Chapter 15. Switching Algorithms in Cisco Routers
The term switching, when used in the context of routers, describes the process of moving packets from one interface to another within a router. Packets in transit arrive at one interface and must be moved to another, based on routing information stored in the router.
Routing is the process of choosing paths and forwarding packets to destinations outside the physical router. Switching is the internal forwarding of packets between interfaces.
Just as there are different routing protocols for determining the external paths for packets, there are different internal methods for switching. These switching algorithms, or paths, are a valuable way to increase (or decrease) a router’s performance.
One of the biggest factors in how fast a packet gets from its source to its destination is the processing delay present in each router along the way. Different switching methods have vastly different impacts on a router’s performance. Choosing the right one—and knowing what to look for when there’s a problem—will help the savvy administrator keep a network running at peak performance.
A router must move packets from one interface to another, just like a switch. The decisions about how to move packets from one interface to another are based on the routing information base (RIB), which is built manually or by Layer-3 routing protocols. The RIB is essentially the routing table (see Chapter 9 for details). Nexus switches may have multiple RIBs called