docs: bridge: add VLAN doc

Add VLAN part for bridge document.
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Documentation/networking/bridge.rst

@@ -148,6 +148,35 @@ called by the kernel when STP is enabled/disabled on a bridge
 stp_state <0|1>``).  The kernel enables user_stp mode if that command returns
 0, or enables kernel_stp mode if that command returns any other value.
 
+VLAN
+====
+
+A LAN (Local Area Network) is a network that covers a small geographic area,
+typically within a single building or a campus. LANs are used to connect
+computers, servers, printers, and other networked devices within a localized
+area. LANs can be wired (using Ethernet cables) or wireless (using Wi-Fi).
+
+A VLAN (Virtual Local Area Network) is a logical segmentation of a physical
+network into multiple isolated broadcast domains. VLANs are used to divide
+a single physical LAN into multiple virtual LANs, allowing different groups of
+devices to communicate as if they were on separate physical networks.
+
+Typically there are two VLAN implementations, IEEE 802.1Q and IEEE 802.1ad
+(also known as QinQ). IEEE 802.1Q is a standard for VLAN tagging in Ethernet
+networks. It allows network administrators to create logical VLANs on a
+physical network and tag Ethernet frames with VLAN information, which is
+called *VLAN-tagged frames*. IEEE 802.1ad, commonly known as QinQ or Double
+VLAN, is an extension of the IEEE 802.1Q standard. QinQ allows for the
+stacking of multiple VLAN tags within a single Ethernet frame. The Linux
+bridge supports both the IEEE 802.1Q and `802.1AD
+<https://lore.kernel.org/netdev/1402401565-15423-1-git-send-email-makita.toshiaki@lab.ntt.co.jp/>`_
+protocol for VLAN tagging.
+
+`VLAN filtering <https://lore.kernel.org/netdev/1360792820-14116-1-git-send-email-vyasevic@redhat.com/>`_
+on a bridge is disabled by default. After enabling VLAN filtering on a bridge,
+it will start forwarding frames to appropriate destinations based on their
+destination MAC address and VLAN tag (both must match).
+
 FAQ
 ===