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Forum Replies

  1. Can you please elaborate on Anycast addresses? The concept is still pretty confusing.

  2. Hi Dhananjay,

    Anycast basically means that we configure the same address on multiple devices and then advertise the address in a routing protocol. For example, you could do this for a DNS server.

    Depending on where you are located, you will always be routed to the closest DNS server.

    You can even “try” this with IPv4. Configure two of your routers to advertise the same loopback interface. Depending on where you are in the network, you will always be routed to the closest loopback.



  3. andrew says:

    There really isn’t a concept of a link-local address in IPv4, but it is critical for IPv6 operation. As the name implies, link-local addresses are significant only on a particular link, and they are the primary vehicle of communication between IPv6 hosts over a shared link. As an illustration of its importance, OSPFv3 will not even function without link-local addresses being present.

    Link Local addresses are all within the FE80::/10 range. They can be manually set (which is the Cisco recommended practice), or they can be created automatically based on the node’s MAC address for that link. The problem with using a MAC, however, is that it is only 48 bits, while an IPv6 address is 128 bits. To solve this problem, extra padding is used. The rule is that you cut the MAC address in half, and insert “fffe” where you made the cut. For example, if you had the MAC address of 1111.2222.3333, the padding would work like this:


    As if that isn’t complicated enough, one additional change has to be made called the EUI-64 bit flip. I won’t go through explaining it because there is a lesson on it. The final result using the bit flip would be:


  4. Great post!

    I only had one question and its not really that important of a question but I was curious.

    If you have to change the L bit to a 1 making it FD then why do they say FC in the first place? why not just call all Unique Local FD which is what it really is anyway because of the rule.

  5. Hello Brian

    MAC addresses when configured have the U/L bit which is the 7th bit of the 48 bit address. This bit, when set to 0 when this address is locally administered and 1 if the address is globally unique. An example is the virtual MAC address that is created by HSRP. This MAC address will always have 0 in the seventh bit, while a hardwired MAC address on a switch or a PC will have the 7th bit 1.

    Now because there is a mechanism of EUI-64 which is used to assign an IPv6 address that is derived from a MAC addresses, this L bit seems to have migrated into the IPv6 format. According to most sources, (and @ReneMolenaar can correct me if I’m wrong), this bit isn’t actually taken into account in most IPv6 applications. Both the FC00 and FD00 blocks are considered unique local addresses.

    The reason why the L should be changed to 1 is because the block FC00 has not been defined yet. It has been proposed to be managed by an allocation authority, but this has not gained acceptance in the IETF.

    I hope this has been helpful!


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