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300-410 Practice Question: A network using IPv6 over IPv4 Teredo tunnels is…
A network using IPv6 over IPv4 Teredo tunnels is experiencing intermittent connectivity. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:0:4136:E378:8000:63BF:3C57:DD0B/128 tunnel source 192.0.2.1 tunnel mode ipv6ip teredo. Router R2 shows: R2# show ipv6 route 2001:0:4136:E378::/64 % Route not found. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between Teredo server and relay roles, leading candidates to incorrectly focus on relay reachability or mode syntax when the missing server configuration is the actual root cause.
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 Teredo server IPv4 address is not configured on R1.
The Teredo tunnel on R1 requires a Teredo server to facilitate the initial configuration and to help the client discover its public IPv4 address and the Teredo relay. Without the 'tunnel teredo server-ip' command specifying the Teredo server's IPv4 address, R1 cannot complete the Teredo setup, leading to an incomplete or non-functional tunnel. This results in the IPv6 prefix 2001:0:4136:E378::/64 not being installed in R2's routing table, as the tunnel interface never becomes fully operational.
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 Teredo server IPv4 address is not configured on R1.
Why this is correct
Without a Teredo server, the router cannot obtain a valid Teredo address or prefix.
- ✗
The tunnel mode should be ipv6ip teredo relay instead of teredo.
Why it's wrong here
The mode is correct for a Teredo client.
- ✗
The IPv4 address of the Teredo relay is not reachable.
Why it's wrong here
The issue is with the server, not the relay.
- ✗
Teredo is not supported on Cisco IOS.
Why it's wrong here
Teredo is supported on some IOS versions.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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