oten- tially be added or the speed of the ports can be increased. However, at one level of hierarchy above, the switch fabric must be replaced with a more powerful one of higher capacity or multiple fabrics must be added in parallel. Redundant switch fabrics are introduced by adding more switch fabric components in parallel on the same or different switch fabric cards in order to increase the reliability and resilience of a switch- ing system by increasing the availability of spare links onto which the system can fold back in case of malfunction. Figure 14.6 illustrates this principle. Appropriate design techniques can ensure that in case of malfunction, the system will automatically switch over to the backup switch fabric card with little or no traffic loss. The context may ensure what carriers call a graceful degradation of serv- ice and the cards may be hot swappable-that is, replaceable or serviceable while the switch is oper- ating. This in-service serviceability is tremendously valuable to carriers who cannot afford to have the entire chassis powered down in order to search for and solve a problem at a specific card. Three types of redundancy are available for switch fabrics: passive, active, and load-sharing-based redundancy. Figure 14.7 illustrates the idea behind passive redundancy. For example, a configuration may have N active switch fabrics (in the drawing N =2) and one extra fabric that behaves as the backup fabric (shown in gray). In such a configuration, the switching system architecture offers N:1 redundancy. If N backup fabrics are operating besides N active fabrics, the configuration is said to be 1:1 redundant. A 1:1 redundancy will for N>1 be more expensive than N:1 redundancy. Therefore, the issue of man- Multiple Potential output buffers switch fabrics for PHY/MAC redundancy TM & NPU TM & NPU PHY/MAC PHY/MAC TM & NPU TM & NPU PHY/MAC PHY/MAC TM & NPU TM & NPU PHY/MAC Line cards FIGURE 14.6 Multiple switch fabrics operating side by side increase the capacity and/or reliability/availability of