The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

17.4. Switch Data Transmit Path Functions

Ultimately, the switch fabric delivers a stream of frames to each output port. These frames may have entered the switch from a single input or from a multitude of input ports. In any case, we need to perform some additional checks and processing before sending them onto the physical link. Figure 17-20 depicts the logical blocks present in the output port (transmit) data path. The sections that follow look at the operations performed within each of these blocks.

17.4.1. Output Filters

For each frame targeted at a given output port, we need to perform two VLAN-related qualification tests before actually queuing it for transmission. As discussed in Chapter 12, "Virtual LANs: The IEEE Standard," the Egress Filter determines whether the output port is in the member set for the VLAN to which the frame belongs. A frame should never be sent on a port that is not in the member set for its associated VLAN. Under most conditions, frames will never be forwarded through the switch fabric to output ports not in the VLAN member set. However, it is possible that, because of inconsistent static entries in the Filtering Database or the operation of Shared VLAN Learning, frames will appear at the output that should not be sent on that port. The Egress Filter detects and prevents this condition.

Figure 17.20. Switch output data flow (transmit path)