"RX over" meaning

[Simon_the_netwo]

Dear community,

I recently found lots of “rx over” counts on some Summit 400 switches when I issue “show port rxerrors”:

Port Rx Error Monitor                                 Wed Mar 31 10:47:48 2021
Port          Link      Rx      Rx      Rx        Rx     Rx        Rx        Rx
              Status    CRC     Over    Under     Frag   Jabber    Align    Lost
======================================================================================
   5            A        0 17568090        0        0        0        0        0
   7            A        0 43528283        0        0        0        0        0

I found a kb article where it is stated that these counter increases when frames larger than 1522 bytes arrive. There are jumbo frames (9126 bytes) configured on those ports.
Does this counter raise when jumbo frames arrive on a port generally or just if there is a problem with them.

Thank you

Simon

[Simon_the_netwo]

Since I enabled the support of jumbo frames on the nic I can find lots of them in my capture. The problem I have now is that I cannot see any frames that are beyond 9014 bytes. Since the switch is able to process frames that have 9126 bytes I cannot see whether there are oversized bytes in my network.

I think I will concentrate on the ISCSI traffic I can see, for now.

[Simon_the_netwo]

Gabriel,

thank you for your detailed answer.

I will check, whether the NIC supports Jumbo Frames.

The switches are Summit400 models and are running version 7.7.e.3.5.

The affected ports are connected to ESX-Hypervisors and some other switches.

Simon

[Gabriel_G]

Hi Simon,

You may not see packets larger than 1482 in a PCAP for multiple reasons (does the capturing device support jumbo frames, have a supported frame size > 1523, etc)

From a quick lab test, that counter should only increment for ‘bad’ frames or frames too large for the switch to handle. Doing a ping test of size 9000, I didn’t see the counters increment with jumbo frames enabled. When I disable jumbo frames, I lost my pings and my counters started incrementing.

Keep in mind this jumbo frame L2 setting is separate from the IP-MTU L3 setting which controls maximum packet (not frame) size. https://extremeportal.force.com/ExtrArticleDetail?an=000078892

Regardless, if that counter is incrementing when you have jumbo frames enabled and the size set to the max (9216, the default), that port is either receiving super-jumbo frames, there is some other type of L1 communication issue causing the switch to think it is receiving large frames, or those counters are just wrong.

What version are you on and are you actually seeing any performance impact? Also, what does this link connect to, another switch or just a client? Are you using supported optics 🙂

Gabriel
 

[Simon_the_netwo]

Alexandr,

I did a packet capture. The largest packet had 1482 bytes. I am a little confused, since I expected lots of jumbo frames.

Kind Regards