BFD control packets are encapsulated in UDP packets, and the destination port number for multi-hop BFD control packets is 4784. This is a standardized port for multi-hop BFD operation .

IS-IS uses a two-level hierarchy:
Level-1 routersoperate within a single area and cannot exchange routing information with routers in other areas directly.
To reach external areas (the IS-IS backbone or other areas), Level-1 routers rely onLevel-1-2 routers, which perform inter-area route redistribution.
Therefore, a Level-1 routermust use a Level-1-2 routeras a gateway to reach destinations outside its own area.
Reference:Huawei HCIP-Datacom-Core Technology Study Guide(Chapter: IS-IS Fundamentals - Section:
Routing Levels and Area Structure)

Comprehensive and Detailed Step-by-Step Explanation:
* Packet Filtering Firewall:
* A packet filtering firewall inspects packets at the network and transport layers based on predefined rules (e.g., ACLs).
* Correct Statements:
* Option B: Packet filtering is performed on a per-packet basis.
* Option D: ACLs define the filtering rules for traffic.
* Incorrect Statements:
* Option A: Packet filtering firewalls do not analyze associated packets. For this, a stateful firewall is required.
* Option C: Application-layer inspection is not supported. This functionality is provided by application-layer firewalls.
References:
* HCIA-Datacom Study Guide, Chapter: Firewalls
* Huawei Firewall Packet Filtering Configuration

The security policy specifies that traffic originating from12.1.1.2and destined for the untrust zone is permitted. Since the source address of the packet (12.1.1.1) does not match this rule, thepacket matches the default implicit deny rule. However, rule 1 does not deny all other traffic explicitly, so the packet is forwarded based on further configurations .

R3, as the DIS in the Level-2 domain, periodically sends Complete Sequence Number Protocol Data Units (CSNPs) to ensure LSDB synchronization among Level-2 routers.
R1 and R2 are in a Level-1 domain, and IS-IS requires the receiving router (R2) to acknowledge an LSP from R1 by sending a Partial Sequence Number Protocol Data Unit (PSNP).
IS-IS LSPs in a broadcast network are sent using multicast, not unicast, making option C incorrect.
For Level-2 LSPs, acknowledgments are not provided using PSNP by R3; CSNPs are sufficient for synchronization .