IPv6 nodes (routers and hosts) on the same link use neighbor discovery (ND) to discover link-layer addresses and to obtain and advertise various network parameters and reachability information. ND combines the services for IPv4 with the Address Resolution Protocol (ARP) and router discovery. In IPv6 ND performs a function similar to ARP (Address Resolution Protocol) in IPv4.
Hosts use ND to discover the routers in the network that you can use as the default routers, and to determine the link-layer address of neighbors attached to local links. Routers also use ND to discover neighbors and link-layer information. ND updates the neighbor database with valid entries, invalid entries, and entries migrated to various locations.
The ND protocol provides the following services:
address and prefix discovery
Hosts determine the set of addresses that are on-link for the given link. Nodes determine which addresses or prefixes are locally reachable or remote with address and prefix discovery.
router discovery
Hosts discover neighboring routers with router discovery. Hosts establish neighbors as default packet-forwarding routers.
parameter discovery
Hosts and routers discover link parameters such as the link MTU or the hop-limit value placed in outgoing packets.
address autoconfiguration
Hosts configure an address for an interface with address autoconfiguration.
duplicate address detection
Hosts and nodes determine if an address is assigned to another router or a host.
address resolution
Hosts determine link-layer addresses (MAC for Ethernet) of the local neighbors (attached on the local network), provided the IP address is known.
next-hop determination
Hosts determine how to forward local or remote traffic with next-hop determination. The next hop can be a local or remote router.
neighbor unreachability detection
Hosts determine if the neighbor is unreachable, and if address resolution must be performed again to update the database. For neighbors you use as routers, hosts attempt to forward traffic through alternative default routers.
redirect
Routers inform the host of more efficient routes with redirect messages.
Neighbor discovery uses three components:
host-router discovery
Host-router discovery performs the following functions:
router discovery
prefix discovery
parameter discovery
address autoconfiguration
host-host communication
Host-host communication performs the following functions:
address resolution
next-hop determination
neighbor unreachability detection
duplicate address detection
route redirect
Note
When a neighbor transitions to the STALE state, to initiate Neighbor Unreachability detection (NUD), a duplicate copy of the traffic destined to this neighbor is sent to the switch Control Processor (CP) on a low priority queue (queue 0). The original packet is forwarded to this neighbor. After NUD is initiated, the hardware records are updated and the traffic is no longer sent to the CP. When a high rate of such traffic is sent to the CP, the switch can drop some of these packets due to the built-in CP rate limiting feature, which protects the CP from DOS attacks.
Use the command show qos cosq-stats cpu-port to view drop statistics on the CPU queue. This design does not result in loss of traffic.
Use the command ipv6 nd reachable-time <0-3600000> to increase the default value of 3000 milliseconds, which in turn delays the scenario of data path sending STALE neighbor destined packets to the CP.
As a best practice, configure a reachable time value of 180000 and retransmit interval of 5000.