La prueba, para que conste:
root@router:~# ifconfig ppp0
ppp0 Link encap:Point-to-Point Protocol
inet addr:X.X.X.X P-t-P:192.168.144.1 Mask:255.255.255.255
inet6 addr: 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf/64 Scope:Global
inet6 addr: fe80::f6f9:11ff:feb3:ebbf/10 Scope:Link
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1492 Metric:1
RX packets:26492750 errors:0 dropped:0 overruns:0 frame:0
TX packets:18176405 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:19012673895 (17.7 GiB) TX bytes:3758056845 (3.4 GiB)
root@router:~# tcpdump -n -i ppp0 ip6
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on ppp0, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
14:28:18.390442 IP6 2001:470:0:35d::2 > 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf: ICMP6, echo request, seq 1, length 24
14:28:18.390482 IP6 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf > 2001:470:0:35d::2: ICMP6, echo reply, seq 1, length 24
14:28:23.393088 IP6 2001:470:0:35d::2 > 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf: ICMP6, echo request, seq 2, length 24
14:28:23.393111 IP6 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf > 2001:470:0:35d::2: ICMP6, echo reply, seq 2, length 24
14:28:28.493182 IP6 2001:470:0:35d::2 > 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf: ICMP6, echo request, seq 3, length 24
14:28:28.493205 IP6 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf > 2001:470:0:35d::2: ICMP6, echo reply, seq 3, length 24
14:28:33.590948 IP6 2001:470:0:35d::2 > 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf: ICMP6, echo request, seq 4, length 24
14:28:33.590971 IP6 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf > 2001:470:0:35d::2: ICMP6, echo reply, seq 4, length 24
14:28:38.693267 IP6 2001:470:0:35d::2 > 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf: ICMP6, echo request, seq 5, length 24
14:28:38.693290 IP6 2a02:9100:e65a:f249:f6f9:11ff:feb3:ebbf > 2001:470:0:35d::2: ICMP6, echo reply, seq 5, length 24
^C
10 packets captured
10 packets received by filter
0 packets dropped by kernel
Obviamente los "reply" no llegan al origen. Ahí es donde filtran. Si no, tendríamos conectividad IPv6