CCNA routing interview questions test your understanding of route selection, dynamic protocols, summarization, and IPv6. Strong answers explain both the concept and the command used to verify it.
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1. What is the difference between a routed protocol and a routing protocol?
A routed protocol, such as IPv4 or IPv6, carries user packets. A routing protocol lets routers exchange reachability information and calculate paths.
2. How does a router choose a route?
A router normally prefers the longest prefix match. If multiple sources offer the same prefix, administrative distance selects the preferred source, and the protocol metric selects among paths learned by that protocol.
3. What is route summarization?
Summarization advertises one broader prefix for several contiguous networks. It reduces routing-table entries and can improve stability.
4. What is OSPF?
Open Shortest Path First is a link-state interior gateway protocol. Routers build a link-state database and use the shortest path first algorithm to calculate routes.
5. What metric does OSPF use?
OSPF uses cost, which is derived from interface bandwidth according to the configured reference bandwidth. Lower total path cost is preferred.
6. What is EIGRP?
Enhanced Interior Gateway Routing Protocol uses the Diffusing Update Algorithm and maintains neighbor, topology, and routing information. Its metric is primarily based on bandwidth and delay by default.
7. What is the size of an IPv6 address?
An IPv6 address contains 128 bits and is written in hexadecimal groups separated by colons. Leading zeros can be omitted, and one continuous run of zero groups can be compressed with ::.
8. Does IPv6 use broadcast?
No. IPv6 uses multicast and anycast mechanisms instead of broadcast. Neighbor Discovery uses ICMPv6 multicast messages.
9. Who fragments IPv6 packets?
IPv6 routers do not fragment transit packets. The source host may use the Fragment extension header when necessary, normally after path MTU discovery.
10. Which commands verify routing?
Use show ip route, show ipv6 route, show ip protocols, protocol-specific neighbor commands, ping, and traceroute.
Routing Interview Checklist
Be ready to read a routing table, identify the longest prefix match, explain administrative distance versus metric, and trace a packet hop by hop. In a lab, remove one route or adjacency and describe the expected symptom before checking command output.
Routing Troubleshooting Example
If a destination is unreachable, confirm the source interface and gateway, then inspect the routing table for the most specific matching prefix. Verify the next hop, outgoing interface, and return path. For dynamic protocols, check neighbor relationships, advertised networks, passive interfaces, timers, and filtering.
IPv6 Verification Checklist
Use show ipv6 interface brief, show ipv6 neighbors, show ipv6 route, ping ipv6, and traceroute ipv6. Be ready to explain link-local addresses, global unicast addresses, multicast, and the purpose of a /64 LAN prefix.