On Tue, 07 Mar 2017 18:27:06 -0500, Dennis Bohn said:
AFAICT, Cisco V6 HSRP (mentioning that brand only because it caused me to try to figure something out, a coincidence that this is in reply to Jakob from Cisco but is based on what he wrote) relies on Link Local addresses. I didn't understand why link locals should be there in the first place seemed klugey and have googled, looked at rfcs and tried to understand why link local addresses were baked into V6. The only thing I found was that it enabled interfaces on point to point links to be unaddressed in V6. (To save address space!??) Can anyone point me in a direction to understand the reasoning for link local addressing?
Because there are a lot of corner cases where you may want to talk to the network before you find out what your network address is. And if it's a stand-alone network, it may not *have* a well-define network prefix to use for SLAAC auto-config addressing. Think about all the places in IPv4 where you toss packets on the net with your MAC address or a bogus placeholder IP address because you don't have an IP address yet (ARP, DHCP for starters). Link-Local is basically the same thing in the IPv6 world.