3Com S7906E - Switch, Configuration Manual

Page: 463 / 2621

1-7
ICMPv6 message Number Function
Redirect message 137
When a certain condition is satisfied, the default
gateway sends a redirect message to the source host
so that the host can reselect a correct next hop router
to forward packets.
The NDP mainly provides the following functions:
Address resolution
Similar to the ARP function in IPv4, a node acquires the link-layer addresses of neighbor nodes on the
same link through NS and NA messages. Figure 1-3 shows how node A acquires the link-layer address
of node B.
Figure 1-3 Address resolution
The address resolution procedure is as follows:
1) Node A multicasts an NS message. The source address of the NS message is the IPv6 address of
the sending interface of node A and the destination address is the solicited-node multicast address
of node B. The NS message contains the link-layer address of node A.
2) After receiving the NS message, node B judges whether the destination address of the packet is its
solicited-node multicast address. If yes, node B learns the link-layer address of node A, and then
unicasts an NA message containing its link-layer address.
3) Node A acquires the link-layer address of node B from the NA message.
Neighbor reachability detection
After node A acquires the link-layer address of its neighbor node B, node A can verify whether node B is
reachable according to NS and NA messages.
1) Node A sends an NS message whose destination address is the IPv6 address of node B.
2) If node A receives an NA message from node B, node A considers that node B is reachable.
Otherwise, node B is unreachable.
Duplicate address detection
After node A acquires an IPv6 address, it will perform duplicate address detection (DAD) to determine
whether the address is being used by any other node (similar to the gratuitous ARP function of IPv4).
DAD is accomplished through NS and NA messages. Figure 1-4 shows the DAD procedure.