Problem: IGRP Not Using Unequal-Cost Path for Load Balancing-Cause: variance Command Is Missing or Misconfigured

Problem: IGRP Not Using Unequal -Cost Path for Load Balancing ‚ Cause: variance Command Is Missing or Misconfigured

To use the variance feature (unequal-cost-path load balancing), it must be configured under the router igrp command. By default, IGRP does not do unequal-cost-path load balancing. Also, when the variance factor is multiplied by the current best metric, the resulting number is compared with other available path metrics. Any available path metric that is under this resulting number will be used for unequal-path load balancing.

Figure 5-39 shows the network setup susceptible to this problem. The network 155.155.0.0/16 is known through two paths, but only one is in the routing table.

Figure 5-39. Network Setup Conducive to Load-Balancing Problems

Figure 5-40 shows the flowchart to follow to solve this problem.

Figure 5-40. Problem-Resolution Flowchart

Debugs and Verification

Example 5-97 shows the routing table entry on R1 showing that R1 is using only one path to reach 155.155.0.0/16.

Example 5-97 R1 Routing Table Entry Shows That Only a Single Path Is Used to Reach the Destination Network

 R1#  show ip route 155.155.0.0  Routing entry for 155.155.0.0/16  Known via "igrp 1", distance 100, metric 8976  Redistributing via igrp 1   Advertised by igrp 1 (self originated)   Last update from 131.108.6.2 on Serial2, 00:00:03 ago   Routing Descriptor Blocks:   * 131.108.6.2, from 131.108.6.2, 00:00:03 ago, via Serial2       Route metric is 8976, traffic share count is 1       Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 0 

Example 5-98 shows the interface configuration on both Serial2 and Serial3. Band -widths are equal in this example, but they could have different values in different scenarios.

Example 5-98 R1's Serial2 and Serial3 Interface Configurations

 R1#  show interface serial2  Serial2 is up, line protocol is up   Hardware is HD64570   Internet address is 131.108.6.1/24   MTU 1500 bytes, BW 1544 Kbit,  DLY 400000 usec  , rely 255/255, load 1/255 R1#  show interface serial3  Serial3 is up, line protocol is up   Hardware is HD64570   Internet address is 131.108.7.1/24   MTU 1500 bytes, BW 1544 Kbit,  DLY 20000 usec  , rely 255/255, load 1/255 

Example 5-99 shows the IGRP configuration on R1. The variance command is configured, but the multiplier has the wrong value. The metric from Serial3 is more than five times larger, which is why the variance is not working. If you multiply the metric value of Serial2 (which is 8976) by 5, you get 44,880, which is still not enough because the metric for Serial3 is 46,976.

Example 5-99 Improperly Configured Variance Value

 R1#  !   router igrp 1    variance 5    network 131.108.0.0  

Solution

To solve this problem, choose a variance factor that yields a metric that is higher then the one being used by another unequal-cost path. For example, when you multiply the current metric of 8796 by 6 instead of 5, you get a value of 52,776. So, any link that has a metric value of less than 52,776 will be used in this unequal-cost-path load balancing. In this example, Serial3 has a metric value of 46,976. Because this number is less than 52,776, it is used for load balancing.

In Example 5-99, the second link metric is more than five times larger than the current metric. Example 5-100 shows that by changing the variance value to 6, IGRP starts including the second path.

Example 5-100 Correcting the Variance So That IGRP Uses Both Paths

 R1#  !   router igrp 1   variance 6   network 131.108.0.0  

Example 5-101 shows that R1 is installing both paths in the routing table, but with the traffic share count ratio equal to 5.

Example 5-101 R1 Routing Table Shows the Traffic Share Count Ratio

 R1#  show ip route 155.155.0.0  Routing entry for 155.155.0.0/16   Known via "igrp 1", distance 100, metric 8976   Redistributing via igrp 1   Advertised by igrp 1 (self originated)   Last update from 131.108.7.2 on Serial3, 00:00:07 ago   Routing Descriptor Blocks:  * 131.108.6.2, from 131.108.6.2, 00:00:07 ago, via Serial2   Route metric is 8976, traffic share count is 5  Total delay is 25000 microseconds, minimum bandwidth is 1544 Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 0  131.108.7.2, from 131.108.7.2, 00:00:07 ago, via Serial3   Route metric is 46976, traffic share count is 1  Total delay is 405000 microseconds, minimum bandwidth is 1544 Kbit       Reliability 255/255, minimum MTU 1500 bytes       Loading 1/255, Hops 0 

The traffic share count ratio is calculated by dividing the worst metric by the metric of a path. In this case, the path from Serial3 is worse and has a value of 46,976.

So, in this example, the ratio is 5:1. After every five packets forwarded on Serial2, Serial3 will be used for one packet.