Setting MTU on a 3750 switch

[pweinhold]

I've started this discussion a couple times already and I haven't really gotten a straight answer, so I'll try one more time...

The question is: Will a 3750 switch route Jumbo frame sizes (e.g. 9000 MTU)?

We know that we can change the System MTU to 9000, and someone on a previous thread said that we can change the Routing MTU to 9000 as well, although I couldn't figure out how to do that.  However, regardless of how we configure the System MTU and Routing MTU, I don't think we're able to adjust the MTU on L3 VLAN interfaces, so if we want to actually "route" between VLANs on the switch, we're limited to 1500 MTU.

Our situation is that we have a customer connecting to our 3750 switch, and this customer wants to use Jumbo frames.  The customer connects to our 3750 switch via their own VLAN, with their own L3 VLAN interface configured on the switch.  The customer will point their traffic towards the L3 VLAN interface, then we want to route them onto another VLAN, via a different L3 VLAN interface, before forwarding their traffic.  Because of the limitations noted above, specificially regarding the routing between these VLANs via L3 interfaces, I do not think we'll be able to support 9000 MTU frames on this 3750 switch.  I think the L3 VLAN interfaces will limit us to 1500 MTU, regardless of what we configured via the global System MTU and Routing MTU settings.

Can anyone confirm this or has anyone tested this?

Thanks.

[Sebastian Helmer · Accepted solution]

Hello Like I see the 3750 don't support it. Only X & E Series supports it.

I found these statements in a Cisco documentation and hope that helps:

"

Jumbo Frame Support

The  Cisco StackWise technology supports granular jumbo frames up to 9 KB on  the 10/100/1000 copper ports for Layer 2 forwarding. Layer 3 forwarding  of jumbo packets is not supported by the Cisco Catalyst 3750. However,  the Cisco Catalyst 3750-E and Catalyst 3750-X. do support Layer 3 jumbo  frame forwarding.

"

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps5023/prod_white_paper09186a00801b096a.html

regards,

Sebastian

[pweinhold]

So that's probably why someone had resonpded saying that you can set the Routing (L3) MTU, but I couldn't find a way to do it.  He was probably working on a different version of the switch.

Thanks very much for the response - that's the answer I kept looking for but couldn't find.

Thanks,

Pat

[Sebastian Helmer]

Pls. rate

[smalep]

5 switch stack of Cisco 3750-X switches

IOS VERSION:   122-55.SE3

TOR(config)#system mtu ?

  <1500-1998>  MTU size in bytes

  jumbo        Set Jumbo MTU value for GigabitEthernet or TenGigabitEthernet

               interfaces

  routing      Set the Routing MTU for the system

TOR(config)#system mtu jumbo ?

  <1500-9198>  Jumbo MTU size in bytes

TOR(config)#system mtu routing ?

  <1500-1500>  MTU size in bytes

It appears that several things are apparent,  1) Jumbo frames are global even on Cisco 3750-X.  You can only enable them globally and not per interface. Also for routing on MTU of 1500 is supported

Thanks,

P