| IPv6 Network - Neighbor Discovery |
IPv6 Network - Neighbor Discovery
When a system wants to send an IPv6 packet to another system connected to the same subnet or link, it needs to know what MAC address (or “link address” in the new IPv6 terminology) it should address the packet to, unless the interface in question is a point-to-point interface. Neighbor discovery allows systems to discover each other’s MAC addresses, similar to ARP on Ethernet with IPv4.
Each IPv6 system joins the “solicited node” multicast group that corresponds to each of its addresses. Because the solicited node group address consists of the prefix ff02:0:0:0:0:1:ff00::/104 followed by the bottom 24 bits of the address in question, addresses in different prefixes based on the same interface identifier (including the link-local address) all map to the same solicited node address. Whenever a system needs to find out the link address for another system residing on the same link, it sends a neighbor solicitation to the solicited node address that the IPv6 address of the remote system maps to. For good measure, the source host includes its own MAC address in the neighbor solicitation, so the neighbor knows where to send the reply. Because multiple IPv6 addresses can map to the same single solicited node address, a system receiving a neighbor solicitation message will first check whether the request is indeed for one of its addresses. If so, the system sends back a neighbor advertisement with its link address in it. At the same time, the system stores the IPv6/MAC address combination from the request in its neighbor discovery mapping table, also called “neighbor cache.”
You can list and manipulate the system’s list of IPv6 neighbors with the ndp command under FreeBSD and MacOS and show the neighbor list with show ipv6 neighbors under Cisco IOS. See Chapter 10 for more information about using the ndp command.
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