Westermo U200 Operator manal - Manual (Page 9)

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U/R/T200 series
- 9 -
4 Switch Operation
4.1 Introduction
A switch has to forward and receive packets from one LAN or device to another. The switch
could forward all packets, but if this was the case it would have similar behavior to a hub.
It would be more intelligent if the switch only forwarded packets which need to travel from one
LAN or device to another. To do this, the switch must learn which devices or LANs are
connected to each port. In simplistic terms; it needs to learn the destination and source ports
of each and every packet received on each individual Switch port. Once learnt, any identically
addressed packet will be automatically be forwarded.
4.2 Error Detection
The switch stores every incoming packet and scans this for errors, usually by checking the
frame CRC (cyclic redundancy check sum). If any errors are found or detected the packet is
discarded. In addition each frame is checked for size. Undersized packets (less than 64
Bytes) and oversized packets (more than 1518 bytes (*)) are also discarded.
Once these basic checks have been carried out the switch can then start learning packet
source and destination information.
(*) When implementing Ethernet MAC tagging maximum Ethernet packet length is increased
to 1522 bytes.
4.3 Flooding
The switch needs to make a decision regarding which port(s) the packet is to be forwarded to.
This decision is based upon the MAC tables that are maintained and updated automatically
by the Switch. The process is known as Layer 2 Switching.
When first powered on the MAC tables within the Switch are empty. When a packet is
received on a port the Switch does not know where the destination MAC address is located.
The Switch learns the address by ‘flooding’ the packet out to all ports. Eventually, the
destination node responds, the address is located and the Switch remembers the destination
port. In simplistic terms; when a Switch receives a packet on a port it stores the source MAC
address in the MAC table that corresponds to that Port. The flooding technique is always
used with Broadcast and Multicast packets. If the switch is equipped with multicast
management then multicast packets will not be flooded.
4.4 MAC Table and Packet Memory
.
The MAC table can hold up to 8 K entries with a MAC aging interval of five minutes. MAC
aging means that a MAC address learned on a given port will be removed from the MAC table
if no packets with this MAC address as the source MAC address are received on the port for
five minutes.
The total packet memory is 1Mbyte. This means that 657 (maximum packet length - 1522
bytes) to 15625 (minimum packet length - 64 bytes) packets. The packet memory is used to
handle short high load/overload situations. Exceeding the packet memory means that the