Continuing our tour of routers with filtering turned on, we examine the Cisco 2651.Like the other routers, it moved short packets at line rate, but only in the baseline case with no filters or dynamic routing applied. Once those features were turned on, throughput really tumbled. At best, throughput was less than 30% of the theoretical maximum once filtering was activated. With routing and filtering set, throughput in some cases dropped to just 7% of the two T-1s' capacity.Cisco's documentation warns that access lists can have an adverse effect on performance. These results put numbers to that warning.For example, when handling 256-byte frames, the Cisco 2651's throughput fell off from line rate, but not as badly as with 64-byte frames; this time the worst-case rate was 29% of the maximum, compared with 7% for short frames.The 2651 moved data at line rate with no routing and up to 64 filters configured, both without routing and with small routing tables. However, rates dropped to 29% of the theoretical limit when we loaded large routing tables on the 2651.Throughput with 1,518-byte frames was line rate in all cases.