tailieunhanh - The Complete IS-IS Routing Protocol- P49

The Complete IS-IS Routing Protocol- P49: 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. | 482 16. Network Design Figure . The resolver needs to track and map BGP next-hops to the shortest path resulting from the SPF calculation The forwarding state change of tens of thousands of routes may stress several sub-systems of an Internet core router. It turns out that changing a forwarding state is one of the most expensive operations in a router. Meanwhile both Juniper and Cisco have found a way to pass on third party next-hop information to the line-cards and retain the dependency of BGP routes to IS-IS speakers to forwarding interfaces. More on passing on third party nexthop information and why it is not always a good idea to attempt to fully resolve a route to its forwarding next-hop can be found in Chapter 10 SPF and Route Calculation . Router Stress 483 CPU and Memory Usage The two main things that utilize the CPU most in an IS-IS router are the SPF calculation and the resolver. SPF calculation puts a short burden on the system but even in large topologies that burden does not last more than 200 ms using modern route processors. As discussed in the previous section the far bigger CPU hog is the resolver which maps BGP routes to forwarding next-hops. SPF execution runtime is ultimately a non-issue however the burden that the resolver can put on the system needs to be carefully examined. In the 1990s during the explosive growth of the Internet routers were constantly short of memory. Since then network service providers are cautious about the memory usage of their routing protocols. There is almost no IS-IS-related documentation regarding memory consumption. The majority of IS-IS implementations use memory in three areas 1. Link-state database 2. SPF result table 3. Storing neighbour information The link-state database size is the easiest to predict. It contains mostly raw data that was extracted from the TLVs in an IS-IS PDU. There are also overhead and index structures so the IS-IS software can quickly traverse the database when it is looking