[c-nsp] mtu issues on 3750

[Michael K. Smith]

This is a really general question, but why not set any and all MTU's to
their maximum values?  Are there system resources that get eaten up in doing
so?  It seems to me that it's "easier" to set the MTU at its highest value,
but I'm curious to know why it's not "better".

Thanks,

Mike


On 4/19/06 6:54 AM, "Johannes Resch" <jr at xor.at> wrote:

> sthaug at nethelp.no schrieb:
>>> Have you tried configuring IP MTU on SVI/L3 interface?
>> 
>> Won't work. This is a known limitation of using the 3750 for L3. It
>> works fine with large MTUs for L2, but not for L3.
> 
> with recent IOS, you can set "system mtu routing <x>" to set the L3 MTU
> for all routed ports/SVIs. particularly useful if you need L2 MTU >
> 1500, but want to keep L3 MTU at 1500 byte.
> 
> -jr
> 
> 
>> Steinar Haug, Nethelp consulting, sthaug at nethelp.no
>> 
>>> Tassos Chatzithomaoglou wrote:
>>>> 3750 switches are said to support mtu > 1500 (1500-9000) on all GE ports
>>>> according to this:
>>>> 
>>>> http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuratio
>>>> n_example09186a008010edab.shtml#c3
>>>> 
>>>> 3750#sh system mtu
>>>> 
>>>> System MTU size is 1500 bytes
>>>> System Jumbo MTU size is 1600 bytes
>>>> 
>>>> Routed port
>>>> -----------
>>>> 3750#sh int g1/0/10
>>>> GigabitEthernet1/0/10 is up, line protocol is up (connected)
>>>>    MTU 1600 bytes, BW 1000000 Kbit, DLY 10 usec,
>>>> 
>>>> 3750#sh ip int gi1/0/10
>>>> GigabitEthernet1/0/10 is up, line protocol is up
>>>>    MTU is 1500 bytes
>>>> 
>>>> SVI
>>>> ---
>>>> 3750#sh int g1/0/10
>>>> GigabitEthernet1/0/10 is up, line protocol is up (connected)
>>>>    MTU 1600 bytes, BW 1000000 Kbit, DLY 10 usec,
>>>> 
>>>> 3750#sh ip int vlan 99
>>>> Vlan99 is up, line protocol is up
>>>>    MTU is 1500 bytes
>>>> 
>>>> 
>>>> So you can't have L3 mtu > 1546 (= system mtu) if the GE ports are
>>>> configured as 
>>>> L3 (Routed/SVI) ports. Any idea how to overcome this?
>>>> 
>>>> --
>>>> Tassos
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