IPv6CCNA 200-301

IPv6 Traffic Not Routing — Missing ipv6 unicast-routing Command

Presenting Symptom

IPv6 traffic is not being routed between subnets; devices can ping their default gateway but cannot reach devices on other VLANs or remote networks.

Network Context

A small branch office with a Cisco 4321 ISR router running IOS XE 16.9, connected to two VLANs (10 and 20) via a Layer 2 switch. IPv6 addresses are configured on the router's subinterfaces (GigabitEthernet0/0/0.10 and .20), and hosts have IPv6 addresses and default gateways. The router is also connected to an upstream ISP via a native IPv6 link. The problem occurs after a recent router reload; IPv6 static routes are configured but traffic fails to forward.

Diagnostic Steps

Check IPv6 interface status and addresses

show ipv6 interface brief

GigabitEthernet0/0/0.10   [up/up]
    FE80::1
    2001:DB8:10::1
GigabitEthernet0/0/0.20   [up/up]
    FE80::1
    2001:DB8:20::1
GigabitEthernet0/0/1       [up/up]
    FE80::2
    2001:DB8:100::2

All interfaces show up/up and have IPv6 addresses. This confirms Layer 1/2 and IPv6 addressing are correct, so the problem is not at the interface level.

Verify IPv6 routing table for routes

show ipv6 route

IPv6 Routing Table - default - 5 entries
Codes: C - Connected, L - Local, S - Static
C   2001:DB8:10::/64 [0/0]
     via GigabitEthernet0/0/0.10, directly connected
L   2001:DB8:10::1/128 [0/0]
     via GigabitEthernet0/0/0.10, receive
C   2001:DB8:20::/64 [0/0]
     via GigabitEthernet0/0/0.20, directly connected
L   2001:DB8:20::1/128 [0/0]
     via GigabitEthernet0/0/0.20, receive
S   2001:DB8:100::/64 [1/0]
     via 2001:DB8:100::2, GigabitEthernet0/0/1

The routing table shows connected and static routes. However, note the absence of any default route or dynamic routes. The static route is present, but if the router cannot forward packets, the issue may be that IPv6 routing is disabled globally.

Check if IPv6 unicast routing is enabled

show running-config | include ipv6 unicast-routing

No output (the command returns nothing)

The absence of 'ipv6 unicast-routing' in the running config indicates that IPv6 routing is disabled globally. Without this command, the router will not forward IPv6 packets between interfaces, even though IPv6 addresses are configured.

Confirm by checking IPv6 packet forwarding statistics

show ipv6 traffic

IPv6 statistics:
  Rcvd: 100 packets, 0 forwarded
  Sent: 50 packets
  Forwarded: 0 packets

The 'Forwarded: 0 packets' field confirms that the router is not forwarding any IPv6 traffic, even though it receives packets. This is consistent with IPv6 unicast routing being disabled.

Root Cause

The global configuration command 'ipv6 unicast-routing' is missing from the router's configuration. This command is required to enable IPv6 routing on Cisco IOS devices. Without it, the router will not forward IPv6 packets between interfaces, even if IPv6 addresses and routes are correctly configured.

Resolution

Enable IPv6 unicast routing globally: 1. Enter global configuration mode: configure terminal 2. Enable IPv6 routing: ipv6 unicast-routing 3. (Optional) Save the configuration: copy running-config startup-config This single command enables the IPv6 forwarding engine, allowing the router to route IPv6 packets between interfaces.

Verification

After applying the fix, verify with: 1. show running-config | include ipv6 unicast-routing Expected output: ipv6 unicast-routing 2. show ipv6 traffic Expected output: Forwarded: (non-zero value, e.g., 10 packets) 3. Perform a ping from a host on VLAN 10 to a host on VLAN 20 or to the upstream ISP address. The ping should succeed.

Prevention

1. Always include 'ipv6 unicast-routing' in the router's initial configuration template. 2. Use configuration management tools (e.g., Ansible, Cisco DNA Center) to enforce that IPv6 routing is enabled on all routers that need to forward IPv6 traffic. 3. After any router reload or configuration change, verify IPv6 routing is enabled as part of the standard post-reload checklist.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears in troubleshooting questions, often as a multiple-choice or drag-and-drop where you must identify why IPv6 traffic is not being routed. The exam tests the candidate's knowledge that 'ipv6 unicast-routing' is a mandatory global command to enable IPv6 routing. A common distractor is that IPv6 addresses and static routes are configured, leading candidates to overlook the missing global command.

Exam Tips

Memorize that 'ipv6 unicast-routing' is required to enable IPv6 routing; without it, the router acts as an IPv6 host and will not forward packets.

In troubleshooting scenarios, always check the running configuration for 'ipv6 unicast-routing' if IPv6 interfaces are up but traffic is not forwarded.

The 'show ipv6 traffic' command is useful to confirm zero forwarded packets, which strongly indicates IPv6 routing is disabled.

Commands Used in This Scenario

show ipv6 interface brief

Displays a summary of IPv6 interface status and addresses, useful for quickly verifying IPv6 configuration and interface operational state.

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.

show running-config

Displays the current active configuration in DRAM, showing all non-default settings.

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