Computer Networks -CS6250 Exam 3 Questions and
Answers
What are the main components of a router? - ✔✔-Input/Output ports
-The switching fabric
-The routing processor
What do the input ports do for a router? - ✔✔-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? - ✔✔Moves packets from input to output ports. 3 Types:
memory, bus, and crossbar
What do the output ports do? - ✔✔Receive and queue the packets that come from the
switching fabric and send them over to the outgoing link
Routers control plane functions - ✔✔implements routing protocols, maintains the routing
tables, computers the forwarding table
Router Lookup (Task 1 of 6) - ✔✔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) - ✔✔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) - ✔✔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) - ✔✔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) - ✔✔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) - ✔✔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 - ✔✔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 - ✔✔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 - ✔✔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) - ✔✔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
Answers
What are the main components of a router? - ✔✔-Input/Output ports
-The switching fabric
-The routing processor
What do the input ports do for a router? - ✔✔-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? - ✔✔Moves packets from input to output ports. 3 Types:
memory, bus, and crossbar
What do the output ports do? - ✔✔Receive and queue the packets that come from the
switching fabric and send them over to the outgoing link
Routers control plane functions - ✔✔implements routing protocols, maintains the routing
tables, computers the forwarding table
Router Lookup (Task 1 of 6) - ✔✔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) - ✔✔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) - ✔✔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) - ✔✔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) - ✔✔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) - ✔✔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 - ✔✔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 - ✔✔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 - ✔✔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) - ✔✔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
