Undesized packages occur when VoIP connected

Answered Question

Hi,

I have a problem.

I use 3560 PoE switches to connect IP phones to our network. When connected, my switch ports start generating undersized packages. If just a regular pc is connected on a similar port these don't occur. Any idea how to solve this problem? Thanks.

Bart van Es

Correct Answer by Michael Owuor about 8 years 11 months ago

Bart,


UnderSize Frames are described as the total number of packets received that were less than 64 octets long (excluding framing bits, but including FCS) and have a good FCS value.


What you are seeing is due to a limitation in the 3550 ASIC in which 802.1q packets of the length 64-67 bytes are counted as undersized packets even though they are legal packets according to the IEEE standard. This is only an accounting problem and it does not affect data forwarding. In other words, these packets are still forwarded correctly by the switch, and only the stats are wrong. The problem is cosmetic.


http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCed73388


Hope this helps.


Regards,

Michael.

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Michael Owuor Sun, 08/24/2008 - 13:20
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Hi Bart,


For clarification, what specifically indicates that undersized packages are being generated?


Thanks,

Michael.

Reference message: Hi Michael,


On a standard configured port undersized packages are registered when a VoIP telephone is connected. When I just connect a PC to the same port these undersized packages do not occur. We use Avaya 1600 phones and as I understood these phones also do something with package sizes.


Bart

-------------------------

Additional info: It occurs on WS-3560 switches. Regards, Bart


Correct Answer
Michael Owuor Wed, 08/27/2008 - 07:05
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  • Cisco Employee,

Bart,


UnderSize Frames are described as the total number of packets received that were less than 64 octets long (excluding framing bits, but including FCS) and have a good FCS value.


What you are seeing is due to a limitation in the 3550 ASIC in which 802.1q packets of the length 64-67 bytes are counted as undersized packets even though they are legal packets according to the IEEE standard. This is only an accounting problem and it does not affect data forwarding. In other words, these packets are still forwarded correctly by the switch, and only the stats are wrong. The problem is cosmetic.


http://tools.cisco.com/Support/BugToolKit/search/getBugDetails.do?method=fetchBugDetails&bugId=CSCed73388


Hope this helps.


Regards,

Michael.

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