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after the bridge classifies its port role by Root port, specified port, non-specified port, creating a loop-free topology is straightforward: the root port and the specified port forward data traffic, and non-specified ports block data traffic. Although forwarding and blocking are the only two used states in a stable network, table 6-3 lists the actual v state of the STP.
table 6-3. STP states
state |
purpose |
Forwarding |
sending/receiving User Data |
Learning |
Building Bridging table |
Listening |
Building "Active" topology |
Blocking |
Receives BPDUs only |
Disabled |
Administratively down |
You can consider the table from bottom to top as the order in which the state of the bridge port changes, starting with the state (disabled and blocking) below the upper table, and then changing the protocol run state to forwarding. disabled status Is the state of the port after the administrator manually disables a port, not a port state that belongs to the protocol during normal dynamic processing. The first port is in blocking status To listen for the arrival of BPDUs packets. After a series of events (such as immediately after the bridge has been started to think that you are the root bridge or not receiving bpdus for a certain period of time), the bridge State may become listening status. at this time, the user data cannot be forwarded-- During this time, the port is determining the network topology by sending and receiving BPDUs, and it is the three-step operation of the initialization convergence discussed in the previous section during the listening state. Those ports that failed in the specified port selection become non-specified ports and their port status is returned to the blocking state. After those ports that become the specified port and root port and remain for 15 seconds (the default time value), their state transitions to the
learning State . This is another 15-second period that does not allow the user packet to pass, but this time the bridge as discussed in chapter three, its forwarding address table is silently learning. Once the bridge receives the frame, the source MAC address and port will be put into the bridge forwarding post, and the learning state reduces the large number of data requests at the beginning of the user's data forwarding. < translator Note: After the switch is initialized, its Mac forwarding is empty, and just beginning once the data is transferred to the switch to start forwarding, the switch forwards the data from all ports (that is, the so-called flooding of the packet) because it fails to look for the destination address in the forwarding publication, thus triggering a large number of packets in the network. The learning state of the STP allows the switch to learn the MAC address without forwarding the user packet, and when the learning state, the switch has until the packet from which the destination MAC address is forwarded out, without flooding the packet, This effectively prevents the initial forwarding of data is a network of large packets of bad situation >. Note:In addition to saving the source MAC address and port information, the Catalyst platform switch learns additional information such as the Source VLAN.
if, after the time period of the learning State expires, those ports that were previously selected to be the specified port and root port have their port status transition to the forwarding state , at this stage, the user data is finally ready to be forwarded. Figure 6-11 shows the status of the port State and its possible transition to. Figure 6-11. Possible Port states and Transitions
Figure 6-12 shows an example of a network port classification and status show, noting that all ports except CAT-C:PORT-1/2 are forwarding states. Figure 6-12. Sample Network with Port states identified
Table 6-4 lists the shorthand symbol tables used in this book instead of the spanning tree state. Table 6-4. STP state and Port Symbols
state/port |
symbol |
Blocking |
B |
Forwarding |
F |
Designated Port |
DP |
Root Port |
RP |
non-designated Port |
NDP |
The classic "Cisco Lan Switching" chapter sixth (vi): Five STP states
