- Store and forward, which buffers and checks the frame prior to
forwarding - Cut-through, which only reads the hardware or MAC address prior to
forwarding - Fragment free, which is a combination of store-and-forward and cut-
through that reads the physical address and performs some error checking - Adaptive switching, which switches among the first three methods as
appropriate
Layer 3
The network layer of the seven-layer OSI model is responsible for end-to-end
packet delivery. Whereas the previous layers may deal with activities with
a network, the network layer involves communications via one or more
networks. Forwarding data packets to different types of network connections
is usually the function of a router, although a Layer 3 network switch can
perform all the functions of a router.
Layer 4
While Layer 4 switches are not well defined they do have some traits com-
mon to the other layers. One is the capability to perform network address
translation (NAT). Typically NAT is used to allow multiple users or devices
to access the Internet using one public IP address. Years ago this type of
translation was a tool to deal with theunavailability of IPv4 addresses. More
recently it is used to hide an internalnetwork structure by making it appear
that all network traffic is originating from the Layer 4 switch rather than the
devices on the internal network.
Because NAT involves translating IP addresses more processing is required
and the model of the end-to-end connectivity across the Internet is somewhat
blemished. Layer 4 switches may also do load balancing. With this feature the
switch resource utilization and traffic throughput is optimized. An example
is a server farm where the network switch may direct traffic across several ser-
vers to balance the load. The switch uses policies or filters to identify and man-
age application specific traffic.
Network Servers
Servers are connected to LANs and provide a variety of resources to
both network users and network administrators. A common application
provides network users with connectivity to the Internet and shared Internet
Data Networks 125