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How does ARP work in IPv6?

Published in IPv6 Networking 3 mins read

In IPv6, the Address Resolution Protocol (ARP) is not used. Instead, the Neighbor Discovery Protocol (NDP) handles the functionality that ARP provides in IPv4.

Understanding the Shift from ARP to NDP

  • ARP's Role in IPv4: In IPv4, ARP maps IP addresses to MAC addresses on the same local network. It uses broadcasts to find the MAC address associated with a specific IP address.
  • NDP's Role in IPv6: IPv6 uses NDP for a similar purpose, but it's far more sophisticated and efficient than ARP. NDP not only performs address resolution, but it also handles router discovery, address autoconfiguration, and other related tasks.

How NDP Replaces ARP

Feature ARP (IPv4) NDP (IPv6)
Functionality Resolves IP addresses to MAC addresses. Resolves IP addresses to MAC addresses, router discovery, address autoconfiguration, etc.
Protocol Type Broadcast-based Multicast-based
Command to view neighbors show arp show ipv6 neighbors
Messages ARP requests and replies Neighbor Solicitation, Neighbor Advertisement, Router Solicitation, Router Advertisement.
Efficiency Less efficient due to broadcasting More efficient due to multicast, improved error handling and security

NDP Key Features

  • Neighbor Solicitation (NS): Like an ARP request, a node sends an NS message to a destination IPv6 address on the local network to request its MAC address. The destination address can be a unicast address or a multicast address.
  • Neighbor Advertisement (NA): The node that owns the solicited address replies with an NA message, providing its MAC address. It can also be sent unsolicited to announce changes in MAC address or status.
  • Multicast communication: NDP primarily uses multicast instead of broadcast, reducing network congestion.
  • Router Discovery: NDP also handles the discovery of routers on the network, using Router Solicitation (RS) and Router Advertisement (RA) messages.
  • Address Autoconfiguration: NDP helps in the automatic configuration of IPv6 addresses using Stateless Address Autoconfiguration (SLAAC).

Practical Insights

  • Command Equivalence: As noted in the reference, the command show arp in IPv4 is replaced by the show ipv6 neighbors command in IPv6. This IPv6 command displays vital neighbor information including the IPv6 address, the associated MAC address, and the interface used.

Example Output of show ipv6 neighbors

IPv6 Address                        Link-layer Address    Interface
2001:db8::2                         00:11:22:33:44:55     GigabitEthernet0/0
2001:db8::1                         AA:BB:CC:DD:EE:FF     GigabitEthernet0/1
fe80::c00:1234:5678:9abc            11:22:33:44:55:66     GigabitEthernet0/0

Conclusion

In conclusion, while ARP is used in IPv4 for address resolution, it is replaced by the Neighbor Discovery Protocol (NDP) in IPv6 which is more efficient and provides additional features.

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