Adjusting MTU for Jumbo Frame

[davidhuynh5]

We currently have a Cisco Catalyst 4948 and I was asked to adjust the switch mtu size to 9000 to support Jumbo frames.

What are the different ways to configure it on this switch and will it have a negative effect on LAN performance?

Thanks in advance.

[Leo Laohoo]

I'm not sure if you can configure per-interface Jumbo frame on a 4948.  This means that Jumbo frames needs to be GLOBAL.

I believe some line cards of the 6500 will support per-interface Jumbo.

[sdheer]

Hi,

The 4900 switch is the same as a 4500. It's a Sup with a 48 port
linecard (exact same hardware) in a fixed chassis. Therefore, the
commands are exactly the same for that IOS.
 
Here is the link which has table as to whether we can enable or not.
http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_note
09186a00801350c8.shtml
<http://www.cisco.com/en/US/products/hw/switches/ps663/products_tech_not
e09186a00801350c8.shtml> 

On the switch 4948, we will have to enable the jumbo frame feature at a
per interface level.
We would be able to set this at the interface level on both L2 and L3
ports.
The system MTU would still show up as 1550 but at the interface level,
it would be what has been configured.

When the support for jumbo frames in configured it should done consistently across devices along the path of the packet ideally to prevent the other switch which is not configured for jumbo frames dropping packets as gaints.

Regards,

Swati

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[pgasparovic]

Very useful even in H2 2019 !

[Shashank Singh]

Hi David,

If the packet’s MTU is larger than the egress SVI’s MTU, the packet is  fragmented by software (if the DF bit is not set), which results in poor  performance. However, software fragmentation only happens for L3 switching. This means that when a packet  is forwarded to an L3 port or an SVI with a smaller MTU, software fragmentation  will occur.

For jumbo frames which need to be switched (not routed), hitting the L2 interfaces, fragmentation happens in hardware which is fast and non impacting.

Hence enabling jumbo frames will certainly improve performance for all types of packets but the effect will be more evident for the L3 packets hitting the SVIs.

Hope this helps,

Shashank

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