25 or 6to4...

which is it?

r1

int lo0
ip add 10.1.1.1 255.255.255.0
ipv6 add fec0::1:1/112

int s1/1
ip add 172.16.12.1 255.255.255.0
clock rat 64000
band 64
no shut

r2

int lo0
ip add 10.1.2.1 255.255.255.0

int s1/1
ip add 172.16.12.2 255.255.255.0
band 64
no shut

int s1/2
ip add 172.16.23.2 255.255.255.0
clock rat 64000
band 64
no shut

r3

int lo0
ip add 10.1.3.1 255.255.255.0
ipv6 add fec0::3:1/112

int s1/2
ip add 172.16.23.3 255.255.255.0
band 64
no shut

r1#ping 172.16.12.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.12.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 1/7/16 ms

r3#ping 172.16.23.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.23.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/8/16 ms

load up on eigrp 

router eigrp 1
no auto-summ
netw 10.0.0.0
netw 172.16.0.0

r3#ping 10.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 8/13/24 ms

r3#sh ip route eigrp

Gateway of last resort is not set

      10.0.0.0/8 is variably subnetted, 4 subnets, 2 masks
D        10.1.1.0/24 [90/41152000] via 172.16.23.2, 00:00:54, Serial1/2
D        10.1.2.0/24 [90/40640000] via 172.16.23.2, 00:00:54, Serial1/2
      172.16.0.0/16 is variably subnetted, 3 subnets, 2 masks
D        172.16.12.0/24 [90/41024000] via 172.16.23.2, 00:00:54, Serial1/2