How do Cisco routers generate Link-Local Addresses?

Cisco IOS routers auto-create an LLA (using EUI-64) the moment a GUA is assigned to the interface — no separate command needed.

Cisco Router LLA Generation:

Default behavior:

• Cisco IOS routers use EUI-64 to generate LLAs
• LLA is automatically created when a GUA is assigned

Example output:

R1# show interface gigabitEthernet 0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
  Hardware is ISR4221-2x1GE, address is 7079.b392.3640

R1# show ipv6 interface brief
GigabitEthernet0/0/0 [up/up]
    FE80::7279:B3FF:FE92:3640  ← EUI-64 from MAC
    2001:DB8:ACAD:1::1

EUI-64 calculation for MAC 7079.b392.3640:

MAC:     70:79:b3:92:36:40
Split:   70:79:b3 | 92:36:40
Insert:  70:79:b3:ff:fe:92:36:40
Flip 7th bit: 72:79:b3:ff:fe:92:36:40
LLA:     fe80::7279:b3ff:fe92:3640

Best practice: Configure a simple LLA like fe80::1 for easier documentation.

Go deeper:

• MAC address — EUI-64 (Wikipedia) — the OUI/U-L-bit detail behind the EUI-64 LLA Cisco IOS auto-generates.