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Re: link-local address calculation
From: Owen DeLong <owen () delong com>
Date: Thu, 7 Apr 2011 05:48:50 -0700


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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