Forum Replies

1. 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:

   F

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2. Rene, I see you keep referring to FA as FFFF 1000. Is that correct or a typo? This was referenced twice that way. This is example on IPv6 EUI-64 explained.

   Example from Notes:

   Let’s look only at the EUI-64 part:

   F816:3EFF:FE60:0217

   We only care about the first two hexadecimal characters:

   F8 = FFFF 1000

   As you can see, the 7th bit has been inverted from 1 to 0.

   2nd Example:

   Here’s the EUI-64 part:

   FA16:3EFF:FE60:0217

   First two hexadecimal characters:

   FA = FFFF 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.

3. Hi Joel,

   First of all, I wouldn’t worry too much about this too much. The bit flipping is just one minor sub-topic of all IPv6 related stuff you can expect in the exam. It’s possible that you don’t get any questions about it so don’t stress about it too much. You can expect a lot of regular subnetting questions so that’s why I highly recommend a “cheat sheet” for that.

   It seems this list would work. It might be a quick method if you’d get 10 bit flipping questions but if you only would get 1 question, it might be just as fast to quickly calculate it?

   hex > bina

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4. Hello Nadav,

   That is no problem, the prefix is different so you will have a unique 128-bit IPv6 address. Here’s an example:

   R1(config)#interface GigabitEthernet 0/1
   R1(config-if)#ipv6 address 2001:DB8:1:1::/64 eui-64 
   R1(config-if)#ipv6 address 2001:DB8:2:2::/64 eui-64 
   R1(config-if)#ipv6 address 2001:DB8:3:3::/64 eui-64
   

   Gets you:

   R1#show ipv6 interface brief | include 2001
       2001:DB8:1:1:F816:3EFF:FE42:B409
       2001:DB8:2:2:F816:3EFF:FE42:B409
       2001:DB8:3:3:F816:3EFF:FE42:B409
   

   Rene

5. Hello Nadav

   This is an excellent question. According to RFC 5375 in section B.2.4 EUI-64 ‘u’ and ‘g’ Bits, it states the following:

   When using subnet prefix lengths other than /64, the interface
   identifier cannot be in Modified EUI-64 format as required by
   [RFC4291]. However, nodes not aware that a prefix length other than
   /64 is used might still think it’s an EUI-64; therefore, it’s prudent
   to take into account the following points when setting the bits.

   The document continues to state several considerations that should be checked carefully. In general, d

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