subject: CCNA Training - Neighbor Discovery for Layer 2 Discovery [print this page] CCNA Training - Neighbor Discovery for Layer 2 Discovery
As a CCNA / CCNP candidate you are expected to understand the purpose and function of the IPv6 Neighbor Discovery protocol.
The CCNA / CCNP's exam will ask of you to determine the purpose of Neighbor Discovery Protocol, what it contains and the role it plays.
In traditional IPv4 network Neighbor discovery for L2 mapping is achieved by employing a the protocol called ARP (Address Resolution Protocol), address resolution is also required in IPv6 to determine the L2 address of devices on the same link.
IPv6 uses a mechanism which discovers the L2 address of a device using ICMPv6 multicast messages, before an IPv6 device can send a packet to another device on the same LAN is see if it has a L3 to L2 mapping within it's neighbor database.
If no mapping exists within it's neighbor database the device will use a special message called a Neighbor solicitation message asking the target host to reply with it's L2 address. The target host will reply with it's own Neighbor Advertisement ICMP unicast back to the device requesting the L2 address.
The original Neighbor Solicitation message uses a special multicast destination address referred to as a "Solicited node Multicast". The device sending the device will construct the Solicited Node Multicast using the last 24 bits of the target host IPv6 address.
The solicited node address is formatted in the following manner FF02::1:FF:0/104. The remaining 24 bits of the solicited node address is created by adding the final 24 bits of the target host.
If there is a target host with the address 2000::4444:4444:1234:1234/64, the requesting device will construct a solicited node address FF02::1:FF:34:1234, now when the requesting device wants to find the L2 address of the it simply constructs the address and sends it out onto the wire, the target host is listening out on the wire for it's address in this example FF02::1:FF:34:1234, when the target device hears it's own address it can reply with it's L2 address.
