Neighbor discovery

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. Once 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 CP, the switch can drop some of these packets due to the in built 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.

Recommended values for reachable time is 180000, and for retransmit interval is 5000.