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How routers learn the way together

Computing · Networks & Security · ages 19-20

Name ______________________ Date ____________

What does a router's forwarding table record?
- A record of packet history for each destination
- A next hop chosen for each destination
- A list of link speeds beside each destination
In a network graph, what does a path describe?
- A route that follows connected links
- A list of every router in the network
- A table entry for one destination
In a distance-vector model, the tables have settled once a round of neighbour exchanges changes no table.
Circle one: True False
A link fails. Why can some routers keep the old route for a while?
- Updates carry the failure outward one hop at a time
- Distance-vector tables never change after startup
- Packets choose longer roads whenever a link fails
Three routers form a line: A, B, C. How does A learn a route to C?
- A sends a direct message to C
- C appears in A's table without any exchange
- B shares its route information with A
Routers A, B, and C form a line. Each link costs 1. After the small model settles, what is the cost from A to C?
Answer: ______________
A student says one router knows the whole network at the start. Which statement corrects this?
- A router starts with local links and neighbours
- Every router starts with the complete network map
- Routers learn from a central map before exchanging
Traffic between two organisations takes a longer road even though a shorter one exists. Why can that happen?
- Distance always chooses the shortest route
- Policy can reject a shorter route between organisations
- Business never affects interdomain paths

Answer key

For grown-ups. Fold this page away before handing over the rest.

How routers learn the way together W1-mt_gyqPDrV6Uo-s1

A next hop chosen for each destination · The table tells the router which next hop to use for a destination it knows.
A route that follows connected links · A path follows connected edges from a starting vertex to a destination vertex.
True · The tables have settled when a round of neighbour exchanges leaves every entry unchanged.
Updates carry the failure outward one hop at a time · In this model, failure information reaches farther routers through later exchanges.
B shares its route information with A · B can tell A about its route to C, allowing A to learn the two-link path.
2 · B advertises distance 1 to C. A adds its link cost to B, so 1 + 1 = 2. A has no other route to C, so the table settles at cost 2.
A router starts with local links and neighbours · No router starts with the whole network. Each one starts with its own links and neighbours, then learns farther routes through neighbour exchanges.
Policy can reject a shorter route between organisations · Operators can accept or reject routes by policy, so hop count alone does not decide every interdomain path.