link-local address calculation

Owen DeLong owen at delong.com
Thu Apr 7 07:48:50 CDT 2011


On Apr 7, 2011, at 1:18 AM, Vikas Sharma wrote:

> Hi,
> 
> How to get link-local address from BIA. I have seen some information
> on Internet but it is not working for all.
> 
Pretty simple:

Split the BIA into two 24 bit chunks:

	cc000b / fc0000

Then insert fffe in the middle:

	cc000b fffe fc0000

Now invert the 0x02 bit of the first octet:

	ce000b fffe fc0000

Prefix with fe80::

	fe80::ce00:0bff:fefc:0000

Which if you shorten it becomes:

	fe80::ce00:bff:fefc:0

> =============
> The BIA in this case is cc00.0bfc.0000. The rules for the modified
> EUI-64 addressing are:
> 
> •FFFE will be put in between the vendor-id (3 most significant bytes)
> and the extension-id (3 least significant bytes), which will lead to
> cc00.0bFF.FEfc.0000
> •Thereafter the seventh bit, known as universal/local bit, gets
> inverted. So first change the cc00 from hex to binary, which leads to
> 110011 0 000000000 and then invert the seventh bit (here a 0): 110011
> 1 000000000. Moved back to hex this will give CE00
> •At last, change all points ‘.’ to colons ‘:’ and delete all leading
> zeros, then you got your link-local IPv6 address:
> FE80::CE00:BFF:FEFC:0
> ============
> 
> So if your BIA is 0008.209a.081b (bia 0008.209a.081b), how link-local
> be calculated. This is the link-local on router =>
>  IPv6 is enabled, link-local address is FE80::208:20FF:FE9A:81B
> 
> Also on XR below is the link local address
> 
> Loopback6 is Up, ipv6 protocol is Up, Vrfid is default (0x60000000)
>  IPv6 is enabled, link-local address is fe80::884c:2dff:fe84:8d75
> 
> I can not see any interface with BIA ending with 75 ...
> 
Have you looked for the BIA on the loopback interface? While it's not
a hardware interface, per se, the device is obviously assigning
a BIA of 8a:4c:2d:84:8d:75 to the loopback6 interface.

> Can someone explain me this?
> 
I think I just did. ;-)

Owen





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