Strange IPv6 reachability problem

[Seth Mattinen]

I'm having a strange IPv6 problem that I hope someone here can help with 
or at least generate some ideas on how to resolve. I have a strange 
partial reachability issue with 2001:500:61:28::70 (debian mirror) and a 
couple others, but I'll just focus on this one for clarity. Both working 
and trouble host are sourced from 2607:fe70::/which I announce via BGP 
and route around in my network.

I can reach it fine from my daily I-work-here-in-SSH server:

source: 2607:fe70:0:beef::a
dest: 2001:500:61:28::70

PING 2001:500:61:28::70(2001:500:61:28::70) 56 data bytes
64 bytes from 2001:500:61:28::70: icmp_seq=1 ttl=57 time=81.3 ms
64 bytes from 2001:500:61:28::70: icmp_seq=2 ttl=57 time=81.3 ms
64 bytes from 2001:500:61:28::70: icmp_seq=3 ttl=57 time=82.4 ms
64 bytes from 2001:500:61:28::70: icmp_seq=4 ttl=57 time=81.3 ms
64 bytes from 2001:500:61:28::70: icmp_seq=5 ttl=57 time=81.3 ms

--- 2001:500:61:28::70 ping statistics ---
5 packets transmitted, 5 received, 0% packet loss, time 4015ms
rtt min/avg/max/mdev = 81.304/81.547/82.434/0.511 ms


I *can not* reach it from a server at my parent's house:

source: 2607:fe70:1002:0:202:b3ff:fe30:f633
dest: 2001:500:61:28::70

PING 2001:500:61:28::70(2001:500:61:28::70) 56 data bytes

--- 2001:500:61:28::70 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4013ms


Now, here's the part that confuses me. After trying a whole bunch of 
basic things I ran nfdump and found something I can't explain:

nfdump filter:
proto icmp6 and host 2001:500:61:28::70
Aggregated flows 3
Top 500 flows ordered by flows:
Date flow start          Duration  Proto      Src IP Addr      Dst IP 
Addr   Packets    Bytes      bps    Bpp Flows
2011-06-19 14:55:00.264     4.012  ICMP6 2001:50..:28::70 
2607:fe..beef::a         5      520     1036    104     1
2011-06-19 14:55:00.760     4.016  ICMP6 2607:fe..beef::a 
2001:50..:28::70         5      520     1035    104     1
2011-06-19 14:56:15.724     4.016  ICMP6 2607:fe..30:f633 
2001:50..:28::70         5      520     1035    104     1

Summary: total flows: 3, total bytes: 1560, total packets: 15, avg bps: 
157, avg pps: 0, avg bpp: 104
Time window: 2011-06-19 14:55:00 - 2011-06-19 14:56:19
Total flows processed: 329214, Blocks skipped: 0, Bytes read: 23941692
Sys: 0.288s flows/second: 1143036.7  Wall: 0.324s flows/second: 1014236.3

For the one that works there's flow records in each direction as 
expected. For the one that doesn't there's no return traffic coming back 
to me. So I'm assuming the problem is not with me since if it was, I'd 
at least see a flow record coming back to me if something in my network 
was dropping it. But there's no record of return traffic. Netflow is 
sampled at the routers that connect to each upstream.

I tried from a second server on 2607:fe70:10::/64 and it dodn't work 
either. Every server/router I try from within 2607:fe70::/48 works fine.

Any ideas?

~Seth