What's ndisc_cache?

Question

Suppose I want to use :

$ ip ntable show dev eth0 
inet arp_cache 
    dev eth0 
    refcnt 4 reachable 20744 base_reachable 30000 retrans 1000 
    gc_stale 60000 delay_probe 5000 queue 31 
    app_probes 0 ucast_probes 3 mcast_probes 3 
    anycast_delay 1000 proxy_delay 800 proxy_queue 64 locktime 1000 

inet6 ndisc_cache 
    dev eth0 
    refcnt 1 reachable 40768 base_reachable 30000 retrans 1000 
    gc_stale 60000 delay_probe 5000 queue 31 
    app_probes 0 ucast_probes 3 mcast_probes 3 
    anycast_delay 1000 proxy_delay 800 proxy_queue 64 locktime 0 

What's ndisc_cache?

Answer 1

In IPv4 networks the neighbour tables are written with usage of the Address Resolution Protocol. Those tables are commonly known as "ARP-tables". They resolve IP-addresses (addresses in the network layer) in MAC-addresses (addresses in the link-layer) and vice-vesa. You can list the entries of this table by the command arp -a or ip neigh show.

On the other side, in the IPv6 internet protocol suite, the functionality of the ARP protocol is provided by a more advanced protocol named Neighbour Discovery Protocol.

The ip ntable show command provides information about the neighbour table of the given network device, therefore:

• arp_cache stands for an ARP-table (ARP Cache) of an IPv4 network.
• ndisc_cache stands for an NDP-table (Neighbor Cache) of an IPv6 network.