Category: Data link layer

IPv6 Neighbor Discovery Protocol

In IPv6 we do not have ARP (address resolution protocol) anymore. ARP is replaced with ICMP based NDP protocol. NDP or ND protocol uses special IPv6 ICMP messages to find and resolve L2 neighbours IPv6 addresses.

It’s a simple way for hosts to learn IPv6 addresses of neighbours on L2 subnet around himself. That includes learning about other hosts and routers on local network. That is the biggest difference between IPv4 and IPv6, there’s no ARP but ICMP takes the function.

NDP is defined in RFC 2461 and this article will introduce you to NDP functions, main features’ lists, and the related ICMPv6 message types.

As the most precise description of NDP is that it belongs to the Link layer of the Internet Protocol suite in TCP/IP model. We can say that Link layer of TCP/IP model is basically a direct combination of the data link layer and the physical layer in the OSI Open Systems Interconnection protocol stack. As in this blog I always try to use OSI model this article was inserted both to Data-link and Physical layer category.

NDP function

In case of IPv6 networks, the NDP Protocol make use of ICMPv6 messages and solicited-node multicast addresses for operating its core function, which is tracking and discovering other IPv6 hosts that are present on the other side of connected interfaces. Another use of NDP is address autoconfiguration.

Let’s discuss some major roles of IPv6 NDP:

• Stateless address autoconfiguration – SLAAC
• Duplicate address detection DAD
• Router discovery
• Prefix discovery
• Parameter discovery link MTU, hop limits 
• Neighbor discovery
• Neighbor address resolution – replaces ARP in IPv6
• Neighbor and router reachability verification

In order to carry out work NDP uses five types of ICMPv6 messages. In the following list you can find the function as well as summary of their goals.

NDP message types:

Inter-Switch Link (ISL) is specifically used for tagging information of VLAN onto an Ethernet frame. This tagging information permits multiplexing of VLANs over a trunk link and this takes place through an external covering procedure or method, as a result of which the switch can easily determine/identify the membership of VLAN frame over the trunked link. When you run ISL, then it is easy to interconnect number of switches and it is also possible to maintain the information of VLAN while the traffic is passing on trunk links between switches.

CDP – Cisco Discovery Protocol is a powerful network monitoring and troubleshooting tool for Cisco devices. CDP is a tool used by network administrators to see information about directly connected Cisco devices. CDP is a tool that enables you to access a summary of protocol and see information about Cisco devices that are connected. Each Cisco device sends periodic messages. These are known as CDP advertisements. Cisco device is sending them to other directly connected Cisco devices. These advertisements contain information about the types of devices that are connected, IP configuration of devices on other side, interfaces they are connected to, the interfaces used to make the connections, and the model numbers of the devices.