IPv6 6in4 Tunnel Not Forwarding Traffic
Presenting Symptom
IPv6 traffic is not being forwarded through a 6in4 tunnel configured between two routers.
Network Context
A small branch office (R1) connects to the main office (R2) over the public IPv4 Internet using a 6in4 tunnel (Tunnel0). Both routers run IOS 15.x. The tunnel is configured with IPv6 addresses and a static route for the remote IPv6 network. The branch office reports that hosts on its IPv6 LAN cannot reach hosts on the main office IPv6 LAN.
Diagnostic Steps
Check tunnel interface status
show interfaces tunnel 0Tunnel0 is up, line protocol is up (connected)
If the interface is down/down, the tunnel is not operational. Check IPv4 connectivity between tunnel endpoints and ensure tunnel source/destination are reachable.
Verify IPv6 routing table for remote network
show ipv6 routeS ::/0 [1/0] via Tunnel0, directly connected S 2001:db8:2::/64 [1/0] via Tunnel0
Look for a route to the remote IPv6 network pointing to Tunnel0. If missing, the static route may be misconfigured or not present.
Check tunnel keepalive and IPv4 reachability
ping 192.0.2.2 source 192.0.2.1Success rate is 100 percent (5/5)
If ping fails, the IPv4 connectivity between tunnel endpoints is broken. Check ACLs, NAT, or routing for the IPv4 transport.
Examine tunnel configuration for mismatches
show running-config interface tunnel 0interface Tunnel0 ipv6 address 2001:db8:1::1/64 tunnel source 192.0.2.1 tunnel destination 192.0.2.2 tunnel mode ipv6ip
Ensure tunnel mode is ipv6ip (6in4). Verify source/destination IPs are correct and match the IPv4 addresses used for transport.
Root Cause
The tunnel interface is administratively down because the 'no shutdown' command was not issued on Tunnel0 after configuration.
Resolution
Verification
Run 'show interfaces tunnel 0' and confirm 'Tunnel0 is up, line protocol is up'. Then test IPv6 connectivity with 'ping 2001:db8:2::2 source 2001:db8:1::1' and expect success.
Prevention
1. Always issue 'no shutdown' on tunnel interfaces after creation. 2. Use configuration templates or scripts to ensure consistent tunnel setup. 3. Implement monitoring to alert on tunnel interface state changes.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why a 6in4 tunnel is not forwarding traffic. The exam tests knowledge of tunnel interface states, IPv6 routing, and basic IPv4 connectivity. Key fact: A tunnel interface must be administratively enabled ('no shutdown') and both endpoints must have IPv4 reachability.
Exam Tips
Remember that 'show interfaces tunnel' output includes line protocol status; 'up/up' means operational.
The exam may present a scenario where the tunnel is configured but 'no shutdown' is missing; look for 'administratively down' in the output.
Know that 'tunnel mode ipv6ip' is required for 6in4 tunnels; other modes like gre ip are for different tunnel types.
Commands Used in This Scenario
ping [ip]
The ping command sends ICMP echo requests to a destination IP address to test network connectivity and measure round-trip time.
show ipv6 route
Displays the IPv6 routing table on a Cisco router, showing all known IPv6 routes and their next-hop information for troubleshooting and verification of IPv6 routing.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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