[j-nsp] Best practice MTU?

[Mark Tinka]

On Friday, April 27, 2012 04:56:02 PM Colin Whittaker wrote:

> 9000 for IP mtu provided to end users / customers is a
> nice round number.
> 
> I have started using 9100 as the internal mtu as it
> leaves 100 bytes for any encap overhead you might want
> from mpls/gre/etc and is easy to remember.

I've always limited vendors to just two, Cisco and Juniper. 

As all the Juniper platforms we've worked with top out at 
9,192 bytes, that's what we've set both the Juniper and 
Cisco kit to (despite the fact that newer Cisco hardware can 
go up to 9,216 bytes).

Junos and IOS XR are in sync. re: how MTU is handled. 
Junos/IOS XR and IOS/IOS XE are out of sync., so this needs 
to be corrected when deploying.

The biggest issue is when you have earlier, low-to-mid range 
Cisco switches that can go no higher than 9,000 bytes if 
you're looking to have the same MTU value supported edge-to-
edge. Newer mid-range Cisco switches (like the 3560-X/3750-
X) now support 9,216 bytes as well.

Also be mindful of older Cisco 100Mbps Fast-E ports that can 
only support 1,500 - 1,988 bytes. Juniper's Fast-E ports 
don't have this issue, however, nor do the Fast-E ports that 
you can install in Cisco's XR 12000/CRS routers (the SPA's).

Mark.