IPv6 interface ID U/L flag flipping

Created: May 14, 2019 12:04:04Latest reply: May 15, 2019 06:53:40 203 5 0 0
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Hi,

when creating EUI-64 format of interface ID from MAC address (inserting fffe in the middle), the key (everywhere described) is to flip the 7th bit, i.e. Universal/Local bit.

The question is, whether to flip the bit (i.e. either from 0 to 1 or from 1 to 0) or to change (set) the bit to 1, because 1 means Local. What is correct?

Short explanation will be appreciated as even RFCs are not clear enough at this point.

Thanks a lot

Petr

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chenhui
Admin Created May 14, 2019 13:05:21 Helpful(0) Helpful(0)

@petr.schorm2015 hi!

Just flip the 7th bit, rather than set it to 1. 

Because on EUI-64, the corresponding bit being 1 means universally unique; 0 means local unique, while this is just converse with the MAC address.
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chenhui
chenhui Admin Created May 14, 2019 13:05:21 Helpful(0) Helpful(0)

@petr.schorm2015 hi!

Just flip the 7th bit, rather than set it to 1. 

Because on EUI-64, the corresponding bit being 1 means universally unique; 0 means local unique, while this is just converse with the MAC address.
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Digibear
Digibear Created May 14, 2019 13:20:59 Helpful(0) Helpful(0)

Hi are you sure? I would say U/L bit = 0 = universal, 1 = local. That is the point: should I set the bit to 1 to indicate it is now locally administered or should I flip the bit no matter what is the initial value (anyway I expect initial value of the UL bit is 0 as MAC address is expected to be global). Petr
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Mohamed_Mostafa
Mohamed_Mostafa Created May 14, 2019 20:13:29 Helpful(0) Helpful(0)

Invert the universal/local (U/L) flag (bit 7) in the OUI portion of the address. Globally unique addresses assigned by the IEEE originally have this bit set to zero, indicating global uniqueness. Likewise, locally created addresses, such as those used for virtual interfaces or a MAC address manually configured by an administrator, will have this bit set to one. The U/L bit is inverted when using an EUI-64 address as an IPv6 interface ID.

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In section 2.5.1 of RFC 2373 explain why :
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Please remember that the scope of the address never changes: global addresses are still global and local addresses are still local. Rather, the meaning of the bit is inverted for convenience, so the value of the bit must be inverted as well.

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chenhui
chenhui Admin Created May 15, 2019 05:51:30 Helpful(0) Helpful(0)

Posted by Digibear at 2019-05-14 13:20 Hi are you sure? I would say U/L bit = 0 = universal, 1 = local. That is the point: should I set the ...
maybe I'm not expressing clearly.
for the IPv6 address in EUI-64 format, 1 = universal, 0 = local, while in MAC address, 1 = local, 0 = universal.
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chenhui
chenhui Admin Created May 15, 2019 06:53:40 Helpful(0) Helpful(0)

Posted by Digibear at 2019-05-14 13:20 Hi are you sure? I would say U/L bit = 0 = universal, 1 = local. That is the point: should I set the ...
let's take an example,
if we just set the bit to 1, then for a local scope MAC address which is specified by the administrator, when translating it to the EUI-64 format ipv6 address, the 7th bit 1 will remain, now, the EUI-64 interface ID turn to be a global scope.
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