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.In RIP,equivalence is parallel WAN paths of equal hops.In Interior Gateway Routing Protocol (IGRP) andOpen Shortest Path First (OSPF), metric equivalence translates into WAN paths of equal bandwidth,where the bandwidth is declared by the network administrator.IGRP also adds the concept of delayto determine metrically equivalent links.To create parallel links for equal-cost paths and to activelyuse these paths, the router must use process switching because all frames sent from one ring toanother have the same IP destination.The following list outlines the capability of supported IP routing technologies to create equal-costpaths:" Static routes For Cisco software releases prior to Software Release 9.1, static routes cannot becreated in parallel; only a single path can be selected.As of Software Release 9.1, static routescan be created in parallel." IGRP and Enhanced Interior Gateway Routing Protocol (Enhanced IGRP) Can use up to fourequal-cost paths in parallel.Ensure that the bandwidth command is correctly configured on alllinks.4-24 Cisco CCIE Fundamentals: Network Design WAN Parallelism" OSPF If paths are of equal declared metrics, OSPF can use up to four equal-cost paths inparallel." RIP RIP can use four equal-cost paths in parallel.Remember that this will not take into accountanything but hops, so even unequal bandwidth links will be evaluated as having equivalent cost.IGRP, Enhanced IGRP, and OSPF can route traffic across equal-cost paths and split SRB trafficacross equal-cost links if the router is process switching.RIP will route across equal-cost paths andit will assume that all WAN links are the same speed regardless of reality.Static routes allow parallelpaths and are a tool for the advanced network designer.A router s capability to use parallel paths is determined in part by the encapsulation method.IfTCP/IP encapsulation is used, parallel paths are used.If FST encapsulation is used under normaloperational conditions, all traffic must use only one of the parallel links.This is because all theRSRB traffic sent to another FST peer goes to a single IP destination address.When using fastswitching, the router might alternate some traffic across parallel links based on destination address.However, because all traffic to a peer router uses only one destination IP address, all RSRB trafficflows across one link.IP Routing over Unequal-Cost Paths Using VarianceThe only routing protocols that can handle intentional unequal-cost path balancing are IGRP andEnhanced IGRP.Using a feature called variance, the router can load balance over unequal-costpaths.Figure 4-24 illustrates one such configuration from A to B.In this figure, load balancing thelink from C to B is assumed to be faster than the link from A to B.Figure 4-24 Unequal-cost load balancing with IGRP.CToken TokenRing RingABVariance has two rules that apply in this or any unequal-cost load balancing situation:" Rule 1 Parallelism must exist in the topology." Rule 2 Packets must make forward progress on any parallel link toward an intended destination.In other words, a router will not forward traffic to another router that has the same (or greater)relative distance metric to a destination.This rule prevents loops.The rule of forward progress isstraightforward.If the next-hop router is closer to the destination (than some other router) a paththrough it will be used as a valid alternative path.If these rules are met and the network administrator adds variance to the IGRP configuration, therouter will load balance over parallel paths for a single IP destination when it is process switching.Figure 4-25 illustrates a case in which variance might be used.Designing SRB Internetworks 4-25 SRB Technology Overview and Implementation IssuesFigure 4-25 Environmental illustrating variance applications.London1.5 Mbps 64 kbpsNew York PragueToken TokenRing Ring2 Mbps56 kbpsParisConsider a set of routers connected via WAN links in a circle, where each WAN link is the samespeed, as illustrated in Figure 4-26.Assume that a data center is at location A and that all the linkspeeds are the same.Consider parallelism from B to A.A parallel link exists from A to B and A toC to D to E to B; however, routing protocols are not intuitive.This topology satisfies the first rulebecause parallelism clearly exists; however, this topology fails the forward progress rule.The way to evaluate the forward progress rule is to examine the obvious short path separately fromthe long variant path, subtracting the first hop.Is C to D to E to B a better path than A to B? Theanswer is no; variance will have no effect in this topology for the problem as described.Figure 4-26 Unequal-cost path and variance implementation example.DC EToken TokenRing RingABNow evaluate the problem from the perspective of A to E.Using the forward progress rule, compareA to B to E with C to D to E.In this topology, these paths are equal and they fail the forward progressrule.If these paths are to pass data in parallel, router A must have two paths: one to C and one to B.If C had variance configured, it would have two paths: one to A and one to D.This leaves thepossibility of C routing to A and A routing to C in a loop.Thus, the variance rule is that the metricof the next-hop router must be less than the metric through the shortest path.In a five-router topologywith equal-cost WAN links, parallelism cannot be achieved.4-26 Cisco CCIE Fundamentals: Network Design WAN ParallelismBy default, variance is not configured.If it is, it must be configured as an integer multiple of theallowable metric variance.Consider the following use of the variance router configurationcommand:router igrp 1343variance 2Using this particular instance of the variance command results in a load-balanced topology with a2:1 ratio of bandwidth [ Pobierz całość w formacie PDF ]