Computer Networks Module 10
Routing interview questions
Interview questions on routing from Computer Networks: short warm-ups, then what each harder question tests, the first step of a good answer and its follow-ups.
Warm-up questions
Short questions with short answers, each answered in full.
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Fresher to mid
What is the difference between a router's RIB and its FIB?
Answered in the lesson Forwarding tables and longest prefix match
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Fresher
A router holds 10.0.0.0/8 from OSPF and 10.1.0.0/16 as a static route. Which one forwards a packet to 10.1.5.9?
Answered in the lesson Forwarding tables and longest prefix match
Questions with model answers
What each question tests, the first step of a good answer and the follow-ups to expect.
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Mid to senior
During a migration you add a static route to 10.20.0.0/16 towards a new firewall. Most of the /16 moves, but traffic to 10.20.30.0/24 still takes the old path. Why, and how do you move it safely?
- What it tests
- Whether the candidate applies longest prefix match before administrative distance, checks the route used for one address, and plans a reversible change.
- First step
- Look up one affected address, for example with ip route get 10.20.30.5 or show ip route 10.20.30.5, and note the prefix and source of the route actually used. Then list every route inside 10.20.0.0/16 that is longer than /16.
- Follow-ups
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- Would lowering the static route's distance, or raising OSPF's, change anything?
- How would you move only 10.20.30.128/25 first?
- The new firewall fails but its link stays up. What happens to the moved traffic?
From the lesson Forwarding tables and longest prefix match
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In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
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Senior to staff
A router must forward 150 million packets a second against 1.2 million IPv4 prefixes. How would you store the forwarding table so that every lookup is fast enough, and what does each choice cost?
- What it tests
- Turning a packet rate into a time budget, and comparing lookup structures by worst-case memory reads, memory size and update cost.
- First step
- Turn the rate into a budget: 1 ÷ 150 million is about 6.7 ns between lookups. A binary trie may need 32 dependent reads, so ask how many reads the budget allows, and pick a structure that bounds the worst case.
- Follow-ups
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- How much memory does a table indexed by the first 24 bits need?
- What changes for IPv6?
- How do you update the table without stopping lookups?
From the lesson Forwarding tables and longest prefix match
Model answer, Pro.In the answer:
- Model answer
- Follow-up answers
- Common mistakes
- Rubric
- Variants
- about 4 minutes
All Computer Networks interview questions · The Computer Networks track