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

[Julien Goodwin]

On 29/05/12 15:41, CiscoNSP_list CiscoNSP_list 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?

With switches doing L2 only it's worth making a difference between L3
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).

There's a few things to remember:
- All L3 devices on a jumbo VLAN must share the same MTU
- Any L2 devices can use a larger MTU
- This may not be the same number in the configuration (eg, IOS vs IOS
XR/JunOS)
- Many devices can't fragment in the fast path, so make sure path MTU
discovery is working

You also should standardise what jumbo means in your enviroment, and
standardise on two/three values (1500, jumbo, and jumbo+QinQ/MPLS). This
makes it much easier to audit and document.


-- 
Julien Goodwin
Studio442
"Blue Sky Solutioneering"

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