On an IPv6 network, a number of important functions happen using discovery processes. These include the discovery of neighboring devices and routers, as well as the maximum transmission unit (MTU) that is supported between a source and destination host. Some of these concepts are similar to ones found on an IPv4 network, while others represent new ways of dealing with traditional IPv4 configuration issues.
Neighbor discovery is the process by which an IPv6 node discovers the link-layer address of systems that it needs to communicate with on the local subnet, and is the method by which a node keeps track of local routers. This neighbor discovery process uses a new version of ICMP – ICMPv6. Ultimately, multicasts and anycasts are used for neighbor discovery functions on an IPv6 network. For example, recall the ARP function on an IPv4 network; a host would send out a broadcast requesting that the host with the specified IP send back its MAC address. In IPv6, ICMP multicasts are used to send out a request looking for the link-layer address associated with a known IPv6 address. This helps to reduce some of the traditional issues associated with broadcast traffic negatively impacting network performance.
Router discovery is a feature of IPv6 that allows an IPv6 node to discover the routers connected to its local link network. Although a similar feature exists in IPv4, it is rarely used with most administrators relying upon manually configured gateway addresses instead. Two main types of router discovery messages are used on IPv6 networks – router solicitations, and router advertisements. A router advertisement is a multicast message periodically sent by an IPv6 router that allows a host to gather valuable information about the network. For example, a router advertisement could contain information about the address configuration method that should be used, the IP prefixes in use on the network (or any changes to them), which router should be considered the default router, and more. Router advertisements will be looked at in more detail in the section on addressing hosts.
In contrast, a node sends out a router solicitation method when it does not have a configured IP address at start up. The purpose of this multicast is to gather information about how the node should be configured, but without the need to wait for the next router advertisement. For example, the result of a router solicitation message could be a reply from a local router specifying that the host should auto-configure its IPv6 address, or perhaps that it should use DHCP instead.
Another very important discovery process on IPv6 networks is maximum transmission unit (MTU) discovery. In previous articles in the CCNA series, we looked at how a router was capable of fragmenting an IPv4 packet when the next network on the path to a destination used a smaller MTU size – for example, when forwarding data from a Token Ring segment to an Ethernet segment. While this method helped to avoid some of the issues associated with interconnecting different network types, it also slowed down the communication process, since a router was not only responsible for reframing and making forwarding decisions, but also fragmenting packets and subsequently reframing them all as well. In IPv6, routers no longer fragment any packets. Instead, the sending node uses a process referred to as MTU discovery to determine the larger possible MTU that is supported between itself and the destination host. If any fragmentation needs to take place, it must be done on the sending node – IPv6 routers stay out of this process completely, leading to greater routing efficiency.
To understand how MTU discovery works, consider the figure below. In it, we see a source host attempting to discover the biggest MTU possible between itself and the destination. In this case, the MTU between Host A and its local router is 1500 bytes, the MTU between Routers A and B is 1400, and the MTU between Router B and the destination is 1200.
To discover the MTU, Host A will sent out a packet to Host B attempting to use its 1500 byte MTU. At Router A, an ICMPv6 error message (packet too big) will be sent back saying that an MTU of 1400 should be used. Host A will then send out another packet with an MTU of 1400, which will be designated as too big by Router B, with a maximum MTU of 1280 specified. Since Host B is connected to a network with an MTU of 1280, Host A now knows that this is the MTU that it should use to communicate with Host B. Although this process may seen cumbersome, it’s worth noting that IPv6 specifies a minimum MTU size of 1280 bytes, and that 1500 is usually the default MTU configured on most internetworking equipment.
