CS 6250 Computer Networks Exam 2 Questions and Answers Latest Update 2025 Already Passed
What are the main components of a router? - Answers -Input/Output ports
-The switching fabric
-The routing processor
What do the input ports do for a router? - Answers -Line termination
-Data link processing unit decapsulate the packets
-Checks with the forwarding table to ensure each packet is forwarded to the appropriate output port
What does the switching fabric do? - Answers Moves packets from input to output ports. 3 Types:
memory, bus, and crossbar
What do the output ports do? - Answers Receive and queue the packets that come from the switching
fabric and send them over to the outgoing link
Routers control plane functions - Answers implements routing protocols, maintains the routing tables,
computers the forwarding table
Router Lookup (Task 1 of 6) - Answers When a packet arrives, router looks at the destination IP address
and determines the output link by looking at the forwarding table or FIB.
Router Switching (Task 2 of 6) - Answers After lookup, the switching system takes over to transfer the
packet from the input link to the output link
Router Queueing (Task 3 of 6) - Answers After switching, it will need to be queued if the link is
congested. The queue maybe simple as FIFO or some other algorithm
Router Header validation and checksum (4 of 6) - Answers The router checks the packet's version
number, decrements the time-to-live field, and recalculates the header checksum
Router Processing (Task 5 of 6) - Answers The routers build their forwarding tables using routing
protocols such as RIP, OSPF, BGP - they are implemented in the routing processors
Protocol Processing (Task 6 of 6) - Answers The routers, in order to implement their functions, need to
implement the following protocols:
-Simple Network Management Protocol which provides counters for remote inspection
-TCP and UDP for remote communication with the router
-Internet Control Message Protocol for sending error messages
, Switching via memory - Answers The ports operate as I/O devices in an operating system. When an input
port receives a packet, it sends an interrupt to the routing processor and the packet is coped to the
processors memory. Process extracts address and looks to table to find output port. Then its copied to
the output port buffer
Switching via bus - Answers When input receives a packet, it puts an internal header that designates the
output port, and sends it to a shared bus. All output ports will receive the packet, but only the
designated one will keep it. Speed of the bus limits the speed of the router.
Switching via Interconnection network - Answers Interconnect network that connects N input ports to N
output ports using 2N buses. Can carry multiple packets at a time
Bandwidth and Internet population scaling (problem 1 of 2 that routers face) - Answers Caused by:
1. Increasing number of devices
2. increasing volumes of network traffic due to new applications
3. new technologies such as optical links that can accommodate higher volumes of traffic
Services at high speeds (problem 2 of 2 that routers face) - Answers New applications require services
such as protection against delays during congestion, and protection during attacks or failures.
Longest prefix matching (Router bottlenecks 1 of 4) - Answers Due to increasing number of internet
hosts and networks, routers need to group destinations into prefixes, but then run into the problem of
more complex algorithms for longest prefix matching
Service differentiation ((Router bottlenecks 2 of 4) - Answers Routers can provide different quality of
service to different packets. This requires routers to classify packets based on complex criteria
Switching Limitations (Router bottlenecks 3 of 4) - Answers Head of line blocking occurs
Bottlenecks about services (Router bottlenecks 4 of 4) - Answers providing performance guarantees in
high speeds is nontrivial.
What is CIDR? Classless internet Domain Routing - Answers Assigns IP addresses using arbitrary-length
prefixes. Decreases the router table size but then brings us a problem of longest-matching-prefix lookup
Why do we need packet classification? - Answers Finer packet handling (TCP flags, source addresses, etc)
3 variants of packet classification - Answers 1. firewalls: routers implement firewalls at the entry and exit
points of the network to filter traffic
2. resource reservation protocols: reserve bandwidth between source and a destination
3. routing based of traffic type: routing based of specific traffic helps avoid delays for time-sensitive
applications
What are the main components of a router? - Answers -Input/Output ports
-The switching fabric
-The routing processor
What do the input ports do for a router? - Answers -Line termination
-Data link processing unit decapsulate the packets
-Checks with the forwarding table to ensure each packet is forwarded to the appropriate output port
What does the switching fabric do? - Answers Moves packets from input to output ports. 3 Types:
memory, bus, and crossbar
What do the output ports do? - Answers Receive and queue the packets that come from the switching
fabric and send them over to the outgoing link
Routers control plane functions - Answers implements routing protocols, maintains the routing tables,
computers the forwarding table
Router Lookup (Task 1 of 6) - Answers When a packet arrives, router looks at the destination IP address
and determines the output link by looking at the forwarding table or FIB.
Router Switching (Task 2 of 6) - Answers After lookup, the switching system takes over to transfer the
packet from the input link to the output link
Router Queueing (Task 3 of 6) - Answers After switching, it will need to be queued if the link is
congested. The queue maybe simple as FIFO or some other algorithm
Router Header validation and checksum (4 of 6) - Answers The router checks the packet's version
number, decrements the time-to-live field, and recalculates the header checksum
Router Processing (Task 5 of 6) - Answers The routers build their forwarding tables using routing
protocols such as RIP, OSPF, BGP - they are implemented in the routing processors
Protocol Processing (Task 6 of 6) - Answers The routers, in order to implement their functions, need to
implement the following protocols:
-Simple Network Management Protocol which provides counters for remote inspection
-TCP and UDP for remote communication with the router
-Internet Control Message Protocol for sending error messages
, Switching via memory - Answers The ports operate as I/O devices in an operating system. When an input
port receives a packet, it sends an interrupt to the routing processor and the packet is coped to the
processors memory. Process extracts address and looks to table to find output port. Then its copied to
the output port buffer
Switching via bus - Answers When input receives a packet, it puts an internal header that designates the
output port, and sends it to a shared bus. All output ports will receive the packet, but only the
designated one will keep it. Speed of the bus limits the speed of the router.
Switching via Interconnection network - Answers Interconnect network that connects N input ports to N
output ports using 2N buses. Can carry multiple packets at a time
Bandwidth and Internet population scaling (problem 1 of 2 that routers face) - Answers Caused by:
1. Increasing number of devices
2. increasing volumes of network traffic due to new applications
3. new technologies such as optical links that can accommodate higher volumes of traffic
Services at high speeds (problem 2 of 2 that routers face) - Answers New applications require services
such as protection against delays during congestion, and protection during attacks or failures.
Longest prefix matching (Router bottlenecks 1 of 4) - Answers Due to increasing number of internet
hosts and networks, routers need to group destinations into prefixes, but then run into the problem of
more complex algorithms for longest prefix matching
Service differentiation ((Router bottlenecks 2 of 4) - Answers Routers can provide different quality of
service to different packets. This requires routers to classify packets based on complex criteria
Switching Limitations (Router bottlenecks 3 of 4) - Answers Head of line blocking occurs
Bottlenecks about services (Router bottlenecks 4 of 4) - Answers providing performance guarantees in
high speeds is nontrivial.
What is CIDR? Classless internet Domain Routing - Answers Assigns IP addresses using arbitrary-length
prefixes. Decreases the router table size but then brings us a problem of longest-matching-prefix lookup
Why do we need packet classification? - Answers Finer packet handling (TCP flags, source addresses, etc)
3 variants of packet classification - Answers 1. firewalls: routers implement firewalls at the entry and exit
points of the network to filter traffic
2. resource reservation protocols: reserve bandwidth between source and a destination
3. routing based of traffic type: routing based of specific traffic helps avoid delays for time-sensitive
applications
