Dell 6224 switch ‘Oversize Packets’ counter
It’s been a while since I’ve written anything on my blog, but following the lack of any hits on Google regarding this, I felt this might well be a useful snippet to those in the same boat as myself.
I’m currently testing & tweaking an iSCSI setup that utilises a Dell 6224 switch. These are very fast switches for the money (about ~£900 if you’ve got a good account manager!) and provide a lot of features, including stacking, if you have more than one. Their drawbacks, however, are mostly in the lack of documentation and/or the same level of user interface ‘polish’ that you receive from other manufacturers. Most people will say; ‘you get what you pay for’, but for the most part they are great switches for the money you pay.
One such “lack of documentation” has had me annoyed today. I’ve been using LACP to bond ports and, at the same time, have raised the MTU to the maximum of 9216 (which can be done per-port, without a reboot or a switchport up/down event, I might add) across all ports. All this, in an attempt to glean a little more performance (i.e. lower processing overhead) from my iSCSI sessions.
And it seemed to work just fine. However, upon inspecting the interface counters, I noticed a stunning amount of packets being regarded as ‘Oversize Packets’:
switch#show interfaces counters port-channel 1
Alignment Errors: ............................. 0
FCS Errors: ................................... 0
Single Collision Frames: ...................... 0
Multiple Collision Frames: .................... 0
Late Collisions: .............................. 0
Excessive Collisions: ......................... 0
Oversize Packets: ............................. 15829678
Internal MAC Rx Errors: ....................... 0
Received Pause Frames: ........................ 0
Transmitted Pause Frames: ..................... 0
I wasn’t sure whether or not to take this as an error or just a simple ‘count’ of packets. “Oversize” would indicate that they’re bigger than the port was expecting, but I was still hitting around 120MB/sec (out of the theoretical 125MB/sec that Gigabit Ethernet can physically provide) which wouldn’t be conducive to a serious string of frame/packet errors.
I couldn’t find anything online, so I contacted Dell ProSupport to raise a ticket. I had to go through the annoying rigmarole of explaining the problem three times over, but eventually a ‘switch expert’ explained that he wasn’t certain on the use of that counter and that its purpose depended on the firmware version currently in use (in my case, this was 3.2.0.9) and needed to check with his colleagues.
He eventually rang back to inform me that this was not a problem with the switch. The “Oversize Packets” counter merely serves to log packets that have a payload in excess of 1518 bytes. A fixed amount. It doesn’t matter than the MTU was set to 9216, it just continues counting the packets. Utterly useless, then!
As some form of consolation, he also mentioned that it didn’t update in real time.. Owing me to believe that there was some form of port stats analysing process running over the real time output. When it’s this useless, could I please have an option to turn it off? Or better yet, don’t bother logging it by default!
