Fast switching vs Process switching

1. Process Switching:

Process switching is the traditional method of packet forwarding used in early routers. When a packet arrives at a router, the router's CPU interrupts the normal processing to examine the packet and determine its destination. This involves a series of steps, including a route lookup in the routing table and ARP resolution to obtain the MAC address of the next-hop device. Once these steps are completed, the router forwards the packet to the appropriate interface.

Process switching involves significant CPU involvement for each packet, making it slow and resource-intensive. It can cause performance bottlenecks, especially during high network traffic, and limits the overall capacity of the router.

2. Fast Switching:

Fast switching was introduced as an improvement over process switching to address its limitations. Fast switching involves the creation of a cache, known as the fast switching cache or fast cache, which stores the results of previous route lookups. When a packet arrives, the router checks the fast cache to see if it has a precomputed entry for the destination IP address. If a match is found, the router uses the cached information to forward the packet quickly, without involving the CPU in time-consuming route lookups.

Fast switching significantly reduces the CPU overhead associated with packet forwarding. Once the cache is populated, subsequent packets with the same destination address can be forwarded at wire speed, resulting in improved forwarding performance and reduced latency. However, it's important to note that the first packet for a particular destination still undergoes the slower process switching to populate the cache.

Differences and Trade-offs:

1. Performance: Fast switching offers superior performance compared to process switching. By minimizing CPU involvement, fast switching enables routers to handle higher packet forwarding rates, resulting in improved overall network performance.

2. Resource Utilization: Process switching consumes more CPU resources because it requires the router's processor to perform complex tasks for every packet. Fast switching, on the other hand, offloads these tasks to the fast cache, reducing CPU utilization and freeing up resources for other critical functions.

3. Flexibility: Process switching allows for greater flexibility in terms of protocol support and packet handling. It can handle complex scenarios that may require special treatment or customization. Fast switching, although more efficient, is relatively limited in terms of customization options.

4. Time to Convergence: In the event of network changes or link failures, process switching can adapt more quickly because it performs a route lookup for every packet. Fast switching, with its reliance on the fast cache, may experience a slight delay before adapting to new routing information.