Addresses That Are Not Unique

In very rare cases, the rightmost 24 bits of the unicast address of the target will not be unique on the link. Solicited-node multicast addresses are used in IPv6 for address resolution of an IPv6 address to a MAC address on a LAN segment. For example, consider two nodes with addresses 2001:DB8:200:300:400:500:aaaa:bbbb and 2001:DB8:200:300:400:501:aaaa:bbbb, where the link prefix is 2001:DB8:200:300::/64. These two nodes, then, would be listening to the same solicited-node multicast address. Each would receive the multicast packet, but only the node whose full address matched the full target address of the multicast packet (embedded in the data field of the multicast packet) would respond with a neighbor advertisement (which includes the actual MAC address). The other node would receive the multicast packet, but upon inspection of the embedded target address would realize that it was not the intended recipient of the request, and would not respond.

The following example describes how this situation would work. Node A has this characteristic:

■ Address 2001:DB8:200:300:400:500:1234:5678 Node B has these characteristics:

■ Address 2001:DB8:200:300:500:AAAA:BBBB

■ Solicited-node multicast address FF02:0:0:0:0:1:FFAA:BBBB (the same as node C) Node C has these characteristics:

■ Address 2001:DB8:200:300:501:AAAA:BBBB

■ Solicited-node multicast address FF02:0:0:0:0:1:FFAA:BBBB (the same as node B)

1. Node A desires to exchange packets with node B. Node A sends a neighbor discovery packet to the solicited-node multicast address of B, FF02:0:0:0:0:1:AAAA:BBBB, as described previously. Inside the packet, in addition to other data, is the full IPv6 address that node A is looking for—2001:DB8:200:300:500:AAAA:BBBB. This is called the target address.

2. Both node B and node C are listening to the same multicast address, so they both receive and process the packet.

3. Node B sees that the target address inside the packet is its own and responds as described previously.

4. Node C sees that the target address inside the packet is not its own and does not respond at all.

In this manner, nodes can have the same solicited-node multicast address on the link without causing neighbor discovery, neighbor solicitation, or neighbor advertisement to malfunction.

© 2006 Cisco Systems, Inc. Implementing IPv6 8-29

Interface ID

128-n Bits

• An IPv6 anycast address is a global unicast address that is assigned to more than one interface.

An IPv6 anycast address is a global unicast address that is assigned to more than one interface. When a packet is sent to an anycast address, it is routed to the "nearest" interface having that address. In a WAN scope, the nearest interface is found according to the measure of distance of the routing protocol. In a LAN scope, the nearest interface is found according to the first neighbor that is learned about. The following describes the characteristics of the anycast:

■ Anycast addresses are allocated from the unicast address space, so they are indistinguishable from the unicast address. When assigned to a node interface, the node must be explicitly configured to know that the address is an anycast address.

■ The idea of anycast in IP was proposed in 1993. For IPv6, anycast is defined as a way to send a packet to the nearest interface that is a member of the anycast group, which enables a type of discovery mechanism to the nearest point.

■ There is little experience with widespread anycast usage. A few anycast addresses are currently assigned: the router-subnet anycast and the Mobile IPv6 home agent anycast.

■ An anycast address must not be used as the source address of an IPv6 packet.

8-30 Building Scalable Cisco Internetworks (BSCI) v3.0

A router on the local link can send network information, such as a 64-bit prefix of the local link network and the default route. It sends this to all the nodes on the local link. A host can autoconfigure itself by appending its IPv6 interface identifier (64-bit format) to the local link prefix (64 bits). This process results in a full 128-bit address that is usable and guaranteed to be globally unique.

A process called duplicate address translation is enabled to detect and avoid duplicate addresses.

Autoconfiguration enables the plug-and-play feature, which allows devices to connect themselves to the network without any configuration and without any servers (like DHCP servers). This key feature enables deployment of new devices on the Internet, such as cellular phones, wireless devices, home appliances, and home networks.

Note Stateless DHCP is a new concept (February 2004) that strikes a middle ground between stateless autoconfiguration and the thick-client approach of stateful DHCP. Stateless DHCP for IPv6 is also called "DHCP-lite." See RFC 3736, Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6.

© 2006 Cisco Systems, Inc. Implementing IPv6 8-31

A Standard Stateless Autoconfiguration

Stage 1: The PC sends a router solicitation to request a prefix for stateless autoconfiguration.

The PC first configures its link-local address and then sends a router solicitation to request a prefix for stateless autoconfiguration.

A Standard Stateless Autoconfiguration (Cont.)

node45.example2.com (initial) 2001 :db8:a::405

• Stage 2: The router replies with a router advertisement.

The router replies with a router advertisement, including prefix information.

8-32 Building Scalable Cisco Internetworks (BSCI) v3.0 © 2006 Cisco Systems, Inc.

IPv6 Mobility

This topic describes how IPv6 simplifies mobile IP connections.

This topic describes how IPv6 simplifies mobile IP connections.

Mobility is a very important feature in networks today. Mobile IP is an Internet Engineering Task Force (IETF) standard available for both IPv4 and IPv6. Mobile IP enables mobile devices to move without breaking current connections. In IPv6, mobility is built in, which means that any IPv6 node can use it as needed. However, in IPv4, mobility is a new function that must be added.

The routing headers of IPv6 make Mobile IPv6 much more efficient for end nodes than Mobile IPv4. Mobility takes advantage of the flexibility of IPv6. For example, binding uses some header options (destination) that are mandatory for every IPv6 device. Also, IPv6 mobility creates a new "mobility" extension header.

Continue reading here: Describing IPv6 Routing

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