Nortel 8610 and MTU value

[ISISMGT]

we have two 8610 connected by IST (4 links) in default vlan (Id=1).

We connect stack of 470-48T by MLT on this core : 2 link activate by stack and all MLT belong set to default vlan.

The two 8610 have an MTU=1950 on all interfaces.
The MTU on the 470 stack is set to 1514 for all interfaces.

We directly connect an OSA card (OS390 IBM) in Fast Ethernet on the 8610.

The MTU on OSA is set to 576 (may be 1492 => I've not yet the information).

We have problems with this configuration : many traffic on the OSA Card and lot of ping are lost (*) when we try to ping the OSA card.

Or the response don't arrive in the time slot selected (option -w of ping, -w was used with 7500 => 7,5 s).

I think the "interface" MTU is too small on the OSA Card ?

Thus, we have connected the OSA card on the stack and it works well but I suppose another team have changed the MTU
ont the OSA card.

On this Forum, there was a previous thread (thread902-1077941 posted by AyrishGrl, problem betwenn Cisco and switch nortel 86xx) but I have not see the result of modify the MTU value on the 86xx side.

Can somebody explain me what is happening and the exact rule to hace a correct MTU between all the LAN (cisco router with FE 100 Mbps, 2 x 8610 and stack of 470-48T) ?

Thank You for your help.

Best Regards.

ISISMGT

PS :

Software release on the 8610 : 3.7.3
One vlan => default vlan ID=1

 

[netmanrick]

Cisco has a different default mtu than Nortel. Subtract 32 from the Nortel mtu to communicate with Cisco.
Nortel to Nortel should not be a problem.

Rick Harris
SC Dept of Motor Vehicles
Network Operations

 

[HungryHouse]

Rick,

I don't believe you can set the MTU to 32 less than the 1950 (1918) on the 8600. It can only be set to one of three values:
either: 1950, 1522, or 9600

And, unfortunately, it can not be set on an Interface level (yet)..it must be done system wide. (Yuck, I know).

HH

 

[Alainv]

I guess, even if you were able to change the mtu on the nortel, it won't make any difference.
The switches just transport the data, they won't add extra bits or bytes on the original packet.
Certainly with pings, if you don't change the size manually, it is way lower then 576.
Sure that is isn't a smlt connection problem?

 

[HungryHouse]

By the way, if anyone is wondering, Nortel technical support has responded to our inquiry/complaint by issuing an ER (enhancement request) in order to be able to change the MTU on an interface level. With the PP8600 supporting so many different networks and interfaces, this would hopefully be a no-brainer. Let's see if the product managers agree. I will post once I hear status of the results of the ER