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300-410 Practice Question: An enterprise uses EIGRP for IPv6 with route…
An enterprise uses EIGRP for IPv6 with route redistribution from a static route. R1 has a static route 2001:db8:0::/32 via Null0 redistributed into EIGRP. R2 receives this route and has a more specific route 2001:db8:1::/32 via a different interface. R2 has an IPv6 ACL applied inbound on the interface facing R1 that permits only EIGRP and denies all other traffic. R2's uRPF is configured in loose mode. Traffic from R2 to 2001:db8:2::1 fails. R2 shows 'show ipv6 route' has both routes, but 'show ipv6 cef' shows the summary route for 2001:db8:2::1 pointing to R1. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between the routing table (RIB) and CEF (FIB), where candidates assume that a more specific route in the RIB will be used for forwarding, but CEF may use a different prefix if the destination does not match the more specific prefix exactly.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1.
R1's static route 2001:db8:0::/32 via Null0 is redistributed into EIGRP, and R2 learns this summary route. When R2 sends traffic to 2001:db8:2::1, the CEF table (show ipv6 cef) shows the longest prefix match for 2001:db8:2::1 is the summary 2001:db8:0::/32 pointing to R1, not the more specific 2001:db8:1::/32. R1 then forwards the packet to Null0 (a virtual interface that drops traffic), causing the failure. The more specific route 2001:db8:1::/32 on R2 is not used because 2001:db8:2::1 does not fall within that prefix.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1.
Why this is correct
R2 forwards traffic to R1 based on the summary route, but R1's static route to Null0 drops it. The ACL is irrelevant to this failure.
- ✗
The ACL on R2 blocks the return traffic from R1, causing asymmetric routing.
Why it's wrong here
The ACL is inbound on R2, so it filters traffic coming into R2, not going out.
- ✗
uRPF loose mode drops the packet because the source address is not in the FIB.
Why it's wrong here
Loose mode only requires a route in the FIB, which exists.
- ✗
EIGRP redistribution of the static route creates a routing loop between R1 and R2.
Why it's wrong here
There is no loop; the traffic is simply dropped at R1.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
OSPF Route Summarization and Filtering
Key term
Route Redistribution Between Protocols
Route redistribution between protocols is the process of taking routes learned from one routing protocol and injecting them into another routing protocol so that networks using different protocols can communicate.
Key term
uRPF
Unicast Reverse Path Forwarding is a network security feature that verifies the source address of incoming packets to prevent IP spoofing attacks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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