MTU settings with VLAN tag

Installing and Using OpenWrt Network and Wireless Configuration
1

Hey guys

I have a question regarding MTU settings and VLAN tags. I currently have Bell WTTH internet which requires a VLAN tag for internet. (35 if anybody is ever in the same boat lol). Decided to play with MTU the other day. Ran some ping tests and discovered my largest unfragmented package is 1412. Correct me if I'm wrong but then you are supposed to add 28 to that for headers which takes me to 1440. However with the VLAN tag then does that make it 1440 or 1444?

I tried setting it to 1440 the other day and everything seemed good for about 24 hours. Then I dropped internet. Interface still showed a public WAN IP but nothing was passing which makes it sound like the VLAN tag was dropped. But why would that happen after 24 hours? I set it back to 1500 for now. Not like it doesn't work that way I just wanted to play and now it's bugging me that I can't figure it out lol.

I've seen other posts regarding internal MTU and VLAN but couldn't find anything regarding this specifically. And I guess the second question is will I really gain anything by doing this? If internally I'm still sending larger packets then does it make sense to fragment them for internet traffic?

2
3

Fair enough, at least that means it should be worth it lol. So that then leaves me with the original question of what IS my optimal MTU?

4

You can use ping as mentioned above to determine the optimal MTU.
However, it depends on the destination for each particular connection/packet:

PMTUD relies on ICMP to notify clients to decrease MTU when necessary.
So, make sure to allow ICMP traffic for both input/forward and zone-to-zone forward.

5

Sorry to keep bugging, just like information lol.

Couldn't get tracepath to work however I used traceroute with the --mtu switch and even upping my MTU in the router still showed F=1440 on the first connection out. So does that mean I set my WAN MTU on the router at 1440 or at 1444 to allow for the VLAN tag? I tried both but the F value didn't change in either case so I'm thinking 1440 is the proper setting.

Appreciate all the help!