HP Cisco MDS 9020 - Fabric Switch Configuration Manual page 1168

About IPv6
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m
Figure 46-7
ICMPv6 type = 135
Src = A
Dst = solicited-node multicast of B
Data = link-layer address of A
Query = what is your link address?
After receiving the neighbor solicitation message, the destination node replies by sending a neighbor
advertisement message, which has a value of 136 in the Type field of the ICMP packet header, on the
local link. The source address in the neighbor advertisement message is the IPv6 address of the node
(more specifically, the IPv6 address of the node interface) sending the neighbor advertisement message.
The destination address in the neighbor advertisement message is the IPv6 address of the node that sent
the neighbor solicitation message. The data portion of the neighbor advertisement message includes the
link-layer address of the node sending the neighbor advertisement message.
After the source node receives the neighbor advertisement, the source node and destination node can
communicate.
Neighbor solicitation messages are also used to verify the reachability of a neighbor after the link-layer
address of a neighbor is identified. When a node wants to verifying the reachability of a neighbor, the
destination address in a neighbor solicitation message is the unicast address of the neighbor.
Neighbor advertisement messages are also sent when there is a change in the link-layer address of a node
on a local link. When there is such a change, the destination address for the neighbor advertisement is
the all-node multicast address.
Neighbor solicitation messages are also used to verify the reachability of a neighbor after the link-layer
address of a neighbor is identified. Neighbor unreachability detection identifies the failure of a neighbor
or the failure of the forward path to the neighbor, and is used for all paths between hosts and neighboring
nodes (hosts or routers). Neighbor unreachability detection is performed for neighbors to which only
unicast packets are being sent and is not performed for neighbors to which multicast packets are being
sent.
A neighbor is considered reachable when the neighbor returns a positive acknowledgment indicating that
it has received and processed packets previously sent to it. A positive acknowledgment could be from an
upper-layer protocol such as TCP indicating that a connection is making forward progress (reaching its
destination) or the receipt of a neighbor advertisement message in response to a neighbor solicitation
message. If packets are reaching the peer, they are also reaching the next-hop neighbor of the source.
Therefore, forward progress is also a confirmation that the next-hop neighbor is reachable.
For destinations that are not on the local link, forward progress implies that the first-hop router is
reachable. When acknowledgments from an upper-layer protocol are not available, a node probes the
neighbor using unicast neighbor solicitation messages to verify that the forward path is still working.
The return of a solicited neighbor advertisement message from the neighbor is a positive
Cisco MDS 9000 Family CLI Configuration Guide
46-8
IPv6 Neighbor Discovery—Neighbor Solicitation Message
ICMPv6 type = 136
Src = B
Dst = A
Data = link-layer address of B
A and B can now exchange
packets on this link
Chapter 46 Configuring IPv6 for Gigabit Ethernet Interfaces
OL-16184-01, Cisco MDS SAN-OS Release 3.x