[c-nsp] ME3600X MTU Notes

[Leigh Harrison]

Hello Group,

Regarding the issue we had with MTU sizes, we implemented a change last night which looks to have cleared the problem. 

Layout is like this:

Server --- L3 switch --- ME3600X --mpls-- 3750ME --L2-- Fortigate --- Internet 

MTU sizes were as follows:
Server --- L3 switch - 1500
L3 Switch --- ME3600X - 1500
ME3600X --mpls-- 3750ME - 1500 on 3600, 9000 on 3750
3750ME --L2-- Fortigate - 1500
Fortigate --- Internet - 1500

Note: the 3750ME switches on older software have a system and jumbo MTU of 1500. After upgrading to the latest code, the 4 gig ports get a jumbo MTU of 9000. 
Note: The ME3600X and the 3750ME are directly connected. 

Issue: Server can ping Internet, but not browse websites when server MTU is set to 1500. After altering server MTU to 1496, web pages load as expected. 

Solution: After the groups advise and various testing, we increased the MTU of the physical ports that connect from the ME3600X down to the 3750ME to 1600. All ME3600X ports are set to an MTU 1500 out of the box, including the ten gig ports. 

This appeared to clear up the issue straight away and normal browsing was back as expected.

Thanks for the advise. 

Leigh