IPv6: Not a Router?

I managed to frustrate myself recently (sadly, not an uncommon occurrence) while working on an IPv6 lab. One of the routers simply refused to pass traffic, and at first I could not figure out why. The solution was simple - and in hindsight, painfully obvious - but I decided to record it here for posterity.

Here we have a simple scenario, three routers connected in series:

R1 and R3 both have a static default IPv6 route installed, pointing toward R2 as the next hop. All interfaces are up, and you would assume they could ping one another.

R1# show ipv6 route
IPv6 Routing Table - 4 entries
Codes: C - Connected, L - Local, S - Static, R - RIP, B - BGP
       U - Per-user Static route, M - MIPv6
       I1 - ISIS L1, I2 - ISIS L2, IA - ISIS interarea, IS - ISIS summary
       O - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2
       ON1 - OSPF NSSA ext 1, ON2 - OSPF NSSA ext 2
       D - EIGRP, EX - EIGRP external
S   ::/0 [1/0]
     via 2001:DB8:0:12::2
C   2001:DB8:0:12::/64 [0/0]
     via ::, FastEthernet0/0
L   2001:DB8:0:12::1/128 [0/0]
     via ::, FastEthernet0/0
L   FF00::/8 [0/0]
     via ::, Null0
R1# ping 2001:db8:0:12::2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:0:12::2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/8 ms
R1# ping 2001:db8:0:23::3
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:0:23::3, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)

No luck. What gives? Let's start an IPv6 packet debug on R2 to hopefully shed some light on the issue.

R2# debug ipv6 packet
IPv6 unicast packet debugging is on

Attempting our ping again from R1 elicits the following output from the debug on R2:

*Mar  1 00:14:14.575: IPV6: source 2001:DB8:0:12::1 (FastEthernet0/0)
*Mar  1 00:14:14.575:       dest 2001:DB8:0:23::3 (FastEthernet0/1)
*Mar  1 00:14:14.579:       traffic class 0, flow 0x0, len 100+14, prot 58, hops 64, not a router?
*Mar  1 00:14:16.591: IPV6: source 2001:DB8:0:12::1 (FastEthernet0/0)
*Mar  1 00:14:16.591:       dest 2001:DB8:0:23::3 (FastEthernet0/1)
*Mar  1 00:14:16.591:       traffic class 0, flow 0x0, len 100+14, prot 58, hops 64, not a router?
...

"Not a router"? I don't want to get into a philosophical debate, I just want to ping, damnit. After a moment, though, the message clicks: R2 is indeed not an IPv6 router. Unlike its well-established predecessor, IPv6 routing does not yet come enabled out of the box. The administrator must enable it, which I forgot to do.

R2(config)# ipv6 unicast-routing

R1# ping 2001:db8:0:23::3

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:0:23::3, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/9/16 ms

Back on R2, our debug confirms R2 is indeed a router now:

*Mar  1 00:19:26.107: IPV6: source 2001:DB8:0:23::3 (FastEthernet0/1)
*Mar  1 00:19:26.107:       dest 2001:DB8:0:12::1 (FastEthernet0/0)
*Mar  1 00:19:26.107:       traffic class 0, flow 0x0, len 100+14, prot 58, hops 63, forwarding

As a footnote, IPv4 can also be toggled on and off with no ip routing; great for pranking your coworkers, especially considering that debug ip packet (the IPv4 packet debug) won't offer any clues.