When configuring extended ACLs on Cisco devices, which of the following statements are true? (Select all that apply.)
Correct Answer: A,B,C
Detailed Explanation: Extended Access Control Lists (ACLs) are powerful tools used in Cisco networks to filter traffic based on a wide range of criteria. Unlike standard ACLs—which filter only based on source IP addresses—extended ACLs can examine source and destination addresses, protocols (such as TCP, UDP, or ICMP), and even specific port numbers. This granularity makes them ideal for scenarios where precise control over traffic flow is required. A. Granular Filtering: Extended ACLs provide the ability to specify rules that match on various fields in the IP header and even in the transport layer headers. For example, an administrator can configure an ACL to allow HTTP traffic (TCP port 80) from a specific source to a specific destination while denying other types of traffic. This level of control (Option A) is a key advantage of using extended ACLs. B. Sequential Processing: When an ACL is applied to an interface, the router or switch processes the ACL entries sequentially from the top down. This means that once a packet matches a rule, no further rules are evaluated. Therefore, the order of statements in an ACL is critical. Misordering can lead to unintended matches and results (Option B). Administrators must plan the ACL rule order carefully to ensure that the most specific rules are evaluated before more general ones. C. Inbound and Outbound Application: Extended ACLs can be applied in both inbound and outbound directions on an interface. An inbound ACL filters packets as they enter the interface, while an outbound ACL filters packets as they exit the interface. This flexibility allows network administrators to control traffic flow in both directions based on the requirements of the network design (Option C). D. Implicit Deny Statement: Cisco ACLs have an implicit "deny all" rule at the end of the ACL. This means that if a packet does not match any of the configured permit statements, it will be dropped by default. There is no need to configure an explicit permit statement at the end of the ACL to allow unmatched traffic; in fact, doing so would defeat the purpose of the ACL if the intention is to filter traffic. Therefore, Option D is incorrect because it states that a final permit is required, which is not the case. Instead, best practice is to explicitly permit traffic only when it is required, and let the implicit deny handle all other traffic. Importance in Network Security: Extended ACLs are a cornerstone in network security because they allow administrators to enforce detailed traffic policies. By carefully crafting and ordering ACL rules, you can ensure that only legitimate traffic is allowed while undesired or potentially harmful traffic is blocked. This is essential for protecting network resources, managing bandwidth, and preventing unauthorized access. Conclusion: Extended ACLs provide granular control (Option A), process entries sequentially (Option B), and can be applied in both directions (Option C). The common misconception that a final permit is required (Option D) is incorrect due to the presence of an implicit deny rule at the end of every ACL. Understanding these principles is essential for configuring effective ACLs and ensuring network security.
This CCNA practice question helps students prepare for Cisco networking certification exams by testing knowledge of network fundamentals, routing, switching, and network security concepts.