Why does OSPFv3 run per link instead of per IPv6 subnet, and why do its Hellos come from link-local addresses?

## What IPv6 means by a link RFC 5340 starts from IPv6's own vocabulary. A **link** is "a communication facility or medium over which nodes can communicate at the link layer": an Ethernet segment, a point-to-point circuit. **Interfaces** attach to links. In IPv6 several **subnets** (prefixes) can be assigned to one link, and two nodes on the same link can exchange packets even when they share no prefix at all. A segment might carry `2001:db8:a::/64` and `2001:db8:b::/64` at once, during a renumbering or by design. OSPFv2 was built around a different unit, the IPv4 subnet, and that assumption is visible in its checks. ## How OSPFv2 is tied to the subnet - **Hello Network Mask**: RFC 2328 requires the `Network Mask` in a received Hello to match the receiving interface's mask, except on point-to-point links and virtual links. A mismatch drops the Hello. - **Source address check**: a packet received on a non-virtual interface must come from an address on the same network as that interface, verified by masking both addresses. Point-to-point links are exempt. - **Neighbour identity**: on broadcast, NBMA and point-to-multipoint links an OSPFv2 neighbour is identified by its IPv4 interface address, and the DR and BDR are named by address in Hellos. All three assume that "the routers on this wire" and "the hosts in this subnet" are the same set. ## What OSPFv3 does instead RFC 5340 replaces "subnet" with "link" throughout and changes the packets to match: 1. The Hello carries **no address information**, and no network mask. Instead it carries an `Interface ID`, a number the router assigns to identify its own interface on the link. 2. The receive checks compare the **Area ID** and **Instance ID** with the receiving interface; the IPv6 source address is explicitly not required to lie in the same subnet. 3. Neighbours are identified by **Router ID** on every link type, and Hellos name the DR and BDR by Router ID. 4. An **Instance ID** in the header lets several OSPF instances share one link; a packet whose Instance ID does not match the interface's is discarded. OSPFv2 had achieved this only in what RFC 5340 calls a haphazard way, through its authentication fields. | Check on a received packet | OSPFv2 (RFC 2328) | OSPFv3 (RFC 5340) | |---|---|---| | Hello mask | must match, except point-to-point and virtual links | no mask in the Hello | | Source address | must be in the receiving interface's subnet, except point-to-point | not restricted to a shared subnet | | Neighbour key | IP address on broadcast, NBMA and point-to-multipoint | Router ID on every link type | | Separating instances | no header field for it | `Instance ID` must match | ## Why link-local addresses RFC 5340 assumes every router has a link-local unicast address (from `fe80::/10`) on each attached link, and on every OSPF interface except virtual links it sends OSPF packets from that address. Several properties make it the right choice: - **It always exists.** An IPv6 interface has a link-local address whether or not any global prefix is configured, so OSPF can run on a link that carries no global prefix of its own. - **It is independent of the link's prefixes.** Adding, removing or renumbering a global prefix changes what OSPF advertises, not which address the neighbours talk to. - **It stays on the link.** IPv6 routers do not forward packets with link-local source addresses, which suits a protocol whose packets mostly travel one hop. - **It is the next hop.** A router learns its neighbours' link-local addresses and uses them as next-hop addresses when forwarding. The address comes from the neighbour's Link-LSA, or on many link types simply from the source of its Hellos. Link-local addresses are allowed in the Link-LSA only; RFC 5340 forbids them in the LSAs that are flooded further, such as Intra-Area-Prefix-LSAs and AS-External-LSAs. ## The exception: virtual links A **virtual link** joins a disconnected area border router to the backbone across a transit area, so its packets cross several routers. A link-local source cannot be forwarded, so RFC 5340 requires a **global** IPv6 address as the source on virtual links. Each end learns the other's global address from the far router's Intra-Area-Prefix-LSA, where it is flagged with the `LA-bit`. ## Mistakes that show up in interviews - Expecting OSPFv3 neighbours to need a common global prefix. They need a common link and matching OSPF parameters. - Saying the Hello's mask was widened to a prefix length. It was removed. - Saying OSPFv3 never uses global addresses for its own packets. Virtual links must. - Saying neighbours are tracked by IPv6 address. They are tracked by Router ID; the source address is stored as the neighbour's address for next-hop use.