PortFast r-STP r-PVSTP A bridge spends 15 seconds in each of the Listening and Learning states by default. There are two types of BPDUs in the original STP specification[7]:63 (the Rapid Spanning Tree (RSTP) extension uses a specific RSTP BPDU): • Configuration BPDU (CBPDU), used for Spanning Tree computation • Topology Change Notification (TCN) BPDU, used to announce changes in the network topology BPDUs are exchanged regularly (every 2 seconds by default) and enable switches to keep track of network changes and to start and stop forwarding at ports as required. When a device is first attached to a switch port, it will not immediately start to forward data. It will instead go through a number of states while it processes BPDUs and determines the topology of the network. When a host is attached such as a computer, printer or server the port will always go into the forwarding state, albeit after a delay of about 30 seconds while it goes through the listening and learning states (see below). The time spent in the listening and learning states is determined by a value known as the forward delay (default 15 seconds and set by the root bridge). However, if instead another switch is connected, the port may remain in blocking mode if it is determined that it would cause a loop in the network. Topology Change Notification (TCN) BPDUs are used to inform other switches of port changes. TCNs are injected into the network by a non-root switch and propagated to the root. Upon receipt of the TCN, the root switch will set a Topology Change flag in its normal BPDUs. This flag is propagated to all other switches to instruct them to rapidly age out their forwarding table entries. STP switch port states: • Blocking - A port that would cause a switching loop if it were active. No user data is sent or received over a blocking port, but it may go into forwarding mode if the other links in use fail and the spanning tree algorithm determines the port may transition to the forwarding state. BPDU data is still received in blocking state. Prevents the use of looped paths. • Listening - The switch processes BPDUs and awaits possible new information that would cause it to return to the blocking state. It does not populate the MAC address table and it does not forward frames. • Learning - While the port does not yet forward frames it does learn source addresses from frames received and adds them to the filtering database (switching database). It populates the MAC address table, but does not forward frames. • Forwarding - A port receiving and sending data, normal operation. STP still monitors incoming BPDUs that would indicate it should return to the blocking state to prevent a loop. • Disabled - Not strictly part of STP, a network administrator can manually disable a port To prevent the delay when connecting hosts to a switch and during some topology changes, Rapid STP was developed, which allows a switch port to rapidly transition into the forwarding state during these situations. Understanding and Tuning Spanning Tree Protocol Timers http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html Spanning Tree Protocol Timers http://facweb.cs.depaul.edu/terbush/TDC511/STPTimersExplained.pdf |