An IPv6 link-local address (LLA) is a unicast address used to communicate with devices on the same network segment. It cannot reach devices across a router, but every IPv6-enabled interface requires one. section 20.5 section 20.5.3
Key properties to remember:
- Uses the fe80::/64 prefix. The broader reserved range is fe80::/10; the following 54 bits are zero. section 20.5.3
- Generates automatically on an IPv6-enabled interface, typically using Modified EUI-64. The ipv6 enable command creates an LLA even without a global unicast address. section 20.5.3 section 20.6
- Identifies one host and works only between devices on the same link. Routers do not forward packets sourced from or destined for LLAs. section 20.5.3 section 21.3.2
- Allows exactly one LLA per IPv6 interface. Manually configuring a new LLA replaces the automatically generated or previous one. section 20.5.3
- Needs no connected or local routing-table entry because LLA traffic never leaves its link. section 21.2.1
Automatic and manual configuration on Cisco IOS
Configuring a global IPv6 address automatically creates an LLA: R1(config)# interface g0/0 R1(config-if)# ipv6 address 2001:db8::1/64 R1(config-if)# no shutdown R1(config-if)# do show ipv6 interface brief GigabitEthernet0/0 [up/up] FE80::260:2FFF:FE62:B801 2001:DB8::1 You can replace the generated address with a simpler one: R1(config-if)# ipv6 address fe80::1 link-local The interface then shows FE80::1 as its LLA while retaining 2001:DB8::1.
LLAs can serve as static-route next hops even though they cannot be route destinations. Because every link uses the same fe80::/64 prefix, IOS requires both the exit interface and next-hop LLA: ipv6 route destination-prefix exit-interface next-hop. The same LLA may be reused on separate links, because uniqueness is required only within each link. section 21.3.2 section 21.4
