The Complete IS-IS Routing Protocol- P12

The Complete IS-IS Routing Protocol- P12: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- P12320 12. IP Reachability Informationlearned from that mistake was to simply define a Reference Bandwidth that will mostlikely not hit the ceiling in the next 10 years: like 100 Terabits per second. But the problemwith setting the Reference Bandwidth too high is that most routing protocols (includingIS-IS and OSPF) have finite and limited Metric fields. This is a problem similar to the6-bit metric space of the original ISO 10589 TLVs, where all the low-speed links getclipped to the maximum value and thereby do not provide any further differentiation. Today the most common Auto-Bandwidth setting for IS-IS is 1 Terabit per second (or1000 Gigabit per second), which can, through the use of 24-bit metric fields, go down to64 KBit/s without clipping the metrics. Manual setting of the Reference Bandwidth is notsupported on IOS. In JUNOS you can configure it through use of the reference-bandwidth command under the protocols isis configurationbranch.JUNOS configurationIn JUNOS you can automatically calculate the metric that IS-IS is using by configuring thereference-bandwidth command.hannes@New-York> show configuration[ … ]protocols { isis { reference-bandwidth 1000g; interface so-0/0/0.0; interface lo0.0; }}[ … ] In very large IS-IS networks the policy to set the routing metric is not as simple as adivision of the Reference Bandwidth. Typically the routing metrics should be controlledmanually to have tighter control about the impact of all kinds of re-routing scenarios.12.3.2 Static Metric SettingService providers do not always want IS-IS to calculate its own routing metrics with a sim-ple formula such as inverse bandwidth. Typically for very expensive links like transat-lantic links, there is an additional de-preference or negative bias expressed in an increasedrouting metric. Consider Figure 12.9 where one of the most important links in the Europeantopology goes down. The SP does not want to heal the European core over the transatlanticlinks, as from a cost-per-bit perspective, it is not very economical to route traffic fromFrankfurt to London via Pennsauken. In order to avoid suboptimal rerouting there are tables similar to the one shown inFigure 12.10, which consider both bandwidth and cost of links. In the first column thedifferent line speeds that are today typically deployed in service provider networks arelisted. In the rows the routing metrics according to that line speed, depending on whatkind of circuit type it is, are shown. If it is an intercontinental (expensive) link a very high New-style Topology (IS-Reach) Information 321 Pennsauken New York London Area 49.0001 Level 2-only Washington Frankfurt ParisFIGURE 12.9. From a cost perspective not every possible path is a feasible pathrouting metric is assigned, if it is a cheap circuit like a pair of fibres inside a POP, it getsa low routing metric. These per circuit-type metrics are not computed linearly. Typicallythere is an exponential function involved, which controls the offset and the gradient ofthe metric curve. The routing metrics shown in Figure 12.10 represent rounded-downvalues which follow an inverse logarithmic curve. Such logarithmic cost/bandwidth322 12. IP Reachability Information Circuit Speed Bandwidth Intra-POP In-country Continental Intercontinental (Mbps) oc-768/STM-256 39808 125 22000 74000 220000 oc-192/STM-64 9952 400 26000 87000 250000 oc-48/STM-16 2488 1300 35000 112000 315000 Gigabit Ethernet 1000 3000 43000 141000 430000 oc-12/STM-4 622 4500 50000 185000 600000 oc-3/STM-1 155 14000 74000 275000 950000 Fast Ethernet 100 20000 117000 370000 T3 45 34000 175000 580000 E3 34 60000 250000 Ethernet 10 100000 435000 E1 2 150000 T1 1,544 220000 Bandwidth cost metric scheme 1000000 Routing metric 800000 600000 400000 200000 0 39808 9952 2488 1000 622 155 100 45 34 10 2 1,544 Circuit speed (Mbps) Intra POP Continental Intercontinental In-CountryFIGURE 12.10. Composite bandwidth/cost metrics are typically for large networks in order tocontrol the re-routing behaviourtables are very common in the service provider community and significantly help to con-t ...