CS 6250 COMPUTER NETWORKS EXAM |
STUDY QUESTIONS AND ANSWERS |
LATEST 2025/2026
The Count to Infinity Problem (definition + solution)
- Correct Answer - A problem in distance vector routing where "bad
news" travels slowly.
Solution: When a link cost increases, send the distance as infinity
initially. Then the network will immediately choose other shortest paths
and converge more quickly.
RIP - Correct Answer - Routing Information Protocol.
A distance-vector routing protocol where edges have unit cost and
"infinity" (for count-to infinity problem) = 16.
Table refreshes occur every 30 seconds or when updates occur.
Every round has a timeout limit of 180 seconds.
In practice, router failure can take minutes to stabilize (slow
convergence).
Split Horizon Rule - Correct Answer - Updates are sent to all neighbors
except for the one who caused the update.
,Associated with RIP.
Link State Routing (definition / overview) - Correct Answer - Each node
distributes a network map to every other node in the network.
Each node then performs a shortest path computation between itself and
all other nodes.
Used in intradomain routing.
Link State Routing (process)
- Correct Answer - 1. Add costs of immediate neighbors D(v) to the table.
All other costs are infinity.
2. Flood costs c(u, v) to neighbors.
3. D(v) = the minimum of D(v) and c(u, w)+D(w) for each w.
Link State Routing:
Two Common Protocols - Correct Answer - Open Shortest Path First
(OSPF)
Intermediate System to Intermediate System (IS-IS)
OSPF - Correct Answer - Open Shortest Path First.
, A link-state routing protocol.
IS-IS - Correct Answer - Intermediate System to Intermediate System.
A link-state routing protocol based on Dijkstra's algorithm. Most
commonly used in large transit networks.
Link State Routing:
Problem - Correct Answer - Scale.
The complexity is O(n^3), where n is the number of nodes in the
network.
Link State Routing:
Coping with Scale - Correct Answer - Hierarchy.
Called "areas" in OSPF or "levels" in IS-IS.
1. The backbone "area" is called "Area 0".
2. Each area not in the backbone has an Area 0 router.
3. Shortest path is computed within areas.
4. To go between areas, use Area 0 routers.
STUDY QUESTIONS AND ANSWERS |
LATEST 2025/2026
The Count to Infinity Problem (definition + solution)
- Correct Answer - A problem in distance vector routing where "bad
news" travels slowly.
Solution: When a link cost increases, send the distance as infinity
initially. Then the network will immediately choose other shortest paths
and converge more quickly.
RIP - Correct Answer - Routing Information Protocol.
A distance-vector routing protocol where edges have unit cost and
"infinity" (for count-to infinity problem) = 16.
Table refreshes occur every 30 seconds or when updates occur.
Every round has a timeout limit of 180 seconds.
In practice, router failure can take minutes to stabilize (slow
convergence).
Split Horizon Rule - Correct Answer - Updates are sent to all neighbors
except for the one who caused the update.
,Associated with RIP.
Link State Routing (definition / overview) - Correct Answer - Each node
distributes a network map to every other node in the network.
Each node then performs a shortest path computation between itself and
all other nodes.
Used in intradomain routing.
Link State Routing (process)
- Correct Answer - 1. Add costs of immediate neighbors D(v) to the table.
All other costs are infinity.
2. Flood costs c(u, v) to neighbors.
3. D(v) = the minimum of D(v) and c(u, w)+D(w) for each w.
Link State Routing:
Two Common Protocols - Correct Answer - Open Shortest Path First
(OSPF)
Intermediate System to Intermediate System (IS-IS)
OSPF - Correct Answer - Open Shortest Path First.
, A link-state routing protocol.
IS-IS - Correct Answer - Intermediate System to Intermediate System.
A link-state routing protocol based on Dijkstra's algorithm. Most
commonly used in large transit networks.
Link State Routing:
Problem - Correct Answer - Scale.
The complexity is O(n^3), where n is the number of nodes in the
network.
Link State Routing:
Coping with Scale - Correct Answer - Hierarchy.
Called "areas" in OSPF or "levels" in IS-IS.
1. The backbone "area" is called "Area 0".
2. Each area not in the backbone has an Area 0 router.
3. Shortest path is computed within areas.
4. To go between areas, use Area 0 routers.
