Node A is using distance vector routing. The current distance table and routing table at Node A are shown below. Current distance table at Node A (rows: to; columns: via B C D E) to B: via B = 2 to C: via C = 1 to D: – to E: via E = 8 Current routing table at Node A dest | next hop | cost B | B | 2 C | C | 1 E | E | 8 What vector, if any, will be sent by A after its distance table has been updated?
Figure required: Figure 1 (network): nodes A, B, C, D, E with link costs A–B 2, A–C 1, B–C 2, B–D 2, D–E 3, A–E 8. The distance and routing tables for Node A are transcribed into the question text.
Try an answer before revealing the guidance below.
Key Concepts
- Send only when the distance vector changes
- Least cost per destination
- Neighbours B, C, E
- Poisoned reverse
Answer Approach
- Compare A’s least costs before and after the update.
- If any least cost changed, A must advertise its new vector.
- List the vector and to whom it is sent.
Full Answer
Answer status: Draft answer (unofficial). Revision notes, not an official marking scheme.
A’s least costs changed (D became reachable at 4 and E improved from 8 to 7), so A sends its new distance vector to its neighbours B, C and E: [A 0, B 2, C 1, D 4, E 7]. (With poisoned reverse, the copy sent to B would advertise D and E as ∞, since A routes to them through B.)
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