Computer Networks CS 6250
Certification Test – Essential Guide
with Verified Content 2025/2026
◉ Computational Complexity of LS Algorithm Answer: O(n^2)
What is the computation complexity of this algorithm? That is, given n nodes
(not counting the source), how much computation must be done in the worst
case to find the least cost paths from the source to all destinations? In the first
iteration, we need to search through all n nodes to determine the node, w, not
in N that has the minimum cost. In the second iteration, we need to check n−1
nodes to determine the minimum cost; in the third iteration n−2 nodes and so
on. Overall, the total number of nodes we need to search through over all the
iterations is n*(n+1)/2, and thus we say that the above implementation of the
link state algorithm has worst case complexity of order n squared: O(n2). (A
more sophisticated implementation of this algorithm, using a data structure
known as a heap, can find the minimum in line 9 in logarithmic rather than
linear time, thus reducing the complexity).
◉ Distance Vector Routing Algorithm Answer: Distance-vector routing
protocol. A distance-vector routing protocol in data networks determines the
best route for data packets based on distance. Distance-vector routing
,protocols measure the distance by the number of routers a packet has to pass,
one router counts as one hop
◉ Distance Vector Routing Count to Infinity Answer: If the link between B and
C is disconnected, then B will know that it can no longer get to C via that link
and will remove it from it's table. Before it can send any updates it's possible
that it will receive an update from A which will be advertising that it can get to
C at a cost of 2. B can get to A at a cost of 1, so it will update a route to C via A
at a cost of 3. A will then receive updates from B later and update its cost to 4.
They will then go on feeding each other bad information toward infinity which
is called as Count to Infinity problem.
◉ Solution to Count to Infinity DV Issue (Route Poisoning - Poison Reverse
Answer: When a route fails, distance vector protocols spread the bad news
about a route failure by poisoning the route. Route poisoning refers to the
practice of advertising a route, but with a special metric value called Infinity.
Routers consider routes advertised with an infinite metric to have failed. Each
distance vector routing protocol uses the concept of an actual metric value
that represents infinity. RIP defines infinity as 16. The main disadvantage of
poison reverse is that it can significantly increase the size of routing
announcements in certain fairly common network topologies.
◉ Routing Information Protocol (RIP) Answer: The Routing Information
Protocol (RIP) is one of the oldest distance-vector routing protocols which
,employ the hop count as a routing metric. RIP prevents routing loops by
implementing a limit on the number of hops allowed in a path from source to
destination. The largest number of hops allowed for RIP is 15, which limits the
size of networks that RIP can support.
◉ Open Shortest Path First Protocol Answer: Link-state routing protocol that
is used to find the best path between the source and the destination router
using its own Shortest Path First
OSPF uses a shorted path first algorithm in order to build and calculate the
shortest path to all known destinations.The shortest path is calculated with
the use of the Dijkstra algorithm. The algorithm by itself is quite complicated.
This is a very high level, simplified way of looking at the various steps of the
algorithm:
Upon initialization or due to any change in routing information, a router
generates a link-state advertisement. This advertisement represents the
collection of all link-states on that router.
All routers exchange link-states by means of flooding. Each router that
receives a link-state update should store a copy in its link-state database and
then propagate the update to other routers.
, After the database of each router is completed, the router calculates a Shortest
Path Tree to all destinations. The router uses the Dijkstra algorithm in order to
calculate the shortest path tree. The destinations, the associated cost and the
next hop to reach those destinations form the IP routing table.
In case no changes in the OSPF network occur, such as cost of a link or a
network being added or deleted, OSPF should be very quiet. Any changes that
occur are communicated through link-state packets, and the Dijkstra
algorithm is recalculated in order to find the shortest path.
The algorithm places each router at the root of a tree and calculates the
shortest path to each destination based on the cumulative cost required to
reach that destination. Each router will have its own view of the topology even
though all the routers will build a shortest path tree using the same link-state
database. The following sections indicate what is involved in building a
shortest path tr
◉ What is the end-to-end (e2e) principle? Answer: When a function has to be
supported in a networked system, the designer often asks if it should be
implemented at the end systems; or should it be implemented within the
communication subsystem that interconnects all the end systems. The end-to-end
argument or principle states that it's proper to implement the function in the end
systems. The communication system itself may provide a partial implementation
but only as a performance enhancement.
Certification Test – Essential Guide
with Verified Content 2025/2026
◉ Computational Complexity of LS Algorithm Answer: O(n^2)
What is the computation complexity of this algorithm? That is, given n nodes
(not counting the source), how much computation must be done in the worst
case to find the least cost paths from the source to all destinations? In the first
iteration, we need to search through all n nodes to determine the node, w, not
in N that has the minimum cost. In the second iteration, we need to check n−1
nodes to determine the minimum cost; in the third iteration n−2 nodes and so
on. Overall, the total number of nodes we need to search through over all the
iterations is n*(n+1)/2, and thus we say that the above implementation of the
link state algorithm has worst case complexity of order n squared: O(n2). (A
more sophisticated implementation of this algorithm, using a data structure
known as a heap, can find the minimum in line 9 in logarithmic rather than
linear time, thus reducing the complexity).
◉ Distance Vector Routing Algorithm Answer: Distance-vector routing
protocol. A distance-vector routing protocol in data networks determines the
best route for data packets based on distance. Distance-vector routing
,protocols measure the distance by the number of routers a packet has to pass,
one router counts as one hop
◉ Distance Vector Routing Count to Infinity Answer: If the link between B and
C is disconnected, then B will know that it can no longer get to C via that link
and will remove it from it's table. Before it can send any updates it's possible
that it will receive an update from A which will be advertising that it can get to
C at a cost of 2. B can get to A at a cost of 1, so it will update a route to C via A
at a cost of 3. A will then receive updates from B later and update its cost to 4.
They will then go on feeding each other bad information toward infinity which
is called as Count to Infinity problem.
◉ Solution to Count to Infinity DV Issue (Route Poisoning - Poison Reverse
Answer: When a route fails, distance vector protocols spread the bad news
about a route failure by poisoning the route. Route poisoning refers to the
practice of advertising a route, but with a special metric value called Infinity.
Routers consider routes advertised with an infinite metric to have failed. Each
distance vector routing protocol uses the concept of an actual metric value
that represents infinity. RIP defines infinity as 16. The main disadvantage of
poison reverse is that it can significantly increase the size of routing
announcements in certain fairly common network topologies.
◉ Routing Information Protocol (RIP) Answer: The Routing Information
Protocol (RIP) is one of the oldest distance-vector routing protocols which
,employ the hop count as a routing metric. RIP prevents routing loops by
implementing a limit on the number of hops allowed in a path from source to
destination. The largest number of hops allowed for RIP is 15, which limits the
size of networks that RIP can support.
◉ Open Shortest Path First Protocol Answer: Link-state routing protocol that
is used to find the best path between the source and the destination router
using its own Shortest Path First
OSPF uses a shorted path first algorithm in order to build and calculate the
shortest path to all known destinations.The shortest path is calculated with
the use of the Dijkstra algorithm. The algorithm by itself is quite complicated.
This is a very high level, simplified way of looking at the various steps of the
algorithm:
Upon initialization or due to any change in routing information, a router
generates a link-state advertisement. This advertisement represents the
collection of all link-states on that router.
All routers exchange link-states by means of flooding. Each router that
receives a link-state update should store a copy in its link-state database and
then propagate the update to other routers.
, After the database of each router is completed, the router calculates a Shortest
Path Tree to all destinations. The router uses the Dijkstra algorithm in order to
calculate the shortest path tree. The destinations, the associated cost and the
next hop to reach those destinations form the IP routing table.
In case no changes in the OSPF network occur, such as cost of a link or a
network being added or deleted, OSPF should be very quiet. Any changes that
occur are communicated through link-state packets, and the Dijkstra
algorithm is recalculated in order to find the shortest path.
The algorithm places each router at the root of a tree and calculates the
shortest path to each destination based on the cumulative cost required to
reach that destination. Each router will have its own view of the topology even
though all the routers will build a shortest path tree using the same link-state
database. The following sections indicate what is involved in building a
shortest path tr
◉ What is the end-to-end (e2e) principle? Answer: When a function has to be
supported in a networked system, the designer often asks if it should be
implemented at the end systems; or should it be implemented within the
communication subsystem that interconnects all the end systems. The end-to-end
argument or principle states that it's proper to implement the function in the end
systems. The communication system itself may provide a partial implementation
but only as a performance enhancement.
