1
CS6250 COMPUTER NETWORK EXAM 2
ACTUAL EXAM 2026/2027 Complete
Questions and Answers Expert Verified Pass
Guaranteed - A+ Graded
SECTION 1: SOFTWARE-DEFINED NETWORKING (19 Questions)
Q1: In an OpenFlow-based SDN network, a switch receives a packet for which it has no
matching flow table entry. Which of the following actions will occur?
A. The switch drops the packet immediately to prevent forwarding loops
B. The switch forwards the packet using traditional MAC learning
C. The switch encapsulates the packet and sends it to the controller
D. The switch floods the packet out all ports except the incoming port
Correct Answer: C [CORRECT]
Rationale: In OpenFlow, when a packet does not match any flow table entry (table-miss), the
switch's default behavior is to send the packet to the controller for processing via the Packet-In
message. This is fundamental to SDN architecture where the controller maintains network-wide
visibility and makes forwarding decisions. Option A is incorrect because dropping is only one
possible configured table-miss action, not the default behavior. Option B is incorrect because
SDN explicitly decouples control and data planes; traditional MAC learning is a distributed
control plane function that OpenFlow switches do not perform autonomously. Option D
describes traditional Ethernet switch behavior using self-learning bridges, which contradicts the
centralized control paradigm of SDN. The controller can then decide how to handle the packet
and potentially install new flow entries to handle subsequent packets of the same flow in the data
plane.
Q2: Which component of the SDN architecture is responsible for translating high-level network
policies into flow table entries?
A. The data plane forwarding hardware
B. The southbound API (e.g., OpenFlow)
C. The network operating system (controller)
D. The physical network topology
,2
Correct Answer: C [CORRECT]
Rationale: The network operating system (controller) such as ONOS or OpenDaylight serves as
the "brain" of the SDN architecture, maintaining a global network view and translating
application-level policies into specific forwarding rules. Option A is incorrect because the data
plane only executes forwarding decisions; it has no policy interpretation capability. Option B is
incorrect because the southbound API is merely a communication protocol between controller
and switches, not a decision-making entity. Option D is incorrect because the physical topology
is a static infrastructure element that informs decisions but does not make them. The controller
uses network graph algorithms and policy engines to compute paths and populate flow tables via
southbound protocols.
Q3: In the context of SDN, what is the primary advantage of separating the control plane from
the data plane?
A. It eliminates the need for routing protocols entirely
B. It enables centralized network management and programmability
C. It increases line-rate forwarding speed by removing control overhead
D. It automatically encrypts all network traffic for security
Correct Answer: B [CORRECT]
Rationale: The separation of control and data planes in SDN enables centralized network
management, global network visibility, and programmatic control through software applications.
This is the defining characteristic of SDN architecture. Option A is incorrect because SDN does
not eliminate routing protocols; it centralizes their execution in the controller rather than
distributing them across routers. Option C is incorrect because while data plane performance is
important, the separation itself doesn't inherently increase forwarding speed—specialized
hardware (ASICs) achieves line-rate forwarding regardless of control plane location. Option D is
incorrect because encryption is not an automatic consequence of SDN architecture; security
policies must be explicitly implemented. Centralization enables consistent policy enforcement,
traffic engineering, and rapid innovation through software updates rather than hardware changes.
Q4: A network administrator using ONOS notices that link failures are not being detected
quickly enough. Which ONOS feature could be tuned to improve failure detection times?
A. Increasing the OpenFlow echo request interval
B. Enabling the Reactive Forwarding application
C. Configuring faster LLDP (Link Layer Discovery Protocol) polling intervals
D. Disabling the distributed core to reduce processing latency
,3
Correct Answer: C [CORRECT]
Rationale: ONOS uses LLDP packets to detect link status between switches. By reducing the
LLDP polling interval, the controller detects link failures faster, though at the cost of increased
control traffic. Option A is incorrect because OpenFlow echo requests maintain switch-controller
connectivity (keepalives) but do not detect inter-switch link failures. Option B is incorrect
because Reactive Forwarding handles packet-in events for unknown flows, not link failure
detection. Option D is incorrect because disabling the distributed core would eliminate ONOS's
high availability and scalability features without improving failure detection. ONOS's Link
Discovery Service uses periodic LLDP transmission and timeout mechanisms to maintain an
accurate network topology graph.
Q5: Which OpenFlow message type does a switch use to inform the controller about a packet
that matched a table-miss flow entry?
A. Flow-Mod
B. Packet-In
C. Port-Status
D. Barrier-Request
Correct Answer: B [CORRECT]
Rationale: The Packet-In message is sent from switch to controller when a packet matches a
table-miss entry or an explicit "send to controller" action in a flow rule. This message contains
the packet header and optionally the full packet payload. Option A is incorrect because Flow-
Mod messages flow from controller to switch to install, modify, or delete flow entries. Option C
is incorrect because Port-Status messages notify the controller about port configuration changes
or state changes (up/down), not packet arrivals. Option D is incorrect because Barrier-Request is
a controller-to-switch message ensuring message ordering and synchronization. The Packet-In
mechanism is crucial for reactive forwarding and learning-based applications in SDN.
Q6: In OpenFlow 1.5, which of the following match fields can be used to perform load balancing
across multiple paths based on transport layer information?
A. Ethernet source and destination MAC addresses only
B. VLAN ID and priority bits
C. TCP/UDP source and destination port numbers
D. MPLS label and traffic class
Correct Answer: C [CORRECT]
, 4
Rationale: OpenFlow 1.5 supports matching on TCP and UDP port numbers (transport layer),
enabling flow-based load balancing where different TCP/UDP flows between the same IP
endpoints can be steered through different paths. Option A is incorrect because MAC addresses
operate at Layer 2 and cannot distinguish between different application flows. Option B is
incorrect because VLAN fields provide Layer 2 segmentation but not transport-layer granularity.
Option D is incorrect because MPLS labels are typically used for traffic engineering across
provider networks, not for fine-grained load balancing based on transport ports. This capability
allows SDN applications to implement equal-cost multi-path (ECMP) or weighted load
balancing at flow granularity rather than just packet granularity.
Q7: What is the primary function of the southbound interface in SDN architecture?
A. To provide REST APIs for network applications
B. To communicate between multiple SDN controllers in a cluster
C. To instruct forwarding devices how to handle packets
D. To manage billing and accounting for network usage
Correct Answer: C [CORRECT]
Rationale: The southbound interface (e.g., OpenFlow, NETCONF, P4Runtime) carries
instructions from the SDN controller to network devices, programming their forwarding behavior
through flow tables or configuration commands. Option A is incorrect because REST APIs are
typically northbound interfaces for application-controller communication. Option B is incorrect
because east-west interfaces handle inter-controller communication for distributed controllers.
Option D is incorrect because billing management is an application-layer function, not a core
southbound interface responsibility. The southbound interface abstracts device-specific hardware
details, allowing the controller to treat diverse network elements uniformly.
Q8: Which of the following is a key difference between OpenDaylight and ONOS controller
architectures?
A. OpenDaylight uses a centralized architecture while ONOS is fully distributed
B. OpenDaylight is based on the OSGi modular framework while ONOS uses a microservices
approach
C. OpenDaylight supports OpenFlow exclusively while ONOS supports multiple southbound
protocols
D. OpenDaylight is designed only for data center networks while ONOS targets service provider
networks
Correct Answer: B [CORRECT]
CS6250 COMPUTER NETWORK EXAM 2
ACTUAL EXAM 2026/2027 Complete
Questions and Answers Expert Verified Pass
Guaranteed - A+ Graded
SECTION 1: SOFTWARE-DEFINED NETWORKING (19 Questions)
Q1: In an OpenFlow-based SDN network, a switch receives a packet for which it has no
matching flow table entry. Which of the following actions will occur?
A. The switch drops the packet immediately to prevent forwarding loops
B. The switch forwards the packet using traditional MAC learning
C. The switch encapsulates the packet and sends it to the controller
D. The switch floods the packet out all ports except the incoming port
Correct Answer: C [CORRECT]
Rationale: In OpenFlow, when a packet does not match any flow table entry (table-miss), the
switch's default behavior is to send the packet to the controller for processing via the Packet-In
message. This is fundamental to SDN architecture where the controller maintains network-wide
visibility and makes forwarding decisions. Option A is incorrect because dropping is only one
possible configured table-miss action, not the default behavior. Option B is incorrect because
SDN explicitly decouples control and data planes; traditional MAC learning is a distributed
control plane function that OpenFlow switches do not perform autonomously. Option D
describes traditional Ethernet switch behavior using self-learning bridges, which contradicts the
centralized control paradigm of SDN. The controller can then decide how to handle the packet
and potentially install new flow entries to handle subsequent packets of the same flow in the data
plane.
Q2: Which component of the SDN architecture is responsible for translating high-level network
policies into flow table entries?
A. The data plane forwarding hardware
B. The southbound API (e.g., OpenFlow)
C. The network operating system (controller)
D. The physical network topology
,2
Correct Answer: C [CORRECT]
Rationale: The network operating system (controller) such as ONOS or OpenDaylight serves as
the "brain" of the SDN architecture, maintaining a global network view and translating
application-level policies into specific forwarding rules. Option A is incorrect because the data
plane only executes forwarding decisions; it has no policy interpretation capability. Option B is
incorrect because the southbound API is merely a communication protocol between controller
and switches, not a decision-making entity. Option D is incorrect because the physical topology
is a static infrastructure element that informs decisions but does not make them. The controller
uses network graph algorithms and policy engines to compute paths and populate flow tables via
southbound protocols.
Q3: In the context of SDN, what is the primary advantage of separating the control plane from
the data plane?
A. It eliminates the need for routing protocols entirely
B. It enables centralized network management and programmability
C. It increases line-rate forwarding speed by removing control overhead
D. It automatically encrypts all network traffic for security
Correct Answer: B [CORRECT]
Rationale: The separation of control and data planes in SDN enables centralized network
management, global network visibility, and programmatic control through software applications.
This is the defining characteristic of SDN architecture. Option A is incorrect because SDN does
not eliminate routing protocols; it centralizes their execution in the controller rather than
distributing them across routers. Option C is incorrect because while data plane performance is
important, the separation itself doesn't inherently increase forwarding speed—specialized
hardware (ASICs) achieves line-rate forwarding regardless of control plane location. Option D is
incorrect because encryption is not an automatic consequence of SDN architecture; security
policies must be explicitly implemented. Centralization enables consistent policy enforcement,
traffic engineering, and rapid innovation through software updates rather than hardware changes.
Q4: A network administrator using ONOS notices that link failures are not being detected
quickly enough. Which ONOS feature could be tuned to improve failure detection times?
A. Increasing the OpenFlow echo request interval
B. Enabling the Reactive Forwarding application
C. Configuring faster LLDP (Link Layer Discovery Protocol) polling intervals
D. Disabling the distributed core to reduce processing latency
,3
Correct Answer: C [CORRECT]
Rationale: ONOS uses LLDP packets to detect link status between switches. By reducing the
LLDP polling interval, the controller detects link failures faster, though at the cost of increased
control traffic. Option A is incorrect because OpenFlow echo requests maintain switch-controller
connectivity (keepalives) but do not detect inter-switch link failures. Option B is incorrect
because Reactive Forwarding handles packet-in events for unknown flows, not link failure
detection. Option D is incorrect because disabling the distributed core would eliminate ONOS's
high availability and scalability features without improving failure detection. ONOS's Link
Discovery Service uses periodic LLDP transmission and timeout mechanisms to maintain an
accurate network topology graph.
Q5: Which OpenFlow message type does a switch use to inform the controller about a packet
that matched a table-miss flow entry?
A. Flow-Mod
B. Packet-In
C. Port-Status
D. Barrier-Request
Correct Answer: B [CORRECT]
Rationale: The Packet-In message is sent from switch to controller when a packet matches a
table-miss entry or an explicit "send to controller" action in a flow rule. This message contains
the packet header and optionally the full packet payload. Option A is incorrect because Flow-
Mod messages flow from controller to switch to install, modify, or delete flow entries. Option C
is incorrect because Port-Status messages notify the controller about port configuration changes
or state changes (up/down), not packet arrivals. Option D is incorrect because Barrier-Request is
a controller-to-switch message ensuring message ordering and synchronization. The Packet-In
mechanism is crucial for reactive forwarding and learning-based applications in SDN.
Q6: In OpenFlow 1.5, which of the following match fields can be used to perform load balancing
across multiple paths based on transport layer information?
A. Ethernet source and destination MAC addresses only
B. VLAN ID and priority bits
C. TCP/UDP source and destination port numbers
D. MPLS label and traffic class
Correct Answer: C [CORRECT]
, 4
Rationale: OpenFlow 1.5 supports matching on TCP and UDP port numbers (transport layer),
enabling flow-based load balancing where different TCP/UDP flows between the same IP
endpoints can be steered through different paths. Option A is incorrect because MAC addresses
operate at Layer 2 and cannot distinguish between different application flows. Option B is
incorrect because VLAN fields provide Layer 2 segmentation but not transport-layer granularity.
Option D is incorrect because MPLS labels are typically used for traffic engineering across
provider networks, not for fine-grained load balancing based on transport ports. This capability
allows SDN applications to implement equal-cost multi-path (ECMP) or weighted load
balancing at flow granularity rather than just packet granularity.
Q7: What is the primary function of the southbound interface in SDN architecture?
A. To provide REST APIs for network applications
B. To communicate between multiple SDN controllers in a cluster
C. To instruct forwarding devices how to handle packets
D. To manage billing and accounting for network usage
Correct Answer: C [CORRECT]
Rationale: The southbound interface (e.g., OpenFlow, NETCONF, P4Runtime) carries
instructions from the SDN controller to network devices, programming their forwarding behavior
through flow tables or configuration commands. Option A is incorrect because REST APIs are
typically northbound interfaces for application-controller communication. Option B is incorrect
because east-west interfaces handle inter-controller communication for distributed controllers.
Option D is incorrect because billing management is an application-layer function, not a core
southbound interface responsibility. The southbound interface abstracts device-specific hardware
details, allowing the controller to treat diverse network elements uniformly.
Q8: Which of the following is a key difference between OpenDaylight and ONOS controller
architectures?
A. OpenDaylight uses a centralized architecture while ONOS is fully distributed
B. OpenDaylight is based on the OSGi modular framework while ONOS uses a microservices
approach
C. OpenDaylight supports OpenFlow exclusively while ONOS supports multiple southbound
protocols
D. OpenDaylight is designed only for data center networks while ONOS targets service provider
networks
Correct Answer: B [CORRECT]
