IPv6 Route Not Being Preferred Over IPv4 Route
Presenting Symptom
IPv6 traffic is not being forwarded over the IPv6 route, but IPv4 traffic works fine for the same destination.
Network Context
A small branch office with a Cisco 4321 ISR router running IOS XE 16.9. The router has both IPv4 and IPv6 connectivity to the corporate HQ via a single WAN link. The network engineer configured a static IPv6 route to a remote subnet, but traffic to that subnet is not using the IPv6 route; instead, it is being forwarded via IPv4.
Diagnostic Steps
Check the routing table for the IPv6 route
show ipv6 route 2001:db8:2::/64IPv6 Routing Table - default - 5 entries
Codes: C - Connected, L - Local, S - Static
...
S 2001:db8:2::/64 [1/0]
via 2001:db8:1::2, GigabitEthernet0/0/0If the route is present, the problem is not a missing route. If the route is missing, check static route configuration.
Verify IPv6 forwarding is enabled globally
show running-config | include ipv6 unicast-routingipv6 unicast-routing
If this command is missing, IPv6 forwarding is disabled. Enable it with 'ipv6 unicast-routing'.
Check the administrative distance of the IPv6 route
show ipv6 route 2001:db8:2::/64 detailRouting entry for 2001:db8:2::/64 Known via "static", distance 1, metric 0 Last update from 2001:db8:1::2 on GigabitEthernet0/0/0
The administrative distance (AD) of the static route is 1. If there is another route (e.g., from a dynamic protocol) with a lower AD, it will be preferred. Compare with IPv4 route AD for the same destination.
Check if the IPv6 next-hop is reachable
ping ipv6 2001:db8:1::2Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 2001:db8:1::2, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5)
If ping fails, the next-hop is unreachable. Check IPv6 connectivity on the interface.
Examine the IPv4 routing table for the same destination
show ip route 192.168.2.0 255.255.255.0Routing entry for 192.168.2.0/24 Known via "static", distance 1, metric 0 Last update from 192.168.1.2 on GigabitEthernet0/0/0
If both IPv4 and IPv6 routes have AD 1, the router may prefer IPv4 due to default behavior. Check if 'ipv6 route' is configured correctly.
Root Cause
The IPv6 route is not being preferred because the IPv4 route to the same destination has a lower administrative distance (1) compared to the IPv6 route (also 1), but the router's default behavior prefers IPv4 when both are equal. Additionally, the IPv6 next-hop may be unreachable due to a missing IPv6 address on the interface or incorrect static route configuration.
Resolution
Verification
Run 'show ipv6 route 2001:db8:2::/64' to confirm the route is present and has the correct AD. Then perform a traceroute to the destination: 'traceroute ipv6 2001:db8:2::1' and verify it uses the IPv6 path.
Prevention
1. Always enable 'ipv6 unicast-routing' when using IPv6 static routes. 2. Use consistent administrative distance values for static routes across address families. 3. Verify IPv6 next-hop reachability before configuring static routes.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where a router has both IPv4 and IPv6 routes to the same destination. The exam tests understanding of administrative distance and IPv6 routing fundamentals. Candidates must know that 'ipv6 unicast-routing' must be enabled and that static routes can have AD configured.
Exam Tips
Remember that IPv6 routing is disabled by default; always check 'ipv6 unicast-routing'.
Administrative distance for static routes can be set per route; lower AD wins.
Use 'show ipv6 route' and 'show ip route' to compare routing tables.
Commands Used in This Scenario
show ip route
Displays the current IP routing table on a Cisco router, used to verify routes, check next-hop addresses, and troubleshoot connectivity issues.
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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