IPv6 creates a link-local address that begins with FE80 and then incorporates the MAC layer 2 address as part of the IPv6 address. I have connected two routers through their serial interfaces. Both routers create the same link-local address because they use the same MAC layer 2 addresses on the interfaces. Serial interfaces do not have MAC addresses, so they are borrowed from a LAN address. I'm not sure where the GNS3 routers are gettting their MAC addresses to create the link-local addresses. However, since both routers end up getting the same link-local address, the serial interface on one of the routers is stalled. Use the show ipv6 interface s0/0 command, for example, to observe " IPv6 is stalled, link-local address is FE80::9CD7:2EFF:FEF0:99FA [DUP]". Any ideas on how I can resolve this issue?

If both interfaces have same MAC address they can't communicate either with IPv4.

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Br,
Kaage

Add one ethernet/fastethernet interface and here you go. You will get different Link Local addresses.

Cheers,