TAILIEUCHUNG - The Complete IS-IS Routing Protocol- P34

The Complete IS-IS Routing Protocol- P34: 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. | o Figure . OSPF leaks all prefixes into all areas which is one of OSPFs scaling harms Domain-wide Prefix Distribution 331 Typically leakage of all routes into the non-zero areas is not necessary. All that the nonzero area needs to know is the area internal routes and a default route that points to the Area Border Routers. IS-IS has much better scaling properties in that respect. Consider Figure . Very much like OSPF IS-IS passes on Level 1 information to Level 2. However the other direction is by default blocked Level 1 routers have to rely on a default route generated by the L1L2 routers. Flooding just a default route is clearly a very scalable approach however the use of the default route as the only routing information pointing towards the ATTached Level 2 router is very unspecific information. Sometimes it is necessary to trade protocol scalability for optimality of traffic flow. The side effect of unspecific information can be suboptimal routing as shown in Figure . Traffic towards 32 gets attracted to the closest L1L2 router which is Router Barcelona due to a lower metric of the default-route 0 0 of 2000 although from a total routing metric perspective sending the traffic straight to the L1L2 Router Milan would be more optimal. The sub-optimal path-cost Madrid-Barcelona-Paris-Frankfurt is 22000. The more optimal path would be MadridMilan-Frankfurt with a composite path-cost of 11500. The use of unspecific routinginformation makes IS-IS have a kind of blind spot and results in sub-optimal routing. RFC 2966 lifts that strict requirement to pass just the default 0 0 route down to Level 1 and allows leaking of prefixes from Level 1 to Level 2. Additionally RFC 2966 allows external routes to exist in Level 1 which was strictly forbidden according to RFC 1195. But allowing the routes to flow from Level 2 to Level 1 is still not enough as shown in Figure . Suppose some router located beyond Paris in Level 2 originates among others .

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