MTU settings on a 3750 switch

[SlipperyPete]

We've been trying to set a Catalyst 3750-12G to forward Jumbo frames (i.e. frames up to 9000 bytes).  It's easy enough to configure, but now that I've configured it I'm wondering about the following output:

3750-12_switch#show 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

As you can see, I've managed to set the "System Jumbo MTU size" at 9000 bytes, and I've confirmed this by checking some of the interfaces:

GigabitEthernet1/0/2 is up, line protocol is up (connected)
  MTU 9000 bytes

However, I'm concerned about this line: "Routing MTU size is 1500 bytes".  What does that mean?  The switch we want to configure this on has multiple L3/SVI VLAN interfaces, so the switch will be routing between those interfaces.  Does this suggest that the MTU can only be set for L2 forwarding, not for L3 routing?

Thanks.

[Shashank Singh]

Hi,

"Routing MTU size is 1500 bytes" means that the MTU for routed ports is 1500 bytes. You can change the MTU size for L3 ports by using the system mtu routing global configuration command.

However, you cannot configure a routing MTU size that exceeds the system MTU size. If you change the system MTU size to a value smaller than the currently configured routing MTU size, the configuration change is accepted, but not applied until the next switch reset. When the configuration change takes effect, the routing MTU size automatically defaults to the new system MTU size.

Example from a sample switch:

Switch#show system mtu

System MTU size is 1504 bytes

Routing MTU size is 1500 bytes

Switch#conf t

Switch(config)#system mtu routing ?

  <1500-1504>  MTU size in bytes

Hope this helps.

Shashank

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[SlipperyPete]

I think, for a 3750 switch, we cannot set the "routing" MTU to anything higher than 1500.  In the output below, you'll see that we set the System MTU to 9000, but the Routing MTU is still set at 1500:

Switch#show 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

When I check the Gig switchports, the MTU is 9000, as expected:

Switch#sh int gi 1/0/3
  MTU 9000 bytes

However, when I check a L3 VLAN interface

Switch#sh int vlan 10
Vlan10 is up, line protocol is up
  MTU 1500 bytes

Also, when I try to configure the L3 VLAN interface to a higher MTU, it won't allow me to go any higher than 1500.  See this output:

Switch#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
Switch(config)#int vlan 10

Switch(config-if)#ip mtu ?
  <68-1500>  MTU (bytes)

So...it certainly appears that a 3750 will only process/forward Jumbo frames at L2, not at L3.  Can anyone confirm this?

[Shashank Singh]

Hi,

Routing MTU can be set to as large as Jumbo MTU (if configured) using the system mtu routing command.

3750#sh system mtu

System MTU size is 1504 bytes

System Jumbo MTU size is 9000 bytes

Routing MTU size is 9000 bytes

ip mtu command is unsupported for any value on 3750:

3750(config)#int vlan 10

3750(config-if)#ip mtu ?

  <68-9000>  MTU (bytes)

3750(config-if)#ip mtu 1500

% ip mtu is not supported on this interface

3750(config-if)#ip mtu 68

% ip mtu is not supported on this interface

3750(config-if)#ip mtu 9000

% ip mtu is not supported on this interface

Cheers,

Shashank

[SlipperyPete]

Hi Shashank,

Is there some specific command to set the Routing MTU?  I'm able to set the System Jumbo MTU, but I'm still not able to set the Routing MTU.  You see in the following output that the System Jumbo MTU is set at 9000, but the Routing MTU is still 1500:

3750#show 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

So then I try to set the Routing MTU, but it won't let me set it at anything higher than 1500:

3750(config)#system mtu routing ?
  <1500-1500>  MTU size in bytes

3750(config)#system mtu routing 9000
                                                   ^
% Invalid input detected at '^' marker.

So how did you set your Routing MTU to 9000?

However, your previous post brings up another question.  Even if I'm able to set the Routing MTU at 9000, you pointed out that we cannot set the MTU on a L3 VLAN interface.  So even if I was able to set the Routing MTU to process Jumbo frames, I still wouldn't be able to route those Jumbo frames between L3 VLAN itnerfaces, right?  In my particular case, I'm trying to accomodate a customer who wants to use Jumbo frames.  This customer connects to one of our 3750 switches and has their own VLAN subnet on this switch, so we would have to route their traffic between L3 VLAN interfaces on the 3750.

Thanks.

[glen.grant]

   Did you reload the switch after you made the system change ?  It will not change without reloading the box..