packets UnderSize

[m-garay]

                   Hi everybody,

Had a question about the rise of packets Undersize when trying to enable a  802.1q  trunk on Catalyst 3750, the other side is a Blade  server BL6000, T4-1B.

My IOS version is C3750E Software (C3750E-UNIVERSALK9-M), Version 12.2(55)SE3

#sh int gi 3/0/17 counters errors

Port        Align-Err    FCS-Err   Xmit-Err    Rcv-Err UnderSize
Gi3/0/17            0          0          0          0     70220

Any help would be welcome.

[Sandeep Choudhary]

Hi,

This may be being caused by duplex mismatch on the Gigabit interfaces.

Can you

show interface g3/0/17 status

at both ends of your links

Regards

please rate if it helps.

[Alessio Andreoli]

Hi m-garay,

i would suggest to read this document:

https://supportforums.cisco.com/docs/DOC-2809

and to perform an extended ping varying the size of your packet to realise which is the max size you can send over the network. Remeber that the MTU won't be anymore 1500 because some headers are added while sending the frame out. generally you should reverify and set MTU to 1522 and to optimize the mss too.

to really check the undersize packets look for the number of "runts" with show interface gigx/y

HTH

Alessio

[Ganesh Hariharan]

m-garay wrote:
                   Hi everybody,
Had a question about the rise of packets Undersize when trying to enable a  802.1q  trunk on Catalyst 3750, the other side is a Blade  server BL6000, T4-1B.
My IOS version is C3750E Software (C3750E-UNIVERSALK9-M), Version 12.2(55)SE3
#sh int gi 3/0/17 counters errors
Port        Align-Err    FCS-Err   Xmit-Err    Rcv-Err UnderSize
Gi3/0/17            0          0          0          0     70220
Any help would be welcome.

Hello,

As stated aove most common cause of the undersize rames can be caused by collisions.  Please check your speed & duplex settings.  Could also be a cabling issue. 

Hope that Helps

Ganehh Iyer

[shulipal]

Hello Team,

Usually any packet smaller than 64Bytes should be counted as an undersized frame. But there is an exception to this rule.When 802.1Q encapsulation is used on a trunk interface on the Catalyst 3750 Switches, runts can be seen on show interface output because valid 802.1Q encapsulated packets which are 61-64 bytes including the q-tag are counted by the Catalyst 3750 Switch as undersized frames, even though these packets are forwarded correctly. This has been documented in bug ID CSCec14238.