[Bug 255749] jumbo frame support of IPv6 vlan interfaces is broken

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255749

            Bug ID: 255749
           Summary: jumbo frame support of IPv6 vlan interfaces is broken
           Product: Base System
           Version: 13.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs at FreeBSD.org
          Reporter: topical at gmx.net

If you have an interface with jumbo frame support enabled and create a vlan,
mtu is not inherited by its IPv6 vlan interface. IPv4 is unaffected.

Setup: change mtu of an interface to 9k, create a vlan sub interface and
(optionally) activate it:

  # ifconfig vtnet0 mtu 9000
  # ifconfig vtnet0.1000 create
  # ifconfig vtnet0.1000 up

Check state with "ifconfig"

  vtnet0: flags=8863<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 9000
  ...
  vtnet0.1000: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu
9000

Check ipv6 mtu:

  # ndp -i vtnet0.1000

  linkmtu=0, maxmtu=1500, curhlim=64, basereachable=30s0ms, reachable=19s,
retrans=1s0ms
  ...

You see that "maxmtu=1500". So, this host never uses IPv6 with Jumbo frames on
this vlan.

To make things worse: Mellanox cards don't like that scenario and have massive
packet loss (but this is another story).

To fix it, you need to temporarily change mtu of vlan interface

  # ifconfig vtnet0.1000 mtu 8000
  # ifconfig vtnet0.1000 mtu 9000
  # ndp -i vtnet0.1000

  linkmtu=0, maxmtu=9000, curhlim=64, basereachable=30s0ms, reachable=19s,
retrans=1s0ms

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