User`s manual
Chapter 1 Overview
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In the half-duplex mode, flow control uses a collision-based scheme to throttle
the connected stations when the free buffer space of the corresponding port is
too low, to avoid discarding frames during network congestion (this approach
is called back pressure). When the buffer space of a port is almost full, its
MAC controller forces a collision in the input port when an incoming frame is
sensed (the alternative, without flow control, is to discard the incoming frame).
In the full-duplex mode, the standard flow control method defined in IEEE
802.3x is used, which is based on pause frames and enables stopping and
restoring the transmission from the remote node. However, this method can
only be used when auto-negotiation is enabled on the port, and the node
attached to the port supports pause frames.
The Ethernet switch internal MAC controllers discard all the received IEEE 802.3x
pause frames, even when full-duplex flow control is disabled or the port is in the
half-duplex mode.
1.6.3.3 Forwarding Algorithms
The Ethernet switch operates as a MAC bridge, that automatically learns the MAC
addresses located on the local LAN, and the port through which frames addressed
to a foreign destination are to be transmitted.
The Ethernet switch LAN table can store up to 1024 MAC address/port number
mappings. Only active MAC address/port number mappings are actually stored;
after a user-defined aging interval, inactive mappings are removed from the switch
memory. However, the user can also add static entries, which are not
automatically removed.
When used in more complex networks, the forwarding algorithm can be extended
to include Spanning Tree Protocol (STP), where the user can select between the
basic STP versions. The selection is separately made for each switch port.
The user can also select the STP parameters, which are used in common by the
whole switch, to fine-tune the performance. To help locating forwarding problems,
the user can read the STP algorithm status and current parameter values.
1.6.3.4 VLAN Support
The Ethernet switch can use VLAN information to select the ports among which
traffic can be forwarded. There are two basic methods: