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Last Weeks CCIE™ Challenge Question

In X.25, when the network permits only a window size of 2, there is a way to increase the effective window size to a specific destination.

a) True
b) False


a) True

Cisco has a parameter x25 nvc n, which lets you work around this limitation when throughput is a major concern. Increasing the NVC count above 1 allows you to create multiple virtual circuits to the same destination. The effect is quite similar to a multilink frame layer, in which the effective window size becomes n times w.

RFC 1356 points out that this method may have unexpected and undesirable characteristics: "Another method of increasing performance is to open multiple virtual circuits to the same destination, specifying the same CUD. Like packet size, this is not always the best method. However, opening multiple circuits also competes more effectively for the physical path, by taking more shares of the available bandwidth. While this may be desirable to the user of the encapsulation, it may be somewhat less desirable to the other users of the path. Opening multiple circuits may also cause datagram sequencing and reordering problems in end systems with limited buffering (e.g., at the TCP level, receiving segments out of order, when a single circuit would have delivered them in order). This will only affect performance, not correctness of operation. Opening multiple circuits may also increase the cost of delivering datagrams across a public data network."

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