The Complete IS-IS Routing Protocol- P21: IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, its well-structured data formats, its flexibility and easy extensibility are all appealing – IS-IS epitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choice in some of the world’s largest networks. | 187 LSP IS Reachability TLV cost 10 cost 10 cost 10 cost 10 cost 10 LSP IS Reachability TLV cost 10 cost 10 cost 10 cost 10 cost 10 LSP IS Reachability TLV cost 10 cost 10 cost 10 cost 10 cost 10 Figure . Five routers on the LAN require O N2 storage space to accommodate all adjacencies LSP IS Reachability TLV cost 10 cost 10 cost 10 cost 10 cost 10 LSP IS Reachability TLV cost 10 cost 10 cost 10 cost 10 cost 10 LSP IS Reachability TLV cost 10 cost 10 cost 10 cost 10 cost 10 188 7. Pseudonodes and Designated Routers IS-IS adjacencies on broadcast LANs Speakers ---- p2p adjacencies to keep on a LAN ---- p2p adjacencies to keep on a LAN with a pseudonode Figure . The number of required IS relationships grows by N2 The IS-IS protocol design team was challenged to turn this N2 problem into a linear problem in order to scale more nicely. The solution to this problem is changing the representation of the LAN in the link-state database. The LAN is represented by so-called pseudonodes. Pseudonodes are comparable to the Network LSA Type 2 in OSPF and are a very common concept in link-state routing protocols. Pseudonode Representation The solution the IS-IS design team came up with is quite straightforward the router-to-router relationship is modelled in the link-state database just like the real physical connection relationship Each router is connected to the LAN The LAN is connected to all the routers So the idea of giving the LAN a nodal representation in the link-state database was born. Figure shows how the LAN is represented in the link-state database as
