Cisco Certified Network Associate (CCNA) Practice Exam

What transport layer protocol should be used to match SMTP traffic in an ACL?

1. UDP

2. TCP

3. ICMP

4. AH

The correct answer is: TCP

The correct choice is TCP because the Simple Mail Transfer Protocol (SMTP) operates over the Transmission Control Protocol (TCP). SMTP is responsible for sending and receiving emails, and it typically uses TCP port 25 for its communication. TCP is a connection-oriented protocol, which means it establishes a reliable connection between the sender and receiver before data can be transmitted. This reliability is crucial for email communication, where packet loss could mean that email messages do not arrive correctly or at all. In contrast, the other protocols listed serve different purposes. UDP (User Datagram Protocol) is connectionless and is typically used for applications where speed is more critical than reliability, such as video streaming or online gaming, making it unsuitable for matching SMTP traffic. ICMP (Internet Control Message Protocol) is used primarily for diagnostic and control purposes, such as ping operations, rather than for data transmission like email. AH (Authentication Header) is an extension of the IP protocol for providing authentication and integrity but does not specifically deal with the transport of data such as SMTP does. Thus, for configuring an Access Control List (ACL) to match SMTP traffic, TCP is the appropriate protocol to specify.