[c-nsp] setting max mtu on switch (Jumbo)

[Saku Ytti]

On (2012-05-29 16:09 +1000), Julien Goodwin wrote:

> > Are there any "issues" with setting mtu at 9000? (We have a mix of routers/switches some of which only support mtu of 1500), so is adding a new switch(or adjusting an existing switch(3560+2960)) going to cause issues if other connecting devices mtu is smaller?

> interface MTU and L2 transit MTU. There's a bunch of switches where the
> two can be different (I remember one switch, can't remember from who,
> doesn't allow L3 above 1500, but L2 can be 9k).

The aforementioned switches are another example of such. They allow only
1998B L3 MTU (fixed in E and X models).

> There's a few things to remember:
One more thing is, that in some platforms like GSR, enabling jumbos can
affect how memory is allocated, causing there to be less memory for
'normal' sized frames than previously.

However generally, unless other information is available, BCP is to set L2
as high as it goes, and L3 ends of L2 link at minimum L2 link or less (less
to achieve standardization of L3 MTU).

-- 
  ++ytti