ror, indicating that the V.35 cable is good. Next, we clear the data port loopback on CSU/DSU (A) and set a local loopback. Again, we perform our tests and all packets return with no errors. We have now eliminated Router A, the V.35 cable, and 90 percent of CSU/DSU (A). Chapter 21: T1 Remote Remote Data port Local payload CSU/DSU CSU/DSU Router A Router B Data port Local Remote line BPV,00F, CRC6] Remote payload Figure 21-12. Loopback testing progression The next step is to clear the local loopback on CSU/DSU (A) and set a remote line loopback on CSU/DSU (B). Because this is a remote loopback, we'll set the loop from CSU/DSU (A). Alternatively, we could have called someone at the (B) location to man- ually set a line loopback on CSU/DSU (B). Again, we run our tests and all results are clean. We have now eliminated Router A, Router A's V.35 cable, CSU/DSU (A), and the T1 line itself (including all telco responsibility). Now we clear the remote line loopback on CSU/DSU (B) and set a remote payload loopback on CSU/DSU (B)-again, administered from CSU/DSU (A). This time when we run our test, CSU/DSU (A) reports many BPV, OOF, and CRC6 errors. We have found the source of the trouble-CSU/DSU (B) is not functioning properly. By sys- tematically moving our loopback point further and further away from one side of the link, we were able determine the point at which the trouble started to appear. Replacing CSU/DSU (B) solves our problem, and we're back in business. Integrated CSU/DSUs T1 WAN interface cards (WICs) with integrated CSU/DSUs are about the coolest thing to happen to routers in the past 15 years. Call me a nerd, but the idea of removing another physical piece of equipment for each T1 installed (as well as those horrible V.35 cables from equipment racks) is the closest thing to geek paradise I've ever seen. The integrated CSU/DSU WICs serve the same purpose as standalone units, and they have the added benefit of being controlled via IOS from the router. Figure 21-13 shows the loopback points for a T1 CSU/DSU WIC 