IPv6 Default Route Missing — No Internet for IPv6 Clients
Presenting Symptom
IPv6 clients on the internal network cannot reach the internet, but IPv4 connectivity works fine.
Network Context
A small branch office with a Cisco ISR 4321 router (IOS XE 16.9) connecting to an ISP via a single Ethernet link. The router has a /64 IPv6 prefix assigned by the ISP and is configured with SLAAC for internal clients. The internal network uses VLAN 10 with a single subnet. IPv6 clients receive addresses via SLAAC but cannot ping or browse external IPv6 hosts.
Diagnostic Steps
Check IPv6 routing table on the router
show ipv6 routeIPv6 Routing Table - default - 4 entries
Codes: C - Connected, L - Local, S - Static, U - Per-user Static route
B - BGP, R - RIP, I1 - ISIS L1, I2 - ISIS L2
IA - ISIS interarea, IS - ISIS summary, D - EIGRP, EX - EIGRP external
ND - ND Default, NDP - ND Prefix, DCE - Destination, NDr - Redirect
O - OSPF Intra, OI - OSPF Inter, OE - OSPF Ext, ON1 - OSPF NSSA Ext Type 1
ON2 - OSPF NSSA Ext Type 2, l - LISP
C 2001:DB8:1::/64 [0/0]
via GigabitEthernet0/0/0, directly connected
L 2001:DB8:1::1/128 [0/0]
via GigabitEthernet0/0/0, receive
S ::/0 [1/0]
via 2001:DB8:1::2Look for a default route (::/0). If missing, IPv6 traffic has no path to the internet. If present, check the next-hop address and interface.
Verify IPv6 default route configuration
show running-config | include ipv6 routeipv6 route ::/0 2001:DB8:1::2
If no output, the default route is not configured. If output shows a route, verify the next-hop address is correct and reachable.
Check IPv6 connectivity to the next-hop
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), round-trip min/avg/max = 1/2/4 ms
If ping fails, the next-hop is unreachable (e.g., ISP issue, wrong address). If successful, the default route is likely missing or misconfigured.
Confirm IPv6 default route is missing
show ipv6 route ::/0% Network not in table
This confirms no default route exists. The router does not know where to send IPv6 traffic destined outside the local subnet.
Root Cause
The router is missing an IPv6 default route (::/0) pointing to the ISP's next-hop address. Without this route, the router drops all IPv6 traffic destined for external networks, causing IPv6 clients to have no internet access.
Resolution
Verification
Run 'show ipv6 route ::/0' to confirm the route is present: S ::/0 [1/0] via 2001:DB8:1::2 Then test from a client: ping ipv6 google.com or ping ipv6 2001:4860:4860::8888 (Google DNS).
Prevention
1. Always include an IPv6 default route when configuring IPv6 on a router connected to an ISP. 2. Use a routing protocol (e.g., OSPFv3) to dynamically learn the default route from the ISP if supported. 3. Document and verify IPv6 routing table after initial configuration.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where IPv6 connectivity fails. The exam tests the ability to identify a missing default route using 'show ipv6 route' and to configure a static IPv6 default route. Candidates must know the syntax 'ipv6 route ::/0 <next-hop>' and understand that ::/0 represents the IPv6 default route.
Exam Tips
Remember that ::/0 is the IPv6 equivalent of 0.0.0.0/0 for IPv4 default route.
The 'show ipv6 route' command is critical; look for the 'S' code indicating a static route.
In the exam, a missing default route is a common cause of 'no internet' for IPv6, even if IPv4 works.
Commands Used in This Scenario
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