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: b...@freebsd.org
Reporter: topi...@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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