valutazioni - 4, GPA: 3.5
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BPDUs are not directly forwarded by the switch, but the receiving switch uses the information in the frame to calculate a BPDU, and, if the topology changes, initiates a BPDU transmission. The communication between switches via BPDUs results in the following: ■ One switch is elected as the root switch. ■ The shortest distance to the root switch is calculated for each switch. ■ A designated switch is selected. This is the switch closest to the root switch through which packets will be forwarded to the root. ■ A port for each switch is selected. This is the port providing the best path from the switch to the root switch. ■ Ports included in the STP are selected. Creating a Stable STP Topology It is to make the root port a fastest link. If all switches have STP enabled with default settings, the switch with the lowest MAC address in the network will become the root switch. By increasing the priority (lowering the priority number) of the best switch, STP can be forced to select the best switch as the root switch. When STP is enabled using the default parameters, the path between source and destination stations in a switched network might not be ideal. For instance, connecting higher-speed links to a port that has a higher number than the current root port can cause a root-port change. STP Port States The BPDUs take some time to pass through a network. This propagation delay can result in topology changes where a port that transitioned directly from a Blocking state to a Forwarding state could create temporary data loops. Ports must wait for new network topology information to propagate throughout the network before starting to forward packets. They must also wait for the packet lifetime to expire for BPDU packets that were forwarded based on the old topology. The forward delay timer is used to allow the network topology to stabilize after a topology change. In addition, STP specifies a series of states a port must transition through to further ensure that a stable network topology is created after a topology change. Each port on a switch using STP exists is in one of the following five states: ■ Blocking - the port is blocked from forwarding or receiving packets. ■ Listening - the port is waiting to receive BPDU packets that may tell the port to go back to the blocking state. ■ . Learning - the port is adding addresses to its forwarding database, but not yet forwarding packets. ■ Forwarding - the port is forwarding packets. ■ Disabled - the port only responds to network management messages and must return to the blocking state first. 75 User’s Manual of FGSW-2620VM / FGSW-2620PVM A port transitions from one state to another as follows: ■ From initialization (switch boot) to blocking. ■ From blocking to listening or to disabled. ■ From listening to learning or to disabled. ■ From learning to forwarding or to disabled. ■ From forwarding to disabled. ■ From disabled to blocking. Figure 4-5-1: STP Port State Transitions You can modify each port state by using management software. When you enable STP, every port on every switch in the network goes through the blocking state and then transitions through the states of listening and learning at power up. If properly configured, each port stabilizes to the forwarding or blocking state. No packets (except BPDUs) are forwarded from, or received by, STP enabled ports until the forwarding state is enabled for that port. 76 User’s Manual of FGSW-2620VM / FGSW-2620PVM 4.5.2 STP Parameters STP Operation Levels The Switch allows for two levels of operation: the switch level and the port level. The switch level forms a spanning tree consisting of links between one or more switches. The port level constructs a spanning tree consisting of groups of one or more ports. The STP operates in much the same way for both levels. On the switch level, STP calculates the Bridge Identifier for each switch and then sets the Root Bridge and the Designated Bridges. On the port level, STP sets the Root Port and the Designated Ports. The following are the user-configurable STP parameters for the switch level: Parameter Description Default Value Bridge Identifier(Not user configurable except by setting priority below) A combination of the User-set priority and the switch's MAC address. The Bridge Identifier consists of two parts: a 16-bit priority and a 48-bit Ethernet MAC address 32768 + MAC. 32768 + MAC Priority A relative priority for each switch - lower numbers give a higher priority and a greater chance of a given switch being elected as the root bridge. 32768 Hello Time The length of time between broadcasts of the hello message by the switch. 2 seconds Maximum Age Timer Measures the age of a received BPDU for a port and ensures that the BPDU is discarded when its age exceeds the value of the maximum age timer. 20 seconds Forward Delay Timer The amount time spent by a port in the learning and listening states waiting for a BPDU that may return the port...
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