After Layer 3 roaming, the STA's traffic is forwarded from the Foreign AP (FAP) to the Foreign AC (FAC), and then to the Home AC (HAC) before reaching the upper-layer network. This ensures that the STA remains on the same subnet without breaking existing sessions .

GRE (Generic Routing Encapsulation) is not a Layer 2 VPN technology. Instead, it is a Layer 3 tunneling protocol used to encapsulate a wide variety of network layer protocols inside point-to-point connections. GRE is commonly used for creating VPN tunnels across IP networks, allowing for the transport of various types of payloads. This misunderstanding about GRE being a Layer 2 technology contradicts its definition and typical application .

In asynchronous mode, BFD control packets are exchanged periodically to detect faults.
In demand mode, once the session is established, BFD stops sending periodic control packets and relies on external mechanisms to verify connectivity.
Asynchronous mode is the most commonly used mode. However, the Echo function is supported in asynchronous mode, making Option B incorrect .

RPF (Reverse Path Forwarding) checks are used to prevent multicast loops. An RPF check uses theunicast routing table(ormulticast static routes, if configured) to determine the correct incoming interface for a multicast packet.
Multicast protocols like PIM use the RPF mechanism, and the unicast routing table is the primary reference.
Multicast dynamic routing protocolsdo not maintain their own separate tablesfor RPF by default.
Reference from Huawei HCIP-Datacom-Core Technology Study Guide:
"RPF checks are performed using the unicast routing table or multicast static routes. The interface through which the multicast source is reachable is considered valid for RPF." (Chapter: Multicast - Section: RPF Principle)

Explanation:
The sequence of steps for electing the root port in an STP (Spanning Tree Protocol) network is as follows:
* Bridge ID Comparison: The Bridge ID (BID) is compared between the bridges in the network. A smaller value indicates a higher priority, meaning the bridge with the lowest Bridge ID is elected as the root bridge.
* RPC (Root Path Cost) Comparison: The path cost to reach the root bridge is calculated. The router with the lowest Root Path Cost (RPC) to the root bridge will have a higher priority for the election of the root port.
* Peer BID Comparison: If there is a tie in the Root Path Cost, the Peer BID is compared. A smaller Peer BID indicates a higher priority. This step ensures that if two routers have the same RPC, the one with the lower Peer Bridge ID wins.
* Local BID Comparison: If there is still a tie, the Local BID is compared. A smaller Local BID indicates a higher priority. This final step ensures that the router with the lowest local identifier is selected.
* Bridge ID Comparison:The first step in electing the root port is comparing the Bridge IDs. The bridge with the lowest Bridge ID becomes the root bridge. The Bridge ID is made up of the bridge priority and MAC address. The root bridge is the center of the network for STP, and all other ports will calculate their paths based on this root.