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Link-Local IPv6

What is a Link-Local IPv6 Address – Exclusive Introduction

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Routers configured with dynamic routing protocols such as OSPF or EIGRP need to send and receive routing protocol messages with their directly connected neighbors. The routers exchange messages between neighbors on the same subnet. These messages are always sent from the router’s source IPv4 address. Link-local IPv6 addresses are perfect for this purpose. The address also activates network devices to exchange data with other IPv6-enabled devices on the same link and only on that link.

Packets contain the source or destination link-local IPv6 address and are not routed beyond the link from where the packet originated. The router sends OSPFv3 messages using the source and destination link-local IPv6 addresses. The source link-local address is the address of the existing interface.

The destination IPv6 address can be a unicast address using the neighbor link local IPv6 address. It can also be sent using a multicast address. The FF02::5 address is the all-OSPF router multicast address, and the FF02::6 is the DR/BDR multicast address. The topology in Figure 1 will configure the link-local IPv6 address and OSPFv3 configuration in the coming article.

Link-Local IPv6

Assigning Link Local IPv6 Address

The router creates a link-local IPv6 address using the EUE-64 process or random interface IDs, which makes it difficult to recognize and remember. The IPv6 routing protocols use link-local IPv6 addresses for unicast and next-hop address information in the routing table, so making the address easily recognizable is important.

So, configuring the link’s local address manually makes it recognizable and easier to remember. Furthermore, a router with several interfaces can assign the same link-local address to each IPv6-enabled interface because the link-local address is only needed for local communications.

We can configure the link-local address using the interface mode command <ipv6 address link-local address link-local>; the same command configures IPv6 global unicast addresses only appending the “link-local” keyword to the command. For example, “ipv6 address fe80::1 link-local”

The prefix range of the link-local address is from FE80 to FEBF. When an address begins with the hextet mentioned above, the link-local keyword must follow it. Figure 1 illustrates the link-local IPv6 address configuration on Router1.

Link-Local IPv6

Verifying Link-Local Address

If we configure a global unicast address without configuring a link-local address, we can verify the link-local address using the same “show ipv6 interface brief” command for global IPv6 address verification. It automatically generates a link-local address. Figure 3 illustrates the verification of the IPv6 address.

Unless we configure the link-local address manually, Cisco routers create the link-local address using the FE80::/10 prefix and the EUI-64 process. We already discussed the EUI-64 in the previous lesson. It uses the 48-bit Ethernet MAC address, inserting FFFE in the middle and flipping the seventh bit. For serial interfaces, Cisco devices use the MAC address of an Ethernet interface.

Link-Local IPv6
EUI-64 Process and Randomly Generated IPv6

EUI-64 Process and Randomly Generated IPv6

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After the client receives SLAAC or SLAAC with a stateless Router Advertisement (RA) message, the client must generate its own Interface ID. The client gets the prefix portion of the IP version 6 address from the Router Advertisement (RA) message; however, the RA message does not contain the client’s interface ID information. So, the client creates its own Interface ID. The Interface ID can be derived from the MAC address or a randomly generated 64-bit number.

EUI-64

EUI-64 Process

EUI, also known as the EUI-64 process defined by IEEE, EUI is the abbreviation of Extended Unique Identifier. The method uses a client’s 48-bit Ethernet MAC address and inserts an extra 16 bits in the middle of the 48-bit MAC address to create a 64-bit Interface ID. Ethernet MAC addresses are represented in hexadecimal containing two parts:

  • Organizationally Unique Identifier (OUI)– This is the first part of the MAC address called OUI. An OUI is a 24-bit number uniquely identifying a device vendor or manufacturer. The organizations purchased an OUI from the IEEE. The first three octets of a MAC address are Organizationally Unique Identifier (OUI).
  • Device Identifier– The last three octets of the MAC address uniquely identify the device. It is also a 24-bit (6 hexadecimal digits) value within a common OUI. The device identifier bits assigned by the organization to their device.

An EUI-64 Interface ID has three parts represented in binary:

  • 24-bit Organizationally Unique Identifier (OUI) from the client MAC address with a reversed 7th bit. For example, if the 7th bit of the OUI is 1, it becomes a 0, and vice versa in the EUI interface ID.
  • 24-bit Device Identifier from the client’s MAC address
  • Hexa decimal value FFFE (16 bits) between Organizationally Unique Identifier (OUI) and Device Identifier.

EUI-64 process using the MAC address of BC:78:ba:b1:f8:55.

  • Get the MAC address of the host or device, for example, in this case, BC:78:ba:b1:f8:55
  • Insert ff:fe in the middle: bc:78:ba:ff:fe:b1:f8:55
  • Reorder the above hexadecimal digits in IP version 6 notation. bc78:baff:feb1:f855
  • You can see that now it’s 4 hextet; convert the first octet from hexadecimal to binary: BC-> 10111100
  • Flip the 7th bit: 10111100 ->10111110
  • convert changed octet back to hexadecimal:01000111 ->BE
  • Now change the first octet with the new value one: bc78:baff:feb1:f855
  • Insert the link-local prefix at the beginning : fe80:: bc78:baff:feb1:f855

Advantages and disadvantages of the EUI-64 Process

EUI-64 process is an easy way to find out that the address was likely created from the MAC address. We can quickly locate FFFE in the middle of the Interface ID. We can also use the MAC address to find out the interface ID. The network administrators also easily track an ID on an end device using the unique MAC address. The main disadvantage of deriving an IP version 6 address from the MAC address is privacy among users because packets can be traced to the actual physical computer. So, we can use a randomly generated Interface ID in its place.

Randomly Generated Interface IDs

Depending on the operating system, we can also configure devices to use a randomly generated Interface ID instead of the MAC address and the EUI-64 process. For example, Windows Vista uses a randomly generated Interface ID instead of one created with the EUI-64 process. Windows XP and previous Windows operating systems used EUI-64.

When Interface ID is established through the EUI-64 process or through random generation, it can be combined with an IP version 6 prefix in the RA message to create a global unicast address. To avoid IP address duplicate addressing, the client can use DAD (duplicated address detection). This is similar to the Address Resolution Protocol (ARP) request for its address.