A client of mine has an organizational network that is comprised of a lot of Ubiquity gear, primarily switches and wireless routers to power their network, with a Sophos XG firewall at the border.
Every once in a while, on the network we provide for 'guest' / 'partner' access, we have someone come in with a laptop that is having problems with 'network speed' and it being slow or nonresponsive for them while connected. This is on an isolated VLAN from the rest of the corporate network and runs on a separate line. This VLAN lives only on the switch and the Router, and we have NUMEROUS devices that work fine, but every once in a while have someone coming by with an issue.
The IT people at this client's site have a tendency to say "Oh, the system is using MTU 1500, this is causing packet fragmentation, you SHOULD be using
1492 instead of
1500." and then say "that's the only way to fix this problem."
My understanding of networking infrastructure (as a network security and design person by trade) and my knowledge of how networks work at the system levels is that the MTU of 1500 is NOT usually the leading cause of network speed / instability problems on wifi networks, because the MTU is the max packet size not the max payload size and the various systems' network stacks should be fragmenting packets which have a payload size that, with packet size overhead, would produce a packet larger than 1500, and in turn would fragment the packets. And that the system sees an MTU of 1500 and has a different thing - an MSS - for determining how to fragment packets based on payload size.
I'm fairly certain that changing the MTU should be the LAST thing that a helpdesk or general support staff should be doing, and instead additional tests on configurations, etc. on the system itself should come first.
Is my knowledge in this correct, or am I confused? It's my understanding that 1500 is the default MTU on every off-the-shelf network device, and should NOT be assumed to be the sole/only cause of issues and probably should NOT be the first thing that IT support staff point fingers at as the problem.