The Routers configured with dynamic routing protocol 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 always sent from the source IPv4 address of the router. The 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 not routed beyond the link from where the packet originated. The router sends OSPFv3 messages using the source and destination link local IPv6 address. Source link local address is the address of exiting interface.
The destination IPv6 address can be a unicast address using the neighbor link local IPv6 address. They can also be sent using a multicast address. The FF02::5 address is the all OSPF router address multicast address and the FF02::6 is the DR/BDR multicast address. The topology in figure1 would be used; to configure the link local IPv6 address and OSPFv3 configuration in the coming article.
Assigning Link Local IPv6 Address
The router creates link local IPv6 address using EUE-64 process or random interface IDs. Therefore it’s difficult to recognize and remember those addresses. The IPv6 routing protocols use link local IPv6 addresses for unicast addressing and next-hop address information in the routing table, so, it is important to make it an easily recognizable address.
So, configuring the link 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 used to configure 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 link local address has from FE80 to FEBF. When an address begins with the above mention hextet the link-local keyword must follow the address. The figure-1 illustrates the link local IPv6 address configuration on Router1
Verifying Link-Local Address
We can verify the link-local address using the same “show ipv6 interface brief” command as for global IPv6 address verification. If we configure a global unicast address without configuring a link-local address. It automatically generates a link-local address. The figure3 illustrates the verification of ipv6 address.
Unless we configure link-local address manually, Cisco routers create the link-local address using FE80::/10 prefix and also using the EUI-64 process. We already discuss the EUI-64 in the previous lesson. Which use 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.