Hi Daniel,

danielseijas3010 · ‎06-30-2016

Hi!,

I am slightly confused as to what IP to use in the Neighbor Solicitation (NS) as the source and in the Neighbor Advertisement packet (NA) as the destination.

If the client is requesting for the MAC of a specific IPv6 address, it sends an NS out destined to the Solicited Multicast Address, but which one of its two IPs (Global Unicast Address or Link Local Address) should it use? Same question applies for DAD and IND.

The internet shows examples for both cases: NS packets sourced from an LLA and a Global Unicast Address.

Best Regards

Daniel Seijas

CCNA

Peter Paluch · ‎07-01-2016

Hi Daniel,

RFC 4861 Section 7.2.2 says:

   If the source address of the packet prompting the solicitation is the
   same as one of the addresses assigned to the outgoing interface, that
   address SHOULD be placed in the IP Source Address of the outgoing
   solicitation.  Otherwise, any one of the addresses assigned to the
   interface should be used.

This description is somewhat hard to digest, so let's do an example: Assume that your interface has a link-local IPv6 address of FE80::1 and a global address 2001:DB8:1:1::1/64. Now, assume you have a data packet whose next hop (perhaps the final destination, perhaps just the nearest router) is the IPv6 address X, and you need to translate X into its MAC address. This packet is not a Neighbor Solicitation itself , but because of this packet, you need to send a Neighbor Solicitation for X. Now:

If this packet is sourced from 2001:DB8:1:1::1/64, then you would send out a Neighbor Solicitation for host X, and this solicitation would be also sourced from 2001:DB8:1:1::1/64
If this packet is sourced from FE80::1, then you would send out a Neighbor Solicitation for host X, and this solicitation would be also sourced from FE80::1
If this packet is sourced from, say, 2222:DEAD:BEEF::CAFE (a different host, and you are just routing the packet), then according to the rules above, the Neighbor Solicitation can be sourced from any address this interface has. On Cisco routers, it seems that the link-local address is used as the source of the Neighbor Solicitation message in this case.

Best regards,
Peter

L2 Mapping (NDP), DAD and IND question