Do I need specific hardware that supports VLAN tagging? If so, what is required? A specific NIC chipset?
YES, and YES. you can find the supported drivers by searching the manual. I searched by googling (manufacturers name and openBSD, but I will admit the information was all over the place).
OSes can implement VLAN tagging in software
~ They can... but only with supporting drivers for the hardware.
It looks like at this point OpenBSD has a lot of driver support for different manufacturers NIC's. They don't quite show all the VLAN hardware support in one list, but rather in each individual driver section in the manual. I gathered a couple searches from the chip-set drivers in the manual as an example.
The VLAN driver itself in openBSD has a caveat section that states:
Some Ethernet chips will either discard or truncate Ethernet frames that are larger than 1514 bytes. This causes a problem as 802.1Q and 802.1ad tagged frames can be up to 1518 bytes. Most controller chips can be told not to discard large frames and/or to increase the allowed frame size.
em — Intel PRO/1000 10/100/Gigabit Ethernet device
The em driver provides support for PCI, PCI-X and PCI Express Gigabit Ethernet adapters based on the Intel 82540EM, 82540EP, 82541EI, 82541ER, 82541GI, 82541PI, 82542, 82543GC, 82544EI, 82544GC, 82545EM, 82545GM, 82546EB, 82546GB, 82547EI, 82547GI, 82562V, 82563EB, 82564EB, 82566DC, 82566DM, 82571EB, 82571GB, 82572EI, 82572GI, 82573E, 82573L, 82573V, 82574L, 82575EB, 82575GB, 82576EB, 82577LC, 82577LM, 82578DC, 82578DM, 82579LM, 82579V, 82580DB, 82580EB, 82583V, I210, I211, I217, I218, I219, I350, I354 (Even longer list of products omitted)
The Intel Pro drivers support:
The em driver supports IPv4 receive IP/TCP/UDP checksum offload and transmit TCP/UDP checksum offload on all but 82542-based adapters
VLAN tag insertion and stripping, and jumbo frames on all but 82562V, 82566DC/82566DM and 82573E/82573L/82573V-based adapters.
- The 82562V Ethernet controller chip only supports 10/100 media types.
- (Known bugs) There are known performance issues with this driver when running UDP traffic with jumbo frames.
There is a known compatibility issue where time to link is slow or link is not established between 82541/82547 controllers and some switches. Known switches include:
I-O Data ETG-SH8
re — Realtek 8139C+/8169/816xS/811xS/8168/810xE 10/100 /1000 Ethernet devices
The re driver provides support for various NICs based on the Realtek RTL8139C+, RTL8169, RTL816xS, RTL811xS, RTL8168 and RTL810xE PCI and PCI Express Ethernet controllers, (Lots of products omitted)
NICs based on the 8139C+ and 810xE are capable of 10 and 100Mbps speeds over CAT5 cable. NICs based on the 8169, 816xS, 811xS and 8168 are capable of 10, 100 and 1000Mbps operation.
All re NICs support IPv4 transmit/receive IP/TCP/UDP checksum offload, VLAN tag insertion and stripping, and use a descriptor-based DMA mechanism.
The 8102E(L), 8103E(L), 8168C/8111C, 8168D/8111D, 8168DP/8111DP and 8168E/8111E chips also support IPv6 receive TCP/UDP checksum offload.
Nvidia chipsets (nfe)
nForce, nForce 2, nForce 3, CK804, MCP04, MCP51, MCP55, MCP61, MCP65, MCP67, MCP73, MCP77, MCP79 and MCP89 Ethernet controller chips.
The nfe driver supports:
- IPv4 transmit/receive IP/TCP/UDP (no listed ipv6)
- checksum offload with the nForce 3, CK804, MCP55, MCP77, MCP79 and MCP89 chipsets and VLAN tag insertion and stripping with the MCP55 chipset. (so only the MCP55 will work with VLANs)
The nfe driver additionally supports Wake on LAN (WoL).
As you can see, different cards/models have varying levels of support, and some do not support vlans. you may want to look into what is actually supported in the manual for vlans via the official source before you buy hardware. My opinion, go with Intel chip-sets, they are well supported, reasonably priced, and rock solid for every application I've used them with.
information was copied from https://man.openbsd.org/
the emphasis is my own.