This issue will serve as an umbrella issue for tracking the current gaps in the code for full VLAN and adjacent technologies. As there is quite a lot of implications and code sections that are affected I'll gradually fill the issue with bullet points.

• Fix VLAN TCI bit handling in ethernet.VLANTag
  • Correct PCP, DEI, and VID extraction to match 802.1Q layout.
  • Add constructor/validation for VID 1–4094, PCP 0–7, DEI.
• Add VLAN tag mutation helpers
  • Add zero/allocation-conscious helpers to insert and remove 802.1Q tags in raw Ethernet frame buffers.
  • Cover length checks, short buffers, existing tags, and preservation of inner EtherType.
• Expand VLAN EtherType support
  • Treat both customer VLAN 0x8100 and service VLAN 0x88a8 consistently where appropriate.
  • Decide whether service VLAN/QinQ is supported, rejected, or parsed as a known-but-unsupported case.
• Improve Ethernet frame VLAN parsing
  • Make header length, payload slicing, validation, and inner EtherType handling robust for tagged frames.
  • Avoid dispatching VLAN frames by outer EtherType when the inner EtherType is what protocol handlers need.
• Add VLAN-aware Ethernet stack support
  • Add a StackEthernetVLAN or equivalent API for a local stack participating in a configured VID.
  • Ingress should filter by MAC and VID, then demux by inner EtherType.
  • Egress should emit Ethernet + VLAN headers and preserve MTU/CRC/min-frame behavior.
• Add multi-VLAN / trunk local stack design
  • Decide whether the local CPU stack supports one VID per stack, multiple VIDs, or a trunk-style abstraction.
  • Needed for managed-switch CPU-port use cases.
• Add VLAN-aware forwarding database
  • Add MAC learning keyed by (MAC, VID).
  • Support dynamic entries, static entries, lookup, aging, flush-by-port, flush-by-VLAN or equivalent management operations.
  • Keep forwarding hot paths allocation-free.
• Add egress tag policy support
  • Track whether each VLAN exits each port tagged, untagged, or dropped.
  • Provide clear caller contract for whether the switch mutates frames itself or returns egress actions for the caller to apply.
• Improve PCAP/debug capture VLAN decoding
  • Decode VLAN tag fields explicitly.
  • Continue parsing the inner EtherType instead of treating the remainder as opaque VLAN payload.
  • Consider nested-tag behavior: unsupported, single-level only, or recursive.
• Add VLAN-focused tests
  • Unit tests for tag construction/extraction.
  • Frame insert/remove tests.
  • Tagged ingress/egress stack tests.
  • FDB learning/aging/static-entry tests.
  • Switch access/trunk/PVID/membership/flooding tests.
  • PCAP inner-EtherType decode tests.
  • Negative tests for invalid VID, malformed/truncated frames, and unsupported service/QinQ cases.
• Add VLAN docs/examples
  • Document supported VLAN scope and explicitly unsupported items.
  • Add examples for a tagged local stack and a small access/trunk switch setup.
• Decide scope for "advanced" VLAN features
  • QinQ / stacked VLANs.
  • STP/RSTP.
  • LACP.
  • GVRP/MVRP.
  • 802.1X integration.
  • PCP/DEI QoS/drop behavior.
  • Multicast group management beyond flooding/static FDB entries.