TAILIEUCHUNG - The Complete IS-IS Routing Protocol- P30

The Complete IS-IS Routing Protocol- P30: 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. | 278 10. SPF and Route Calculation 40 K prefixes the total convergence time until the last prefix is updated takes about 40 seconds if not minutes depending on the router hardware. Updating forwarding tables under load is hard and quite expensive to achieve. Hierarchical Forwarding Table The alternate solution does not map the prefixes directly to the forwarding next-hops. There is instead the notion of an indirect next-hop. An indirect Next-hop is the originator of a prefix that is not directly connected to the router. It is a different way of modelling the dependency from BGP routes to IS-IS routes at a forwarding table level. Consider the sample network again where there are six BGP speakers. Therefore the number of indirect next-hops equals six. Figure shows how the forwarding table is implemented. The 120 000 prefixes do not point to the forwarding next-hop directly anymore but rather to their corresponding indirect next-hops. The indirect next-hops finally point to the forwarding next-hop. This is the key point interfaces point to six indirect next-hops which point to only three physical interfaces. What is gained from that except another memory lookup in the forwarding path to find out the indirect next-hop Actually a lot is gained once it comes down to changing the fowarding state. Look at the previous example where the link between Washington and Frankfurt fails and assume the forwarding table is structured as in Figure . After LSP propagation a full SPF run and BGP recursion some BGP speakers most notably Washington are found to have a different path. Previously the link outage traffic to Washington has been routed straight but now it is re-routed via Paris. The good news is that now at least pointers that point from the prefixes to the forwarding next-hops do not have to be changed. The only thing that the router needs to do is flip one pointer The hierarchical scheme does trade some forwarding lookup latency less than 1 .

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