S Layer 2 Managed Switch User Manual
陈泽科技有限公司
www.stephen-tele.com
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Assign switch ID:
Combined with switch priority and MAC address.
Assign port ID:
consist of assigned port priority and port number.
Assigned message life:
current configure message can still live for how long time. MessageAge.
Assigned message MaxAge:
this configure message living Maxage from sending out.
Assigned message sending cycle:
configure message sending time interval.
Port status transmission delay: Fowarddelay.
STP Spanning tree realization process
1)
Configuration BPDU
When switches start, every switch thinks itself as root switch. Every switch sends configuration BPDU from all port. It
calculates tree topology through communication of BPDU message. Every configuration BPDU contains following information:
Root switch’s bridge ID that acknowledged by sending switch
Expense to root switch
Bridge ID of sending switch
Age of message
Sign of output port
Timer’s value: hello, forward-delay, max-age etc.
Every assigned port in the network will send BPDU message as Hello_Time time interval.
If switch receives superior (smaller BID, less path expense) configuration BPDU message, it will reserve the message for this
port. If this BPDU message is received from switch’s root port, switch will forward this BPDU message to all LAN that assigned
by this switch.
If switch receives inferior configuration BPDU message (compare with original message from this port) from some port, it will
drop this BPDU message. If switch receives inferior BPDU message from some assigned switch’s LAN, switch will send the
reserved latest BPDU message to this LAN. Inferior BPDU message is dropped and superior BPDU message is spread to the
whole network through this mode.
BPDU message exchange will result to:
One switch is selected to be root switch. To every VLAN, switch which get highest priority (smallest priority value) is
selected to be root switch. If priority is the same, switch which owns smallest MAC address will be root switch.
Every switch need to choose a root port besides root switch. When switch forwards data to root switch, this port provide
a least expense path.
Every switch calculates the shortest path to root switch according to path expense.
Every LAN section needs to choose a designated switch. When data from forwarding LAN section reach to root switch,
designated switch provides a least expense path. LAN section port that connected with designated switch is called
designated port.
2)
Topology Change Notification (TCN) BPDU
The designated switch in LAN which network topology has changed will generate a TCN BPDU message and send this out
through Root Port. After the designated switch receives TCN BPDU message, it will confirm this message (if not, the source of
this message will keep on sending this TCN BPDU message) and regenerate this TCN BPDU message. Send it out to Root Port
through its Root Port as well. Spanning tree estimates if the path need to be changed or not. After STP converged, there is no
TCN BPDU message in the network.
After Root Switch receives TCN BPDU message, Root Switch will configure TC indicating bit of configuration BPDU message to
1 and send this message to other switches. The duration that Root Switch send this kind of message (TC indicating bit is 1) is
FMaxAge. Meanwhile other switches receive this kind of BPDU messages, they shorten their MAC addresses item
