[c-nsp] Giants and mtu-size

[Mikael Abrahamsson]

On Fri, 31 Oct 2008, Arne Larsen / Region Nordjylland wrote:

> I know that giants is packets that are discarded because they exceed the 
> medium's maximum packet size. But how can I verify the right mtu size.

Please share MTU-related config from both ends and we'll be able to help.

Short story:

Set MTU to whatever you want the highest MTU to be (normally what you want 
MPLS to use). Set clns mtu (if you're running ISIS) and ip mtu if other 
end needs it.

So, for instance if you're running a LAN with mixed MPLS enabled devices 
and some not, and some only supports 1500 ip mtu:

int gi1/0
mtu 1546
ip mtu 1500
clns mtu 1497
ipv6 mtu 1500

As far as I have been able to discern from the cisco docs (and after being 
pointed out to me actually on this list), the only time "mpls mtu" 
(tag-switching mtu) should be used is on old 7200 FE based ports.

There are platforms with cosmetics bugs where all jumbos are counted as 
giants in interface counters, you might want to check the bug database.

-- 
Mikael Abrahamsson email: swmike at swm.pp.se