A vlan's whole MTU over a front port, on the XGS 136 and both XGS 3300s, 2026-11-06
1052: on an XGS 136 running
v0.1.0-alpha.12, four vlan(4) interfaces over the front port npup8 took an MTU of 1,496, not 1,500;
ifconfig npup8 mtu 1504 was refused with "Operation not supported", vlanmtu changed nothing, and a
TLS handshake whose first segment was full-size stalled. Measured on the lab on 2026-11-06 (UTC): the
port's refusal is FreeBSD's for a member of a bridge, not the driver's; the coprocessors of both
families carry a tagged frame of 1,518 bytes, and of 9,018; and the four bytes went because neither
driver offered VLAN_MTU. With VLAN_MTU on every front port, a vlan over a port of 1,500 takes 1,500,
and its full-size frames cross both ways - on the XGS 136 and on both XGS 3300s, at 1,500 and at
9,000.
The whole page, with how it was taken and what it does not show: A vlan's whole MTU over a front port, on the XGS 136 and both XGS 3300s, 2026-11-06.
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