Does anyone here know why routing in IPv6 prefer to use link-local address to build adjacency with neighbor router instead of using the global unicast address or it's original address as source address on hello packet? For example in OSPFv3 every hello packet contains link-local address (FE80:xxxx:xxx::) on its source address while OSPFv1 use original address on source interface. Thank you
Refer RFC5340 which states that in IPV6 protocol processing is per-link and not per subnet. so OSPF uses link local address for adjacency with neighbor. Please find the details below (extract from RFC)
2.1. Protocol Processing Per-Link, Not Per-Subnet
IPv6 uses the term "link" to indicate "a communication facility or medium over which nodes can communicate at the link layer" ([IPV6]). "Interfaces" connect to links. Multiple IPv6 subnets can be assigned to a single link, and two nodes can talk directly over a single link, even if they do not share a common IPv6 subnet (IPv6 prefix). For this reason, OSPF for IPv6 runs per-link instead of the IPv4 behavior of per-IP-subnet. The terms "network" and "subnet" used in the IPv4 OSPF specification ([OSPFV2]) should generally be replaced by link. Likewise, an OSPF interface now connects to a link instead of an IP subnet.
This change affects the receiving of OSPF protocol packets, the contents of Hello packets, and the contents of network-LSAs.
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