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Fat Fingers From the Past

We've all done it: fat fingered a config and not caught it until packets started disappearing. We realize the error in a few days, if not hours, of that last "wr mem" on the router.

But thanks to our Green Bay, Wis., lab's unique setup--designed to enable multispeed testing without massive reconfigs--we didn't notice one misconfiguration for nearly two years.

Sound impossible? Not in this case. The lab's core backbone sits between a Cisco Catalyst 6500 and an Extreme Summit7i. The two are connected with 1-GB fiber and 100-Mbps Cat5e. Two Cisco 7200 VXRs provide T1 connectivity between the core routers. The core routers determine which of the three backbones to use, based on availability and routing metrics. This lets us drop a link and the routers fail over to the next available speed.

A couple of years ago, I reconfigured both routers for a BGP-based test. Afterward I restored the original configuration, but apparently reversed the metrics on the 100-Mbps and T1 routes on the Summit7i. And no one noticed. Not until recently, when my lab partner (and husband) Don MacVittie started testing WAFS products that required dropping the network down to the 100-Mbps backbone (see "BuzzCut: We Know How Hard IT Is--Trust Me").

Don saw an inordinate number of retransmits, incorrect checksums and duplicate ACKs. Tracing the route from client to server showed that outbound packets went the way of the T1 and came back over the 100-Mbps backbone. However, the outbound path ignored the Shunra Storm he'd interposed in the 100-Mbps backbone to introduce latency and congestion for his testing, causing us all to scratch our heads.

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