# Link Type 100+ Mbps Token Ring 10-Mbps Ethernet Serial Load and Reliability in Objective-C Encoding PDF 417 in Objective-C Link Type 100+ Mbps Token Ring 10-Mbps Ethernet Serial Load and Reliability

Link Type 100+ Mbps Token Ring 10-Mbps Ethernet Serial Load and Reliability
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Load is not normally used, but when it is, it is based on the exponentially weighted average of how saturated the link is during a given sample period The sample period is normally five minutes, and the load calculation is updated every five seconds (in other words, every five seconds, the load calculation will change to reflect the load on the link in the previous five minutes) Load is measured from 1 to 255, with 1 being minimal use and 255 being fully saturated (100 percent used) Reliability is similar to load in that it is calculated using an exponentially weighted average that is taken from activity occurring in the previous five minutes and is updated every five seconds It is also not normally used Reliability, however, uses a reverse scale from load (I
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am convinced this was done solely to cause confusion), with 255 being completely error-free and 1 being unusable Calculating the Formula Now that you have a better idea of how each component is calculated, take a look at that formula again: metric = K1 x Be + (K2 x Be)/(256 load) + K3 x Dc) x (K5/(reliability + K4) Well, you now know what the Be and Dc are, but what about all those Ks K is a constant, referred to as a weight Weights allow you to assign different priorities to different metric components, which is useful when classifying traffic and routing packets based on the classification For instance, for voice traffic, latency (delay) is most important, with bandwidth being secondary For file transfers, however, delay is pretty unimportant, while bandwidth reigns supreme The original idea was to use IGRP as a routing mechanism that could use packet classifications to choose the best route for specific types of traffic, but this concept was never fully implemented, so choosing weights ends up coming down to choosing which metric component you feel is most important Simple Metric Calculations Usually, it is recommended that you simply use the defaults, which are K1 (bandwidth weight) = 1, K3 (delay weight) = 1, K2 (load weight) = 0, K4 (secondary reliability weight) = 0, and K5 (primary reliability weight) = 0 With the default weights, only K1 and K3 are used The rest of the weights are set to zero, which, instead of being inserted as zero into the equation (and giving a resulting metric of zero for every route), means that part of the equation is removed After removing the unused sections of the equation, the formula simplifies to this: metric = K1 x Be + K3 x Dc, or, even more simply (because the default weights for K1 and K3 are 1), metric = Be + Dc With the default weights, metric determination is fairly simple For instance, examine the network shown in Figure 24-3
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Figure 24-3: Network for the basic metric calculation example In this example, calculating the metric for the route from Miura to Net 4 is fairly simple First, you find the lowest bandwidth in the path, which is the serial link between Urraco and Jalpa You then divide 10,000,000 by the bandwidth in bps of that link to create the final bandwidth figure, 19,531 You then add up all of the delays on all of the outgoing interfaces to create a delay score of 40,630 (20,000 + 20,000 + 630) You then divide this delay score by 10 (because delay is calculated for the metric in units of ten microseconds) for a total of 4,063 Finally, you add the bandwidth to the delay to achieve a total metric of 23,594
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and for the T1, the delay figure is caused by the route having to be sent through 30+ routers, all with a delay of 20,000 leading to a massive cumulative delay To reduce the delay figure on the T1, you would either need to purchase a PVC directly to the HQ (to reduce the number of hops in the path) or change the delay figures on all of the routers in the path (a solution with unintended consequences, because other routers in the WAN that do not have this specific problem also use these delay figures) However, if you change the bandwidth (K1) weight on the remote router, it will not adversely affect other routers and it will accomplish the goals By changing the K1 to 8, you would change the metrics as follows:
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ISDN link: (78,125 x 8) + 2,000 = 627,000 Sat link: (1,000 x 8) + 120,000 = 128,000 T1 link: (64767 x 8) + 78,000 = 129,813
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Bingo Initial problem solved The sat link is now preferred, with the T1 second, and the ISDN is a long last However, now you need to take load into account If you change the load (K2) weight to a nonzero integer, you end up with the following formula: (K1 x bandwidth) + (K2 x bandwidth)/(256 load) + delay With a K2 of 1, your metrics would look like this under a minimal load:
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ISDN link: (78,125 x 8) + (78,125/(256 1)) + 2,000 = 627,306 Sat link: (1,000 x 8) + (1,000/(256 1)) + 120,000 = 128,004 T1 link: (6,4767 x 8) + (6,4767/(256 1)) + 78,000 = 129,839
So, under minimal load, nothing really changes Well, let's see what happens if you put the sat link under full load Remember, the goal was to have the T1 be used if the sat link was saturated Under full load for only the sat link, the metrics would be as follows:
ISDN link: (78,125 x 8) + (70,000/(256 1)) + 2,000 = 627,306 Sat link: (1,000 x 8) + (1,000/(256 255)) + 120,000 = 129,000 T1 link: (6,4767 x 8) + (6,4767/(256 1)) + 78,000 = 129,839
re-advertise the routes to HQ back to any other routers (due to split horizon), so increasing the K2 won't cause any update or holddown problems Once you increase the K2 to 10,000, the metrics with the sat link at a 217 load and the other links at 1 become the following:
ISDN link: (78,125 x 8) + ((10,000 x 70,000)/(256 1)) + 2,000 = 3,690,725 Sat link: (1,000 x 8) + ((10,000 x 1,000)/(256 217)) + 120,000 = 384,410 T1 link: (6,4767 x 8) + ((10,000 x 6,4767)/(256 1)) + 78,000 = 383,801
Now let's see what happens if the T1 is saturated as well (Remember, the goal was to have the ISDN link become the primary route if the T1 and the sat link were saturated) Again, you are looking for the route through the ISDN line to become the primary if the sat link is over 99-percent saturated (253 load) and the T1 is over 92-percent saturated (235 load) The numbers, if both the T1 and sat link were saturated, are as follows:
ISDN link: (78,125 x 8) + ((10,000 x 70,000)/(256 1)) + 2,000 = 3,690,725 Sat link: (1,000 x 8) + ((10,000 x 1,000)/(256 253)) + 120,000 = 3,461,333 T1 link: (6,4767 x 8) + ((10,000 x 6,4767)/(256 235)) + 78,000 = 3,213,949
You are very close, but still no cigar At this level of saturation, the route through the T1 would still be the primary, the route through the sat link would be the secondary, and the ISDN would be a close third So what do you need to do That's right, increase the K2 yet again Like before, due to the logarithmic nature of the load calculation, you will have to increase the K2 by a very large number to achieve your goals In this case, you will have to increase K2 to around 300,000 Once you increase the K2 to 300,000, the metrics under the previous load values are as follows:
ISDN link: (78,125 x 8) + ((300,000 x 70,000)/(256 1)) + 2,000 = 92,538,765 Sat link: (1,000 x 8) + ((300,000 x 1,000)/(256 253)) + 120,000 = 100,128,000 T1 link: (6,4767 x 8) + ((300,000 x 6,4767)/(256 235)) + 78,000 = 92,653,870
Whew! It's a good thing the infinite metric in IGRP is 4 billion! Now, let's add the final two weights (K4 and K5) and put reliability into the equation In this case, you would like to use the sat link unless it is unreliable enough that the T1 will have a higher throughput (due to retransmissions required on the sat link) Again, with just reliability taken into account, the sat link would have to reach an 85-percent error rate for the route through the T1 to be faster An 85-percent error rate on the sat link ends up as a 38 reliability rating (Remember, reliability is backward, with 255 being the best and 1 being the worst) Because the T1 is a reliable link, you don't need to be too concerned about reliability issues with it Therefore, this equation will be a bit simpler than the load equation because you do not need to worry about failover to the ISDN due to T1 reliability Note In actuality, IGRP metrics are 24-bit, making the infinite metric 167 million, not 4 billion (which is the infinite value for EIGRP) However, this is a miniscule point in most environments, where the metrics typically never reach either maximum value I chose to use 4 billion as the maximum value in this chapter in order to illustrate the use of extremely large metrics Just keep in mind that metrics larger than 16 million are not actually valid in IGRP Before modifying the reliability weights for the example, let's look at the details of the reliability calculation When modifying the K4 and K5 weights, you need to realize the
purpose of each and the mathematical relationship each has with the rest of the metric The composite metric calculated up to this point is modified in its entirety by the reliability portion of the equation Basically, the relationship boils down to this: (all of the other factors) x (reliability factors) = metric So, reliability will modify everything else As far as the reliability portion of the equation is concerned, K4 and K5 influence each other Just for review, the formula in the reliability portion of the equation is K5/(reliability + K4) You can use K4 to decrease the metric range based on reliability (in other words, K4 sets how important reliability is), and K5 to increase or decrease the final metric For instance, if your base metric (before setting K4 and K5) is 500,000, you set K4 and K5 to be 1, and then you compute the metric at maximum reliability (255), your final formula will be (500,000) x (1/(255 + 1)), for a final metric of 1,953 If you set K5 to be 2, however, your formula becomes (500,000) x (2/(255 + 1)), for a final metric of 3,906 To understand K5, think of it as a metric scalar, meaning that it can scale the final metric up or down to match it to your environment K4, on the other hand, tells you how important reliability is in the final metric calculation For instance, in the previous example, with a base metric of 500,000 and K5 and K4 values of 1, the metric with a reliability rating of 255 was 1,953 However, with a reliability rating of 1, the metric increases to a whopping 250,000! If you set K4 to be 1,000, however, the metric with a 255 reliability is 398, while the metric with a 1 reliability is 500 In this manner, K4 determines how much reliability matters in the final metric You must be very careful when matching K4 and K5 in your metric calculations, because if you set K5 too low and K4 too high, you will end up with extremely low metrics For instance, with a base metric of 500,000, if you set K4 = 1,000,000 and K5 = 1, your metric will always be 1, regardless of the reliability of the link Now that you have seen how K4 and K5 modify the metric, let's configure the reliability settings for our "Complex Metric Calculation" example So far, in my example, I have decided on the following weights: K1 = 8, K2 = 300,000, K3 = 1 These weights provide the following metrics during periods of low congestion (load of 1):