I have a question
Let me quote my cisco book:
The following list and Figure 2-3 illustrate the basic concept of moving frames from the received port or ingress, to the transmit port(s) or egress using a shared bus architecture:
Frame received from Host1? The ingress port on the switch receives the entire frame from Host1 and stores it in a receive buffer. The port checks the frame's Frame Check Sequence (FCS) for errors. If the frame is defective (runt, fragment, invalid CRC, or Giant), the port discards the frame and increments the appropriate counter.
Requesting access to the data bus? A header containing information necessary to make a forwarding decision is added to the frame. The line card then requests access or permission to transmit the frame onto the data bus.
Frame transmitted onto the data bus? After the central arbiter grants access, the frame is transmitted onto the data bus.
Frame is received by all ports? In a shared bus architecture, every frame transmitted is received by all ports simultaneously. In addition, the frame is received by the hardware necessary to make a forwarding decision.
Switch determines which port(s) should transmit the frame? The information added to the frame in step 2 is used to determine which ports should transmit the frame. In some cases, frames with either an unknown destination MAC address or a broadcast frame, the switch will transmit the frame out all ports except the one on which the frame was received.
Port(s) instructed to transmit, remaining ports discard the frame? Based on the decision in step 5, a certain port or ports is told to transmit the frame while the rest are told to discard or flush the frame.
Egress port transmits the frame to Host2? In this example, it is assumed that the location of Host2 is known to the switch and only the port connecting to Host2 transmits the frame.
I have question about 2nd step where it is said:
"A header containing information necessary to make a forwarding decision is added to the frame. "
I want to know about that header, what fields it contains?
thanks a lot!
that header is :
never exposed to outer world
(you cannot capture if with a sniffer it is internal)
it is a sort of tag that is used for forwarding.
probably it is just enough
vlan-id number /color
type of destination address: unicast, multicast, broadcast
for multicast : a flow number to identify the multicast group or other way to decide if the port is part of the list (in case of usage og IGMP snooping or the older CGMP)
for unicast: a flag for unknown unicast
Hope to help