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Catalyst 3750-E and 3560-E Switch Software Configuration Guide
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Chapter 39 Configuring IPv6 Unicast Routing
Understanding IPv6
These addresses are defined by a global routing prefix, a subnet ID, and an interface ID. Current
global unicast address allocation uses the range of addresses that start with binary value 001
(2000::/3). Addresses with a prefix of 2000::/3(001) through E000::/3(111) must have 64-bit
interface identifiers in the extended universal identifier (EUI)-64 format.
• Link local unicast addresses can be automatically configured on any interface by using the link-local
prefix FE80::/10(1111 1110 10) and the interface identifier in the modified EUI format. Link-local
addresses are used in the neighbor discovery protocol and the stateless autoconfiguration process.
Nodes on a local link use link-local addresses and do not require globally unique addresses to
communicate. IPv6 routers do not forward packets with link-local source or destination addresses
to other links.
See the “IPv6 Address Type: Unicast” section in the “Implementing Basic Connectivity for IPv6”
chapter in the Cisco IOS IPv6 Configuration Library at this URL:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5187/products_configuration_guide_chapter0918
6a00801d65f5.html
Each IPv6 host interface can support up to three addresses in hardware (one aggregatable global unicast
address, one link-local unicast address, and zero or more privacy addresses).
DNS for IPv6
IPv6 introduces new Domain Name System (DNS) record types that are supported in the DNS
name-to-address and address-to-name lookup processes. The new DNS AAAA resource record types
support IPv6 addresses and are equivalent to an A address record in IPv4. The switch supports DNS
resolution for IPv4 and IPv6.
Path MTU Discovery for IPv6 Unicast
The switch supports advertising the system MTU to IPv6 nodes and path MTU discovery. Path MTU
discovery (RFC 1981) allows a host to dynamically discover and adjust to differences in the MTU size
of every link along a given data path. In IPv6, if a link along the path is not large enough to accommodate
the packet size, the source of the packet handles the fragmentation. The switch does not support path
MTU discovery for multicast packets.
ICMPv6
The Internet Control Message Protocol (ICMP) in IPv6 (RFC 2463) functions the same as in IPv4. ICMP
generates error messages, such as ICMP destination unreachable messages, to report errors during
processing and other diagnostic functions. In IPv6, ICMP packets are also used in the neighbor discovery
protocol and path MTU discovery. A value of 58 in the Next Header field of the basic IPv6 packet header
identifies an IPv6 ICMP packet.
Neighbor Discovery
The switch supports Neighbor Discovery Protocol (NDP) for IPv6 (RFC 2461), a protocol running on
top of ICMPv6, and Static Neighbor Discovery for IPv6 stations that do not support NDP. The IPv6
neighbor discovery process uses ICMP messages and solicited-node multicast addresses to determine the
link-layer address of a neighbor on the same network (local link), verify the reachability of the neighbor,
and keep track of neighboring routers.
