KSZ8873MLL_13 Datasheet, PDF (39/115 Pages) Micrel Semiconductor – Integrated 3-Port 10/100 Managed Switch with PHYs

Micrel, Inc.
KSZ8873MLL/FLL/RLL
Rapid Spanning Tree Support
There are three operational states of the Discarding, Learning, and Forwarding assigned to each port for RSTP:
Discarding ports do not participate in the active topology and do not learn MAC addresses.
Discarding state: the state includs three states of the disable, blocking and listening of STP.
Port setting: "transmit enable = 0, receive enable = 0, learning disable = 1."
Software action: the processor should not send any packets to the port. The switch may still send specific packets to the
processor (packets that match some entries in the static table with “overriding bit” set) and the processor should discard
those packets. When disable the port’s learning capability (learning disable=’1’), set the Register 2 bit 5 and bit 4 will flush
rapidly the port related entries in the dynamic MAC table and static MAC table.
Note: processor is connected to port 3 via MII interface. Address learning is disabled on the port in this state.
Ports in Learning states learn MAC addresses, but do not forward user traffic.
Learning state: only packets to and from the processor are forwarded. Learning is enabled.
Port setting: “transmit enable = 0, receive enable = 0, learning disable = 0.”
Software action: The processor should program the static MAC table with the entries that it needs to receive (e.g., BPDU
packets). The “overriding” bit should be set so that the switch will forward those specific packets to the processor. The
processor may send packets to the port(s) in this state, see Tail Tagging Mode section for details. Address learning is
enabled on the port in this state.
Ports in Forwarding states fully participate in both data forwarding and MAC learning.
Forwarding state: packets are forwarded and received normally. Learning is enabled.
Port setting: “transmit enable = 1, receive enable = 1, learning disable = 0.”
Software action: The processor should program the static MAC table with the entries that it needs to receive (e.g., BPDU
packets). The “overriding” bit should be set so that the switch will forward those specific packets to the processor. The
processor may send packets to the port(s) in this state, see Tail Tagging Mode section for details. Address learning is
enabled on the port in this state.
RSTP uses only one type of BPDU called RSTP BPDUs. They are similar to STP Configuration BPDUs with the exception
of a type field set to “version 2” for RSTP and “version 0” for STP, and a flag field carrying additional information.
Tail Tagging Mode
The Tail Tag is only seen and used by the port 3 interface, which should be connected to a processor. It is an effective
way to retrieve the ingress port information for spanning tree protocol IGMP snooping and other applications. The Bit 1
and bit 0 in the one byte tail tagging is used to indicate the source/destination port in port 3. Bit 3 and bit 2 are used for the
priority setting of the ingress frame in port 3. Other bits are not used. The Tail Tag feature is enabled by setting Register 3
bit 6.
Figure 7. Tail Tag Frame Format
September 20, 2013
39
Revision 1.6