Switch uplink port MTU

[Dan]

Hello,

I am currently seeing output drops on several uplink interfaces to other switches from a cisco 3560. I have monitored the controllers utilization and can see these interfaces occasionally spiking and this is when the output drop count increases. It doesn't seem to be continuously oversubscribed. I am not seeing output drops on the 2950's, only the core 3560.

The 3560 interfaces are gigabit whereas the 2950's are 10/100.

I suspect it is being caused by an MTU mismatch. The 3560 has the system mtu configured to 9000 whereas the 2950's are all set to 1500. Can anyone confirm my thinking or is it likely to be something else?

The IOS versions:

Cisco 3560 version Catalyst L3 Switch Software (CAT3K_CAA-UNIVERSALK9-M), Version 03.06.03E

Cisco 2950 version 12.1(22)

Thanks in advance

[Mark Malone]

Hi Dan

Yes that could be the cause , you need to be careful enabling jumbo frames on a local network

good ref explaining it

https://kb.netgear.com/25091/Guidance-on-the-use-of-jumbo-frames

Caveats

Performance problems arise when the MTU of one device is different from the MTU of another. Making casual changes to MTU to optimize performance is likely to have the opposite effect of decreasing performance.

If most of your local network devices and applications can be configured for the same jumbo frame size, your network might benefit from larger packets, less fragmentation, and less overhead. In modern equipment, this optimization is becoming less of a problem due to LSO/LRO and TCP offload engines, especially on higher end server grade NICs.

All equipment on the same layer 2 network, which means the same LAN or VLAN, should support the same frame size. By default, this is 1500 bytes. If you increase the MTU on one of your end devices, your switch needs to be able to pass these larger frames (same frame size or larger, than default MTU), and the receiving end needs to handle these larger frames as well. If this change is not coordinated, your network might actually run slower, or break.

 

When should jumbo frames be used?

Use jumbo frames only when you have a dedicated network or VLAN, and you can configure an MTU of 9000 on all equipment, to increase performance. A good example of this approach is a separate SAN or storage network. In all other situations, the effort of configuring jumbo frames everywhere on your network is not worth the marginal improvement, and has the potential of slowing down or breaking non-jumbo frame clients.

[Dan]

Thanks for the responses guys.

QOS is not configured.

I think jumbo frames has been implemented on the 3560 due to iscsi being used.

I will speak to our server team about the implications of dropping the mtu to 1500.

I will post my findings.

Thanks

[Joseph W. Doherty]

iSCSI is just the kind of application that microbursts will exhaust interface buffers.

I had a 3750 that had two SAN devices attached to it which those ports were showing multiple drops a second.  Tuning its QoS buffers settings got the drop rate to to a few drops a day.

[Joseph W. Doherty]

Indirectly, perhaps.  Jumbo frames might exhaust interface buffers quicker during a transient burst.  However, if your down stream (L2) 2950 switches aren't configured for jumbo frames, why interfaces to those switches have any jumbo frames on them, would be a question that needs answering.

Is QoS globally enabled on your 3560?  If so, is it using default settings?  If so, 3560 (and 3750) QoS, especially with default settings, often exhausts interface buffers.  Disabling QoS, if not truly needed, or tuning QoS settings, often reduces drops due to buffer exhaustion.