Encapsulation Ad Absurdum
It’s not uncommon to see _three _ or more recursive overlay encapsulations
E.g. K8s VXLAN overlay on a hypervisor-networking GENEVE overlay on a data-center VXLAN overlay
That’s triple the risk of getting your math wonky while calculating what the nominal IP interface MTU should be.
Illustrated
pending.
Failure to Implement PLPMTUD
- PLPMTUD solves all of the problems we’ve discussed
- But, only at the packetization-layer, not at the IP Layer.
- It has been part of the TCP implementation in Linux and Windows for years, but it is not enabled by default.
- If your device has a TCP implementation and it’s not doing PLPMTUD?
- Shame! Shame; shame; shame.
Jumbo Gumbo
- It’s a (not un-)reasonable practice to set the gateway’s IP MTU to 9000 when enabling jumbo frames on an Ethernet broadcast domain.
- It’s a (not un-)common practice to do this without requiring every host on the same subnet to:
- Have an L2 MRU of 9022(-ish) bytes
- Have an L3 MRU of 9000 bytes
- This creates a PMTUD blackhole if any of those hosts don’t happen to have L2/L3 MRUs at least that high