hardMultiple ChoiceObjective-mapped
300-410 Practice Question: A large enterprise network is experiencing…
A large enterprise network is experiencing intermittent IPv6 connectivity between two remote sites connected via an IPv6-in-IPv4 manual tunnel. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::1/64 tunnel source 192.0.2.1 tunnel destination 198.51.100.1 ipv6 route 2001:DB8:2::/64 Tunnel0. Router R2 shows: R2# show ipv6 route 2001:DB8:1::/64 % Route not found. The tunnel interface is up/up on both routers, and IPv4 reachability between tunnel endpoints is verified. What is the root cause?
⚠ Common exam trap
Cisco often tests the misconception that if the tunnel is up/up and IPv4 reachability is verified, IPv6 routing will automatically work, but they intentionally omit the reciprocal static route to trap candidates who overlook the need for explicit routing on both sides.
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
✓
R2 is missing a static route for 2001:DB8:1::/64 pointing to its Tunnel0 interface.
The issue is that R2 has no route to the 2001:DB8:1::/64 network, which is the tunnel subnet on R1's side. Even though the tunnel is up/up and IPv4 reachability exists, R2 cannot return traffic to R1's tunnel endpoint address because it lacks a static route pointing to its own Tunnel0 interface. This is a common misconfiguration in point-to-point IPv6-in-IPv4 manual tunnels where both sides must have reciprocal static routes for the remote tunnel networks.
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 tunnel destination on R1 is incorrect and should be 198.51.100.2.
Why it's wrong here
The tunnel destination is the IPv4 address of R2's tunnel source, which is correct as given.
- ✓
R2 is missing a static route for 2001:DB8:1::/64 pointing to its Tunnel0 interface.
Why this is correct
Without a return route, R2 cannot forward packets back to R1's IPv6 network.
- ✗
The tunnel mode must be changed to ipv6ip for IPv6-in-IPv4 tunneling.
Why it's wrong here
The default tunnel mode for manual tunnels is ipv6ip, which is correct.
- ✗
IPv6 unicast-routing is disabled on R2.
Why it's wrong here
While necessary, the show command indicates the route is missing, not that routing is disabled entirely.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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