Chapter 33 IPv6 Commands
Ethernet Switch CLI Reference Guide
144
Neighbor Discovery Protocol (NDP)
The Neighbor Discovery Protocol (NDP) is a protocol used to discover other IPv6 devices and
track neighbor’s reachability in a network.
An IPv6 device uses the following ICMPv6 messages types:
• Neighbor solicitation: A request from a host to determine a neighbor’s link-layer address
(MAC address) and detect if the neighbor is still reachable. A neighbor being “reachable”
means it responds to a neighbor solicitation message (from the host) with a neighbor
advertisement message.
• Neighbor advertisement: A response from a node to announce its link-layer address.
• Router solicitation: A request from a host to locate a router that can act as the default
router and forward packets.
• Router advertisement: A response to a router solicitation or a periodical multicast
advertisement from a router to advertise its presence and other parameters.
IPv6 Cache
An IPv6 host is required to have a neighbor cache, destination cache, prefix list and default
router list. The Switch maintains and updates its IPv6 caches constantly using the information
from response messages. In IPv6, the Switch configures a link-local address automatically,
and then sends a neighbor solicitation message to check if the address is unique. If there is an
address to be resolved or verified, the Switch also sends out a neighbor solicitation message.
When the Switch receives a neighbor advertisement in response, it stores the neighbor’s link-
layer address in the neighbor cache. When the Switch uses a router solicitation message to
query for a router and receives a router advertisement message, it adds the router’s information
to the neighbor cache, prefix list and destination cache. The Switch creates an entry in the
default router list cache if the router can be used as a default router.
When the Switch needs to send a packet, it first consults the destination cache to determine the
next hop. If there is no matching entry in the destination cache, the Switch uses the prefix list
to determine whether the destination address is on-link and can be reached directly without
passing through a router. If the address is onlink, the address is considered as the next hop.
Otherwise, the Switch determines the next-hop from the default router list or routing table.
Once the next hop IP address is known, the Switch looks into the neighbor cache to get the
link-layer address and sends the packet when the neighbor is reachable. If the Switch cannot
find an entry in the neighbor cache or the state for the neighbor is not reachable, it starts the
address resolution process. This helps reduce the number of IPv6 solicitation and
advertisement messages.
Multicast Listener Discovery
The Multicast Listener Discovery (MLD) protocol (defined in RFC 2710) is derived from
IPv4's Internet Group Management Protocol version 2 (IGMPv2). MLD uses ICMPv6
message types, rather than IGMP message types. MLDv1 is equivalent to IGMPv2 and
MLDv2 is equivalent to IGMPv3.
MLD allows an IPv6 switch or router to discover the presence of MLD listeners who wish to
receive multicast packets and the IP addresses of multicast groups the hosts want to join on its
network.
MLD snooping and MLD proxy are analogous to IGMP snooping and IGMP proxy in IPv4.
MLD filtering controls which multicast groups a port can join.
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