[c-nsp] TCP Throughput / MTU problem ? with Cisco 7304 and MPLS VPN's

[Peter Rathlev]

On Sat, 2008-01-12 at 08:30 +0100, Mikael Abrahamsson wrote:
> On Sat, 12 Jan 2008, Peter Rathlev wrote:
> > But you still can't (shouldn't) use an interface MTU lower that the MPLS 
> > MTU. Your interface MTU should be larger than the "X" MTU for any "X".
> 
> Why?

As Łukasz writes, they're changing the commands in newer IOS releases.
That's one argument for doing it.

In my head, the interface MTU is the maximum data link frame size
without the data link headers. IP/MPLS/CLNS/whatever packets are
encapsulated in a data link header + tail, and the resulting packet
should fit within the data link maximum frame size.

The "show controllers <int>" shows a "max_mtu" that seems to follow
interface MTU with room for headers. It says "max_mtu = 1528" on my 1500
byte interface MTU Ethernet interface. Adding a few bytes to interface
MTU changes the "max_mtu". On 12.3(6) I can't adjust IP MTU up unless I
change interface MTU:

Router(config)#interface GigabitEthernet0/2
Router(config-if)#do show interface GigabitEthernet0/2 | incl MTU
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec, 
Router(config-if)#ip mtu ?
  <68-1500>  MTU (bytes)

Router(config-if)#mtu 1520
Router(config-if)#do show interface GigabitEthernet0/2 | incl MTU
  MTU 1520 bytes, BW 1000000 Kbit, DLY 10 usec, 
Router(config-if)#ip mtu ?
  <68-1520>  MTU (bytes)

Router(config-if)#

So to sum it up: Having the interface MTU lower than any of the Layer 3
(or 2.5) protocol MTUs is possible and often works, but it's not
"right".

Regards,
Peter