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Sep 2, 2010, 4:53 AM
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Problem with IPv6 anycast
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Hello,
I seem to have issues understanding IPv6 anycast on NI MLX/XMR.
My understanding is, that any router that does have an IPv6 address in a
subnet should be listening on the Subnet-Router anycast address (RFC3513
section 2.6.1).
When I configure an IPv6 unicast address on a MLX VE it shows that it
joins the Subnet-Router anycast address (2001:db8:f1:c:: [Anycast]):
Interface VE 200 is up, line protocol is up
[...]
IPv6 is enabled, link-local address is fe80::20c:dbff:fee1:2f00
[Preferred]
Global unicast address(es):
2001:db8:f1:c::2 [Preferred], subnet is 2001:db8:f1:c::/64
2001:db8:f1:c:: [Anycast], subnet is 2001:db8:f1:c::/64
2001:db8:f1:c::1 [Anycast], subnet is 2001:db8:f1:c::/64
2001:db8:f1:c:: [Anycast], subnet is 2001:db8:f1:c::/64
Joined group address(es):
ff02::1:ff00:1
ff02::1:ff00:0
ff02::1:ff00:2
ff02::1:ffe1:2f00
ff02::2
ff02::1
[...]
Now when I try to ping that address from a client within the network
2001:db8:f1:c::/64, I get a neighbour advertisement from the MLX announcing
it's UNIcast address:
16:02:40.548719 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32)
2001:db8:f1:c::160 > ff02::1:ff00:0: [icmp6 sum ok] ICMP6, neighbor
solicitation, length 32, who has 2001:db8:f1:c::
source link-address option (1), length 8 (1): 00:16:3e:19:16:a9
0x0000: 0016 3e19 16a9
16:02:40.548719 IP6 (class 0xc0, hlim 255, next-header ICMPv6 (58) payload
length: 32) 2001:db8:f1:c::2 > 2001:db8:f1:c::160: [icmp6 sum ok] ICMP6,
neighbor advertisement, length 32, tgt is 2001:db8:f1:c::2, Flags [router,
solicited]
destination link-address option (2), length 8 (1):
00:0c:db:e1:2f:00
0x0000: 000c dbe1 2f00
This causes the asking client to create an entry in its neighbour cache
for the routers unicast address, but not realising, that this was the
answer to its question, hence it keeps on sending solicitation messages for
the anycast address.
In my understanding the neighbour advertisement should have the unicast
address as source, but should announce the anycast address.
If I take a Linux box and enable ip6 forwarding in the kernel, it also
joins that very same anycast group. If I do the same test again, you can
see,
that the Linux box answers with its unicast address, announcing the
anycast address:
12:50:36.194948 IP6 (hlim 255, next-header ICMPv6 (58) payload length: 32)
2001:db8:f1:c::160 > 2001:db8:f1:c::21: [icmp6 sum ok] ICMP6, neighbor
advertisement, length 32, tgt is 2001:db8:f1:c::, Flags [router, solicited]
destination link-address option (2), length 8 (1):
00:16:3e:19:16:a9
0x0000: 0016 3e19 16a9
This works and the client creates an entry in its neighbour cache for the
anycast address and can now communicate with the address.
Same goes for the explicitly configured anycast address 2001:db8:f1:c::1
you can see in the output of the interface.
If it matters, I do have ipv6 nd suppress-ra active on that VE.
So what does this mean, is it
a) Me not understanding the concept of the anycast address (basicaly I
want to use it as a default gateway)?
b) A problem with my configuration (which is pretty plain for v6 right
now)?
c) A bug?
d) ??
Does anyone have experience with that they are able to share?
Thanks for any hints,
Philipp
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Problem with IPv6 anycast
