The Complete IS-IS Routing Protocol- P10

The Complete IS-IS Routing Protocol- P10:IS-IS has always been my favourite Interior Gateway Protocol. Its elegant simplicity, itswell-structured data formats, its flexibility and easy extensibility are all appealing – IS-ISepitomizes link-state routing. Whether for this reason or others, IS-IS is the IGP of choicein some of the world’s largest networks. Thus, if one is at all interested in routing, it is wellworth the time and effort to learn IS-IS.
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The Complete IS-IS Routing Protocol- P10258 10. SPF and Route Calculation10.3.1 Full SPF RunThe full SPF run is the heavyweight of SPF flavours. It both re-computes the topologicalgrid in an area as well as re-computes the reachable IP prefixes. Full SPF runs are typi-cally 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 isdelayed for a configurable minimum amount of time. The most typical event from theabove list is a new or updated LSP. In IS-IS networks, as in any other network runninglink-state routing protocols, there is a general observation that single LSP updates arevery rare. They are almost always accompanied by other LSPs, which follow shortlyafter the first LSP shows up. The reason behind this is very clear: if a link fails there arealways two routers that need to re-originate their LSPs. So it is better to wait a couple ofmilliseconds before starting an SPF calculation, which may tie the router down on theorder 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-statedatabase and does the SPF calculation. Freezing means that during this time, no LSPadditions or changes can be made.10.3.1.1 Link-state Database LockingIt is absolutely mandatory for an IS-IS implementation to freeze the database during anSPF calculation run. An LSP change inserted during a run of the SPF calculation mayresult in bogus routes. Consider Figure 10.10 to get an idea what will happen if the link-state database is not locked. We are in the middle of an SPF calculation. The early stagesof the SPF calculation considered the path through Washington the best path in the network.Now it is exploring the network downstream from Washington. Suddenly, the link betweenWashington and New York goes down. Unfortunately, the New York–Washington path isour best-path candidate. The SPF calculation does not backtrack through path candidatesto see if the path properties have changed. If the router does not lock the link-state databasethen the result will be most likely bogus routes. Of course, IOS and JUNOS both lock thedatabase (as any serious IS-IS implementation has to) and queue any incoming LSPs forinsertion once the database is unlocked. After the SPF calculation has completed, the router starts an SPF hold-down timerwhich blocks further SPF runs for self-protection reasons. SPF Calculation Diversity 259 Pennsauken oc192/STM-64 oc48/STM-16 26000 315000 UNKNOWN List London- Frankfurt 22000 Frankfurt- London 22000 New York London oc48/STM-16 Frankfurt- Paris 87000 New York 315000 Paris- Frankfurt 87000 oc768/STM-256 Area 49.0001 oc768/STM-256 22000 Level 2-only 22000 TENTative List oc192/STM-64 Washington Frankfurt LSDB entry cost cost to root 250000 Pennsauken- London 315000 315000 Washington D.C.- Paris 600000 648000 oc12/STM-4 oc192/STM-64 600000 87000 PATH List Paris Destination via cost to root New York New York 26000 Washington D.C New York 48000 Frankfurt New York 298000FIGURE 10.10. If the contents of the LSDB are not locked during the SPF computation, bogusroutes will result10.3.1.2 Self-protectionThe purpose of hold-downs is to allow the IS-IS router to work less. Consider Figure 10.11to see why SPF hold downs make sense. If there were no hold-down for SPF calculation,then the average utilization of the control plane CPU would be very high. During an SPFcalculation (100–200 ms) the CPU utilization jumps to 100 per cent. But shortly there-after it drops down to 0 per cent. If a network is shaky, then additional LSPs triggeringnew SPF calculations will follow, raising the CPU utilization to 100 per cent once againfor a short period of time. By applying SPF hold-down timers, IS-IS keeps the intervalsbetween ...