3Com 10014298 manual Final Stabilized Spanning Tree

Configuring STP 185

calculation is launched again by new events, for example, the link from Switch B to C is down or the port receives a better configuration BPDU.

Ethernet 1/0/1 receives the updated configuration BPDU, {0, 5, 1, e1/0/4}, from Switch B. Since this configuration BPDU is better then the old one, the old BPDU will be updated to {0, 5, 1, e1/0/4}.

Meanwhile, Ethernet 1/0/5 receives the configuration BPDU from Switch A but its configuration BPDU is not updated and remains {0, 0, 0, e1/0/2}.

By comparison, the configuration BPDU of Ethernet 1/0/1 is elected as the optimum one. Ethernet 1/0/1 is elected as the root port, whose BPDU does not change, while Ethernet 1/0/5 is blocked and retains its BPDU, but it does not receive the data forwarded from Switch A until spanning tree calculation is triggered again by changes, for example, the link from Switch B to C is down.

Thus the spanning tree is stabilized. The tree with the root Switch A is illustrated in Figure 52.

Figure 52 The Final Stabilized Spanning Tree

Switch B with priority 1

Switch A with priority 0

	E1/0/1
	5
E1/0/7
		Switch C
E1/0/4	4	with priority 2

E1/0/1

The root ID and the designated switch ID, in actual calculation, should include both switch priority and switch MAC address. The designated port ID should include port priority and port MAC address. In the updating process of a configuration BPDU, other configuration BPDUs besides the first four items make modifications according to certain rules. The basic calculation process is described below.

Configuring the BPDU Upon the initiation of the network, all the switches regard themselves as the roots. Forwarding Mechanism The designated ports send the configuration BPDUs of local ports at a regular

interval of HelloTime. If it is the root port that receives the configuration BPDU, the switch will enable a timer to time the configuration BPDU, as well as increase MessageAge carried in the configuration BPDU by certain rules. If a path goes wrong, the root port on this path will not receive configuration BPDUs anymore, and the old configuration BPDUs will be discarded due to timeout. Recalculation of the spanning tree will be initiated to generate a new path to replace the failed one, and thus restore the network connectivity.

The new configuration BPDU as now recalculated will not be propagated throughout the network right away, so the old root ports and designated ports, that have not detected the topology change, will continue to forward the data through the old path. If the new root port and designated port begin to forward data immediately after they are elected, a occasional loop may still occur. In RSTP,