# Q7(d) · Destination-based forwarding

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2022-ai-final/q7d

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2022-ai-final--q7d/

Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.

### Q7(d). Router A connects H1 at interface 1, H2 at 2, router B at 3 and router C at 4. B and C each lead to D, and D leads to H3. (i) Show A’s forwarding entry sending all H3 traffic through interface 3. (ii) Can an ordinary destination-based table send H1→H3 through 3 but H2→H3 through 4? Explain. [5 marks]

### Answer

| Destination | A output interface |
| --- | --- |
| H3 (its destination address/prefix) | 3 |

For part(ii), an ordinary destination-only forwarding lookup sees the same destination H3 for both packets, so it cannot make the requested distinction by sender. It would need policy/source-aware forwarding or a more general match-action rule. A simple table sending H3 to3 sends both H1 andH2 that way. The alternate path through4 exists physically but is not selected by source under the stated rule.