How to configure Jumbo frames only on certain Gigabit interfaces

[insccisco]

On a 3750 switch, is it possible to configure jumbo frames only on certain interfaces and leave the other ones as they are?

[satish_zanjurne]

1.It is not possible to set mtu for certain interfaces.

2.you set it for all 10/100 or all Gigabit Ethernet interfaces

3.You can increase the MTU size for all interfaces operating at 10 or 100 Mb/s by using the system mtu global configuration command

4.You can increase the MTU size to support jumbo frames on all Gigabit Ethernet interfaces by using the system mtu jumbo global configuration command

5.You can change the MTU size for routed ports by using the system mtu routing global configuration command.

6.Other work around is , you can modify the mtu for individual vlan under vlan configuration mode

vlan 2

mtu 1800

this way only ports in vlan 2 will have mtu as 1800.

7.If Layer 2 Gigabit Ethernet interfaces are configured to accept frames greater than the 10/100 interfaces, jumbo frames received on a Layer 2 Gigabit Ethernet interface and sent on a Layer 2 10/100 interface are dropped.

HTH..rate if helpful..

[insccisco]

I know I'm a bit late in responding but I am revisiting some similar setups and I need some confirmation.

I have a single 3750 with etherchannels, trunks, and bunch of vlans. If I only want to enable jumbo frames let's say on VLAN20, and I enable it with the above commands, am I actually accomplising just that? Meaning that only the ports that are members of VLAN20 will be affected and the other ports belonging to the other vlans will not be affected??

There are just too many conflicting articles out there and this is why I am back in here to try to get a bit more of some expert confirmation. Unfortunately I can't test it because the network is already in production.

Thank you in advance

[Ven Taylor]

Something else to remember.

If you are configuring jumbo frames, you have to configure them end-to-end.

In other words, every affected interface, be it a vlan interface or a physical interface, must be configured to support jumbo frames.

Example:

You have a device on switch 1, port 5, that requires jumbo.

The server it needs to talk to (using jumbo frames) is on switch 2, port 8.

The L3 vlan interface is vlan50.

Then:

Interface vlan50 needs jumbo frame support, the device ports need jumbo, and the "trunk" or link between the two switches also needs jumbo support.

If you don't do this, you can actually introduce latency into your network.

Ven

[insccisco]

Can you list the exact code to implement this good example?

[Ven Taylor]

Use this link.  It'll show you exactly how to do it and verify for CatOS or NativeIOS.

http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml

Ven

[insccisco]

I read that document and looks like it does not detail the VLAN portion of it.

Would you have exact details?

thank you

[Ven Taylor]

Assuming you're using NativeIOS:

config t

interface vlanXXX

mtu 9198  (or whatever jumbo frame size your application requires)

!

end

Ven

[insccisco]

Great stuff. I just needed a confirmation as I said, I will be applying this to a production environment.

With these commands, then every port that is a member of vlanXXX will be able to talk at jumbo speeds?? provided obviously that the device connected to that port has its NIC set to jumbo frames 9198, and that every other port on that same switch that belong to other vlans will still be connected to mtu 1500??

in other words, by doing this configuration we are effectively only configuring certain ports with jumbo frames right? (in this case the ports that belong only to vlanXXX)

thank you one more time

[Ven Taylor]

Oh yeah... About that.

It's usually best, if possible, to avoid combining non-jumbo and jumbo frame devices on separate subnets.

If you don't, you can increase latency.

We created a separate vlan for jumbo purposes.