[c-nsp] Giants and mtu-size

[Arne Larsen / Region Nordjylland]

Hi Mikael

Here are two interface config that peers.


interface GigabitEthernet1/15
 description B: Uplink to MPLS aasnxc2 gi5/1
 mtu 9216
 ip address xxxxxxxxxxxxxxxxx
 ip router isis
 load-interval 30
 tag-switching mtu 1546
 tag-switching ip


interface GigabitEthernet5/1
 description B: Uplink to MPLS aasnxt1 gi1/15
 mtu 9216
 ip address xxxxxxxxxxxxxxxxxxxxx
 ip router isis
 load-interval 30
 tag-switching mtu 1546
 tag-switching ip

/Arne

-----Oprindelig meddelelse-----
Fra: Mikael Abrahamsson [mailto:swmike at swm.pp.se]
Sendt: 31. oktober 2008 08:01
Til: Arne Larsen / Region Nordjylland
Cc: cisco-nsp at puck.nether.net
Emne: Re: [c-nsp] Giants and mtu-size

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