Bridging 51
T0008-16F Release 5.2M
Transparent Bridging for Ethernet LANs
Forwarder
Functions The Forwarder bridging logic includes decisions such as:
• Which Link(s) the frame should be sent to.
If the destination address of the frame exists in the Fo rwar ding Table, and there is a
link number associated with it, then the Forwarder checks whether filters should be
applied to the frame and cause the frame to be dropped. If all these conditions are
satisfied and no filters apply, the Frame is sent over to the link via the Handler.
• Whether the frame should be broadcast to all ports, even though the frame is
not a multicast or broadcast frame.
If the destination address of the frame did not exist in the Forwarding Table
(such as when this is the first time the bridge learns t he source address of the frame),
the Forwarder attempts to send the frame out over all links which are in Forwarding
state (except the link where the frame arrived). Before the Forwarder sends th e frame
out, it also checks whether filters are to be applied to this frame. The frame may get
filtered at certain links.
• Whether the frame is a multicast or broadcast frame, and if it is, to which links
the frame should be sent out.
The Forwarder attempts to send the multicast or broadcast frame over to all links
which are in forwarding state (except the link where the frame arrived). Before
sending the frame out to each link, the Forwarder checks with the filtering facility to
see if filters are to be applied to the fram e at that link. If not, the Forwarder sen ds the
frame over to the link. A multicast/Broadcast frame is a frame with the Least
Significant Bit of its destination address being set to 1.
• Whether the received non-multicast and non-broadcast frame should be
dropped without sending it out.
The Forwarder drops the frame due to filtering, when the link is unconfigured or the
link is not in forwarding state.
Two types of forwarding database entries that are kept in running memory are:
• Learned (dynamic)
• Permanent (static)
The learned entries of the forwarding database are obtained from the source address
of frames that are received by the bridge entity. This relies on the end stations
sending frames so that the bridge learns the station location from the source address
contained in the frame.
The permanent entries are obtained from a CMEM record that is configurable by the
system administrator. They are loaded into the database whenever the node or the
table is booted. The permanent entries are maintained by the system administrator.
Forwarder
Initialization During Forwarder Module initialization, the Forwarder creates the sockets to
connect to other modules in the node, such as sockets used to communicate with the
network handlers. It also defines MACRO services in the Forwarder Module
Descriptor to export the socket addresses to the outside world.