Security designs should consider a layered approach to address or
protect against a specific threat or to reduce a vulnerability. For example, the use of a packet-filtering router in conjunction with an application gateway and an intrusion detection system combine to increase the work-factor an attacker must expend to successfully attack the system.
The need for layered protections is important when commercialoff-
the-shelf (COTS) products are used. The current state-of-the-art for
security quality in COTS products do not provide a high degree of protection against sophisticated attacks. It is possible to help mitigate this situation by placing several controls in levels, requiring additional
work by attackers to accomplish their goals.
Source: NIST Special Publication 800-27, Engineering Principles for Infor- mation Technology Security (A Baseline for Achieving Security).

File Transfer Protocol (FTP) is the protocol that is used to facilitate file transfer between two machines. TFTP has no directory browsing capabilities. SMTP is generally used to send messages from a mail client to a mail server. Telnet's primary function is terminal emulation.
Source: KRUTZ, Ronald L. & VINES, Russel D., The CISSP Prep Guide: Mastering the
Ten Domains of Computer Security, 2001, John Wiley & Sons, Page 88.

Individuals are authorized access to resources on a network through
specific paths and the enforced path prohibits the user from accessing a resource through a different route than is authorized to that particular user. This prevents the individual from having unauthorized access to sensitive information in areas off limits to that individual. Examples of controls to implement an enforced path include establishing virtual private networks (VPNs) for specific groups within an organization,
using firewalls with access control lists, restricting user menu options, and providing specific phone numbers or dedicated lines for remote access. Answer a is a distracter. Answer c, security perimeter, refers to the boundary where security controls are in effect to protect assets.
This is a general definition and can apply to physical and technical
(logical) access controls. In physical security, a fence may define the security perimeter. In technical access control, a security perimeter can be defined in terms of a Trusted Computing Base (TCB). A TCB is the total combination of protection mechanisms within a computer system.
These mechanisms include the firmware, hardware, and software
that enforce the system security policy. The security perimeter is the
boundary that separates the TCB from the remainder of the system. In
answer "A trusted path" a trusted path is a path that exists to permit the user to access the TCB without being compromised by other processes or users.

Integrated Services Digital Network (ISDN) Basic Rate Interface
(BRI) offers two B channels which carry user data at 64 Kbps each,
and one control and signaling D channel operating at 16 Kbps.
Answer "It offers 23 B channels and 1 D channel." describes ISDN Primary Rate Interface (PRI)
for NorthAmerica and Japan , with 23 B channels at 64 Kbps and one 64 Kbps D channel, for a
total throughput of 1.544 Mbps.
Answer "It offers 30 B channels and 1 D channel." Describes ISDN PRI for Europe , Australia ,
and other parts of the world, with 30 64 Kbps B channels and one D channel, for a total throughput
of 2.048 Mbps.
Answer "It offers 1 B channel and 2 D channels." is a distracter. Source: Internetworking
Technologies Handbook, Second Edition (Cisco Press, 1998).

Explanation/Reference:
Explanation:
Full-interruption tests operate like parallel tests, but they involve actually shutting down operations at the primary site and shifting them to the recovery site. After a parallel test has been completed the next step is to perform a full-interruption test.
Incorrect Answers:
B: The checklist test is one of the simplest tests to conduct. You should perform it before, after, you perform a Parallel test.
C: Simulation tests are similar to the structured walk - through tests, and should be performed before parallel test, after parallel tests.
D: Parallel tests represent the next level in testing compared to a structured walk-through test, not vice versa.
References:
Stewart, James M., Ed Tittel, and Mike Chapple, CISSP: Certified Information Systems Security Professional Study Guide, 5th Edition, Sybex, Indianapolis, 2011, p. 671