Switch Basics

SWITCH BASICS

We saw a variety of ways to build a switch, ranging from a general-purpose workstation with a suitable number of network interfaces to some sophisticated hardware designs. The control processor is responsible for running the routing protocols discussed above, among other things, and generally acts as the central point of control of the router. The switching fabric transfers packets from one port to another, just as in a switch; and the ports provide a range of functionality to allow the router to interface to links of various types (e.g., Ethernet or SONET).

Another consequence of the variable length of IP datagrams is that it can be harder to characterize the performance of a router than a switch that forwards only cells. Routers can usually forward a certain number of packets per second, and this implies that the total throughput in bits per second depends on packet size. Router designers generally have to make a choice as to what packet length they will support at line rate. That is, if (pps) packets per second is the rate at which packets arriving on a particular port can be forwarded, and linerate is the physical speed of the port in bits per second, then there will be some packetsize in bits such that:

packetsize× pps = linerate

This is the packet size at which the router can forward at line rate; it is likely to be able to sustain line rate for longer packets but not for shorter packets. Sometimes a designer might decide that the right packet size to support is 40 bytes, since that is the minimum size of an IP packet that has a TCP header attached. Another choice might be the expected average packet size, which can be determined by studying traces of network traffic.