[c-nsp] 3750, QinQ & Jumbo Frames?

[Tassos Chatzithomaoglou]

Howard,

3750Gs usually do not have any 10/100BaseT ports. They have 10/100/1000BaseT and SFP ones.

As long as the port is 10/100/1000, the actual speed that is running (10 or 100 or 1000) doesn't have any effect on the MTU.

So you can have it running in 10/100 and use the full mtu.

3750G>sh int gi1/0/22 | i media|MTU
   MTU 1600 bytes, BW 100000 Kbit, DLY 100 usec,
   Full-duplex, 100Mb/s, media type is 10/100/1000BaseTX


Also, as you can see below, you can go up to 1998 for FastEthernet ports on 3750Gs, which should be more than enough for 
most applications:

3750G(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

3750G(config)#system mtu jumbo ?
   <1500-9000>  Jumbo MTU size in bytes

--
Tassos

Howard Jones wrote on 15/10/2008 4:34 μμ:
> We're just looking at running QinQ over a network of 3750G switches, and
> while I was investigating enabling jumbo frames, I came across this
> document:
>   
> http://www.cisco.com/en/US/products/hw/switches/ps700/products_configuration_example09186a008010edab.shtml#c3
> which contains:
> "*Note: *If Gigabit Ethernet interfaces are configured to accept frames
> greater than the 10/100 interfaces, jumbo frames that ingress on a
> Gigabit Ethernet interface and egress on a 10/100 interface are dropped."
> 
> It seems to me that I want to have jumbo frames enabled on the trunks to
> 'make room' for the extra dotq header, but if that means that user ports
> can't run at 10/100 that seems like a severe limitation. Also, the
> document is from 2005, and doesn't mention QinQ at all.
> 
> Is this still really the case with a modern 3750G? Is there anything
> that can be done?
> 
> There's a sort-of middle area where actually the frame that leaves the
> 10/100 port would have had the extra layer of dotQ stripped off... is
> that what I'm missing in the above?
> 
> Thanks in advance for any light anyone can shed.
> 
> Howie
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