Sometimes, in case of OSPF, I’m facing with a Host IP address and a prefix /32 in the fields, where I hope to find the network IP address with the prefix /30 and less accordingly. It would be great you could clear up my questions, which correspond to content of some LSA fields in case of point-to-point connection between Routers. Namely:
• Router’s LSA Link Type 3 (Stub Network Link) – Link ID / Link Data.
• Summary Network LSA – Link State ID / Network Mask
In both of cases, according to name - Network, it should be here network’s IP address / mask or prefix and length of prefix. Take an example. There are three point-to-point connection between three routers with assigned subnets and IP addresses for segment’s interfaces:
• 172.16.1.1, 172.16.1.2, mask – 255.255.255.0
• 172.16.2.1, 172.16.2.2, mask – 255.255.255.0
• 172.16.3.1, 172.16.3.2, mask – 255.255.255.0
Let’s suppose, all three reside in Area 0 and router with interface 172.16.1.1, for example, is the ABR and have to propagate information about Subnets to next Area.
First of all, each Router in theirs Router’s LSA have to form the Stub Network Link with network’s addresses and mask in the fields Link ID and Link Data accordingly:
• 172.16.1.0 / 255.255.255.0
• 172.16.2.0 / 255.255.255.0
• 172.16.3.0 / 255.255.255.0
Second, the Router will produce 3 summary-LSA with the same data as Link State ID and Mask.
Well, it is theory and concerning to the Stub Network link of Router’s LSA, it correspond to the RFC 2328 Option 2 case (Page 130). My question here would be following. There is also Option 1 in RFC, where Link ID could be presented as Host’s IP address with 0xffffff mask in the field - Link Data. Is this the case?
Next question. How Router would derive network’s IP address, when nevertheless there would be the network’s one?
I’m asking, because I have collide in my working activity with OSPF implementation where Link ID field contains Host IP address with prefix length /32. I have asked developers if it is correct, because on the next step, above mentioned ABR would produce 6 network summary LSA with host IP address of each Interface for each subnet. In which connection, that an additional aggregation command isn’t implemented yet. According to developers: it is completely correspond to the requirements of RFC2328 and only in case of presence additional aggregation’s command, for example, for each from three subnet, it would be possible to reduce number of the summary LSA to three. Only then, there will be network’s IP addresses and mask in Link State ID and Network Mask fields accordingly. Of course, additional aggregation command for the network 172.16.0.0/16, could reduce the number of summary LSA to single.
Therefore, my question is, if this implementation is correct. I’ve tried to modelling this situation with Cisco Packet Tracer. CISCO IOS acts according to the described scenario. That is, there is exactly the network prefix in the stub link of router’s LSA and also 3 summary LSA is generated by ABR for each point-to point subnet. Of course, without additional aggregation’s command.
RFC2328 doesn’t say definitely, if summary network LSA should consist only network’s address as Link State ID. There is only a description of example in RFC on Page 138, where is mentioned a condensing host routes Ia and Ib into a single summary-LSA. Is this the case? In general, is it difficult for Router, in case of point-to-point segment, to find out the prefix? What take the Router as the source for summary network LSA?
Would you like please to explain this situation?