[f-nsp] Problem with IPv6 anycast

[harbor235]

Good question, RFC2526 hs more meat concerning anycast reserver addresses, I
have a couple questions too;

1) Can you allocate additional anycast addresses out of the unicast space
other than the 127-n bits identified?

2) how does your client machine behave when a reservered anycast address is
used?



On Thu, Sep 2, 2010 at 7:53 AM, Philipp Geschke <foundry-nsp at pgmail.net>wrote:

> 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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