Understanding the network.pdf - Back to Home

Switch Ports and Related Functions
Each interface port (like a bridge) has its own distinct MAC address.
Attached to each interface is a DRAM memory buffer, which is used to
store the packets as they enter the switch port. Switching, like routing
and bridging, is a packet-by-packet process. The packet comes in to
the switch and is stored in its port buffer. A destination lookup is made
using the SAT. If a destination is found, the packet is forwarded on to
the correct port, where it is forwarded to its destination. If a
destination cannot be found, the packet is flooded to each switch port.
The flooding process is also used for SAT generation. When the switch
first comes online, it knows nothing about the network. As it begins to
process packets, it learns addresses the same way a bridge does, by
examining the source addresses of the packets on their way in. The
switching process does result in some latency, for the same reasons
that latency occurs with routing and traditional bridges. The amount
of latency depends on a variety of factors, such as the following:
• The speed of the ports involved
• The size of the packet being processed
• The forwarding scheme used by the switch
• The switch's backplane capacity and forwarding rate
We look at latency in more detail when we discuss the different switch
forwarding methodologies.
Port Configuration Options
Most switches manufactured today autonegotiate between 10 and
100Mbps, half or full duplex. Earlier, we mentioned that, with
switching, Ethernet has evolved into an encapsulation protocol for
point-to-point dedicated connections. With Ethernet (as with FDDI
and Token Ring), this capability requires the port to operate at full
duplex. Full-duplex operation is possible because each port on the
switch is effectively its own collision domain. When a single
end-station (or switch port) is connected to a switch port, full-duplex
operation is possible, because there are only two devices in the
collision domain. CSMA/CD media access is not required because the
stations are communicating with each other simultaneously. In
half-duplex mode, the port operates under CSMA/CD and collisions
occur. Even with only one device, collisions still occur between the
switch and the connected end-station.