I'd like to further clarify this - as I think the subtleties here
between layer 2 and layer 3 MTU may be giving a misleading picture.
The Layer 2 MTU (AKA frame size) is set globally, in the same way as on
a Catalyst floor switch such as a 3560/3750 is done.
This is performed by something like this in the config:
-------------
policy-map type network-qos enable-jumbo-frames
class type network-qos class-default
mtu 9216
system qos
service-policy type network-qos enable-jumbo-frames
--------------
However the Layer 3 MTU is customisable, per VLAN interface:
---------------
SWT01(config)# int vlan 22
SWT01(config-if)# mtu ?
<64-9216> MTU size in bytes
SWT01(config-if)#
interface Vlan22
no shutdown
mtu 9216
-----------------------------
In the majority of cases you will probably want to have a high layer 2
MTU globally, and then perhaps be more careful with your layer 3 MTU
settings at your layer 3 boundaries (the default is almost always 1500
so unless you specifically modify it, that's what it'll be, regardless
of your layer 2 MTU). If you aren't doing any Layer 3 switching on the
device then the layer 3 MTU setting obviously won't be necessary on there.
The config above is from a 5548UP running 5.2(1)N1 (that has the L3
daughter card).
I'm not aware of any negative side effects on this platform from setting
the layer 2 MTU to a high value globally. Things most certainly break
badly when the L2 MTU is set too small, but not usually when it's too big.
Reuben
On 20/09/2012 4:01 PM, Lars Christensen wrote:
Hi Joshua,
You can't set a per interface MTU on the Nexus 5500-series switches.
However, you can do it on per Class of Service.
So in order to do increased MTU for your iSCSI traffic, you should
create a policy-map identifying your iSCSI traffic, place the traffic
in a separate queue with increased MTU.
This is the way to go forward.
Lars Christensen CCIE #20292
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