Hi Jose,
Let’s look at an example:
R1#show interfaces GigabitEthernet 1 | include bia
Hardware is CSR vNIC, address is fa16.3e60.0217 (bia fa16.3e60.0217)
The 7th bit is in the first two hexadecimal characters:
fa = 1111 1010
As you can see, the 7th bit is set to 1 here. Now let’s check the IPv6 address:
R1#show ipv6 interface GigabitEthernet 1 | include link-local
IPv6 is enabled, link-local address is FE80::F816:3EFF:FE60:217
Let’s write down the complete uncompressed address:
FE80:0000:0000:0000:F816:3EFF:FE60:0217
Let’s look only at the EUI-64 part:
F816:3EFF:FE60:0217
We only care about the first two hexadecimal characters:
F8 = 1111 1000
As you can see, the 7th bit has been inverted from 1 to 0.
Let’s see what happens when we change the 7th bit in the MAC address, right now it starts with fa:
fa = 1111 1010
Let’s make it:
f8 = 1111 1000
R1(config)#interface gigabitEthernet 1
R1(config-if)#mac-address f816.3e60.0217
Our IPv6 address has become:
R1#show ipv6 interface GigabitEthernet 1 | include link-local
IPv6 is enabled, link-local address is FE80::FA16:3EFF:FE60:217
The complete uncompressed address is:
FE80:0000:0000:0000:0000:FA16:3EFF:FE60:0217
Here’s the EUI-64 part:
FA16:3EFF:FE60:0217
First two hexadecimal characters:
FA = 1111 1000
As you can see, it got inverted. No matter if the 7th bit of the MAC address is a 0 or 1, it always gets inverted.
Hope this helps!
Rene