Cisco Nexus 7000 Series Configuration Manual page 78

Information About IPv6
S e n d d o c u m e n t c o m m e n t s t o n e x u s 7 k - d o c f e e d b a c k @ c i s c o . c o m .
Figure 3-12
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 is the IPv6 address of the node (the IPv6 address of the node interface that
sends the neighbor advertisement message). The destination address is the IPv6 address of the node that
sent the neighbor solicitation message. The data portion includes the link-layer address of the node that
sends the neighbor advertisement message.
After the source node receives the neighbor advertisement, the source node and destination node can
communicate.
Neighbor solicitation messages can verify the reachability of a neighbor after a node identifies the
link-layer address of a neighbor. When a node wants to verify the reachability of a neighbor, it uses the
destination address in a neighbor solicitation message as 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 a change, the destination address for the neighbor advertisement is the
all-nodes multicast address.
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 a positive acknowledgment is returned from the neighbor
(indicating that packets previously sent to the neighbor have been received and processed). A positive
acknowledgment—from an upper-layer protocol (such as TCP)—indicates that a connection is making
forward progress (reaching its destination). If packets are reaching the peer, they are also reaching the
next-hop neighbor of the source. 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
acknowledgment that the forward path is still working (neighbor advertisement messages that have the
solicited flag set to a value of 1 are sent only in response to a neighbor solicitation message). Unsolicited
messages confirm only the one-way path from the source to the destination node; solicited neighbor
advertisement messages indicate that a path is working in both directions.
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
3-14
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 3 Configuring IPv6
OL-20002-02