A stateful inspection firewall tracks the state of active connections. If a packet matches an existing session in the firewall's state table, it is allowed to pass. The diagram indicates that the TCP packet matches the session state, so the firewall forwards it .

Comprehensive and Detailed In-Depth Explanation:
* Intermediate System to Intermediate System (IS-IS) routers send IS-to-IS Hello (IIH) packets to establish neighbor relationships.
* The Reserved/Circuit Type field in IIH packets indicates which level of IS-IS adjacency the router is trying to establish:
* 00 # Level-0 (Not used in IS-IS).
* 01 # Level-1 router.
* 10 # Level-2 router.
* 11 # Level-1-2 router (Both levels supported).
* Since the question asks for the field value in an L1-L2 router's IIH packets, the correct answer is 11 (B).
# Reference: Huawei HCIA-Datacom Study Guide, IS-IS Neighbor Discovery and IIH Packet Format.

BGP Open Message Components
* The Open message contains the following critical parameters:
* Local AS Number: The autonomous system of the router.
* Router ID: A unique identifier for the router.
* Hold Time: The maximum time the router will wait for Keepalive or other messages from its peer.
Incorrect Option
* C. NLRI: Network Layer Reachability Information is not included in Open messages; it is carried in Update messages.
HCIP-Datacom-Core Reference
* The structure and contents of Open messages are explained in BGP protocol details.

The preference of routing protocols determines the selection order of routes when a router receives multiple routes to the same destination from different routing protocols. For instance, OSPF has a higher preference over RIP, so OSPF routes will be selected first. Huawei routing preferences confirm this behavior .

Comprehensive and Detailed In-Depth Explanation:
* SSH (Secure Shell) provides secure remote login and data encryption over insecure networks.
* A. TRUE # SSH requires user authentication before access is granted.
* B. TRUE # Enabling SSH opens listening ports, which could be scanned by attackers. Proper security measures (e.g., disabling unused SSH access) are recommended.
* C. TRUE # ACLs can be applied to VTY (Virtual Terminal) lines to restrict SSH access based on IP addresses.
* D. FALSE # The default SSH server port is 22, not 23. Port 23 is used for Telnet, which is less secure than SSH.
# Reference: Huawei HCIA-Datacom Study Guide, SSH Configuration and Security Measures.