OSPFv3 LSA Types
Last updated
Last updated
In OSPFv2, this LSA advertises the connected links in an area with their prefixes.
In OSPFv3, this LSA no longer has the prefix information. Also notice that the loopback is no longer included as a link. The router LSA now strictly advertises the links and connected neighbors. Instead of using IP addresses to indetify links, now we see an interface ID used to identify each link. This matches how ISIS treats the topology - the topology is separate from the IP addressing.
In OSPFv2, the DR generates this LSA, advertising the connected routers and the prefix for the broadcast network. The link state ID is the DR’s IP address on the network. The network mask indicates the prefix of the network itself.
In OSPFv3, the DR still generates the LSA, but there is no prefix information. In fact, the LSA looks almost identical to the OSPFv2 LSA, just with the network mask missing, and the link state ID as the interface ID instead of the interface IP.
So what is the point of this change? Why aren’t the IPv6 addresses and prefixes included in these two LSAs like with OSPFv2?
OSPFv3 mimics ISIS now, as these two LSAs simply build the graph of routers and how they connect to each other. This adds stability to the protocol, because with OSPFv2, the graph was based on the IP addresses. If you changed the IP address of a link, this generated a completely new router LSA, and caused routers to have to make a full SPF run after rebuilding their graph.
With OSPFv3, if the IP address of a link changes, the graph has still stayed the same, therefore rebuilding the graph and doing a full SPF run isn’t necessary.
The second advantage is that you can hang IPv4 and IPv6 addresses off of this graph. Now the graph itself is completely indepedent from IP addressing. This closely matches the behaviour of ISIS, as it builds a graph independent of any sort of layer 3 addressing, and then IP addresses are just placed “on top of” or “hung off” the nodes in the graph. In other words, the type 1 and type 2 LSAs are protocol independent in OSPFv3.
OSPFv3 introduces two new LSAs to advertise the IPv6 prefixes that we are missing in the type 1 and type 2 LSAs.
A router advertises a type 8 LSA for every link. This is flooded only on the link itself, it is not flooded among the area.
If you’ve noticed how the next-hops for IPv6 prefixes in the RIB that came from OSPFv3 are always link-local next-hops, this is how that works.
This means that you don’t need to actually configure IPv6 addresses between to neighbors. You can just let the routers auto-generate link local addresses and advertise these to each other using type 8 LSAs.
This is where the prefixes of the links are finally advertised. In this case, CE2 is not the DR for its interfaces, so we simply see one LSA for its loopback which it is advertising into OSFPv3.
If we look at the DR, we can see the prefixes associated with the type 2 LSAs that it originated:
In OSPFv2, these are called Summary LSAs, and are generated by ABRs.
In OSPFv3, these LSAs are the same type (type 3), and function in the same way. The difference is that they are now called “Inter-area prefix LSAs” instead of “Summary LSAs.” The link state ID is different - in OSPFv3 it is now an ID instead of the subnet address. However everything else is practically identical.
