Cisco 3560G and max MTU Size

[matthydras]

Hello

I have an Cisco 3560G with an version 15.0(1)SE2 IOS. I want to forward Jumbo frames (ISCSI packets) through this switch.
On my SAN, each interface have been configured on 9000 bytes.

On this switch, I have this output :

Sw1#sh system mtu

System MTU size is 1500 bytes
System Jumbo MTU size is 9000 bytes
System Alternate MTU size is 1500 bytes
Routing MTU size is 1500 bytes
Sw1#

I would like to know how to increase, if possible, the Jumbo MTU to 9198 bytes. I want to do that because the 9000 bytes ISCSI packets normally will been encapsuled using vlan so the ethernet packet will increase to 9022 (Ethernet 18bytes and extra 4 bytes for vlan).

If the Jumbo MTU reconfiguration is not possible, the ISCSI packets will be fragmented each time it forwarded through the switch isn't it ??

Thanks in advance for your help.

Have a nice day

Matt

[Bilal Nawaz]

Hi Matt,

The 3560 only supports up to the MTU size of 9000, stated in the document below

http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6000-series-switches/24048-148.html#c3

At layer 2 i think it will be dropped (giant), if there is a frame bigger than supported mtu size.

[matthydras]

Hi Bilal

Thanks for your reply.

In the document, Cisco add note on the top of the webpage on the Components Used : "Note: In all the examples in this document, unless specifically mentioned, all values that quote MTU in bytes omit the 18 bytes for the Ethernet header and Frame Check Sequence (FCS)."

So In my mind, the 3560G switch will not drop 9018 ethernet bytes but will normally drop the vlan Jumbo frames (9022 ethernet bytes).

I think Cisco will included the Ethernet header and FCS on the document because networks admins included it and not think to on payload data.

Matt

[Bilal Nawaz]

Hi Matt, you are correct. Configurable maximum transmission unit (MTU) of up to 9000 bytes, with a maximum Ethernet frame size of 9018 bytes (Jumbo frames) as described here:http://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3560-series-switches/product_data_sheet09186a00801f3d7d.html