The Complete IS-IS Routing Protocol- P10: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. Thus, if one is at all interested in routing, it is well worth the time and effort to learn IS-IS. | 258 10. SPF and Route Calculation Full SPF Run The full SPF run is the heavyweight of SPF flavours. It both re-computes the topological grid in an area as well as re-computes the reachable IP prefixes. Full SPF runs are typically triggered by the following events Local configuration change Update to a known LSP which contains an adjacency change Local aged adjacency Receipt of a new unknown LSP New Area-ID in the Level-1 network Link metric change Purging an LSP Periodically for additional robustness every 15 minutes The full SPF run is not scheduled immediately after the above trigger events. Instead it is delayed for a configurable minimum amount of time. The most typical event from the above list is a new or updated LSP. In IS-IS networks as in any other network running link-state routing protocols there is a general observation that single LSP updates are very rare. They are almost always accompanied by other LSPs which follow shortly after the first LSP shows up. The reason behind this is very clear if a link fails there are always two routers that need to re-originate their LSPs. So it is better to wait a couple of milliseconds before starting an SPF calculation which may tie the router down on the order of 100s of milliseconds. So routers delay the SPF calculation. The typical pre-SPF delay value is 100 or 200 ms depending on IOS or JUNOS . After the pre-SPF delay the router freezes the link-state database and does the SPF calculation. Freezing means that during this time no LSP additions or changes can be made. Link-state Database Locking It is absolutely mandatory for an IS-IS implementation to freeze the database during an SPF calculation run. An LSP change inserted during a run of the SPF calculation may result in bogus routes. Consider Figure to get an idea what will happen if the linkstate database is not locked. We are in the middle of an SPF calculation. The early stages of the SPF calculation considered the path through .
