be considered.

1)Store and forward - Full frame inspected for errors - forward if good - drop if bad
2)Cut Through - Only inspect the up to the destination mac then forward - so dodgy frames can be forwarded too.

I remember some old switches -I think they were menu driven 1900s/2800s
when you could select which switching mode to use.

Newer swtches like 3560X & 3750X use only store & forward
but some of the nexus ranges can use cut through

Here is a wee bit of light reading for you 
http://www.cisco.com/c/en/us/products/collateral/switches/nexus-5020-switch/white_paper_c11-465436.html

Regards
Alex

[Leo Laohoo]

Thanks for the rating, Alex.  :)