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Catalyst 2360 Switch Software Configuration Guide
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Chapter 27 Configuring IPv6 Unicast Hosts
Understanding IPv6
128-Bit Wide Unicast Addresses
The switch supports aggregatable global unicast addresses and link-local unicast addresses. It does not
support site-local unicast addresses.
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Aggregatable global unicast addresses are IPv6 addresses from the aggregatable global unicast
prefix. The address structure enables strict aggregation of routing prefixes and limits the number of
routing table entries in the global routing table. These addresses are used on links that are aggregated
through organizations and eventually to the Internet service provider.
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 unique identifier (EUI)-64 format.
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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 (NDP) 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.
For more information, see the section about IPv6 unicast addresses in the “Implementing IPv6
Addressing and Basic Connectivity” chapter in the
Cisco IOS IPv6 Configuration Library
on Cisco.com.
DNS for IPv6
IPv6 supports Domain Name System (DNS) record types in the DNS name-to-address and
address-to-name lookup processes. The 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.
ICMPv6
The Internet Control Message Protocol (ICMP) in IPv6 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 path MTU discovery.
Default Router Preference
The switch supports IPv6 default router preference (DRP), an extension in router advertisement
messages. DRP improves the ability of a host to select an appropriate router, especially when the host is
multihomed and the routers are on different links. The switch does not support the Route Information
Option in RFC 4191.
An IPv6 host maintains a default router list from which it selects a router for traffic to offlink
destinations. The selected router for a destination is then cached in the destination cache. NDP for IPv6
specifies that routers that are reachable or probably reachable are preferred over routers whose
reachability is unknown or suspect. For reachable or probably reachable routers, NDP can either select
the same router every time or cycle through the router list. By using DRP, you can configure an IPv6
host to prefer one router over another, provided both are reachable or probably reachable.
For more information about DRP for IPv6, see the “Implementing IPv6 Addresses and Basic
Connectivity” chapter in the
Cisco IOS IPv6 Configuration Library
on Cisco.com.
