This is the content of STP protocol. The spanning tree instance will select the root bridge. The position of the root bridge in the network topology determines how to calculate the role of the port. The root port exists on a non-root bridge and has the best path to the root bridge. The root port forwards traffic to the root bridge. The root port can populate the MAC table with the source MAC address of the received frame. A bridge can only have one root port. The specified port exists on root bridge and non-root bridge. All switch ports on the root bridge are designated ports. For non-root bridges, the designated port refers to the switch port that receives frames or forwards frames to the root bridge as needed. A network segment can only have one designated port. If there are multiple switches on the same network segment, the designated switch will be determined through the election process, and the corresponding switch port will start to forward frames for the network segment. The specified port can populate the MAC table. Unspecified ports are blocked switch ports, which do not forward data frames and do not populate the MAC address table with source addresses. The unspecified port is not the root port or the specified port. In some variants of STP, non-designated ports are called replacement ports. A disabled port is a switch port that is administratively closed. Disabled ports do not participate in the spanning tree process. Stp involves the following contents. Take a good look, these are the most basic STP elections. All kinds of elections are realized by exchanging BPDU messages, which are directly encapsulated in Ethernet frames. One root bridge per network (each network has only one root bridge) Root bridge election: lowest BID STP assigns a unique identifier to each SW, which is called BID (Bridge ID). Composition of BID: 2 (bridge priority) +6 (MAC) = 8 bytes Default priority: 32768 (0x8000) The VLAN number is added to the value of switches above 2950, and each switch has a reference MAC address. With the following command, you can see that each port of the SW2 # show version base Ethernet MAC address: 00: 0d: 28: 61:35: 00 switch has a MAC address, which is obtained by adding the port number to the basic MAC address. The command Show interface f0/ 1 allows you to view the MAC address and STP information of the switch port-SW2 # show spanning-tree SW3 # show spanning-tree brief (for the lower version) is used for switches below 2900. By default, each VLAN of Cisco SW has an STP, which does not affect each other. The priority of some low-end switches (2900) is not added with vlan number. Set the SW with the best performance as the root bridge SW 1 (configuration) # Spanning tree VLAN 1- 10 root main bridge (24576 = 0x6000) (establish priority, Become the root bridge) sw2 (configuration) # Spanning tree VLAN 1- 10 Root auxiliary bridge (28672 = 0x7000) (standby root bridge, to prevent priority root bridge showdown) Sw 1 (configuration) # Spanning tree VLAN1-/kloc. One root port for each non-root bridge (each non-root bridge has one root port) Root port (RP): The lowest path cost to the root bridge. Each non-root bridge has one and only one root port, and the non-root bridge has the lowest cost to reach the root bridge. (Forwardable traffic) Method of selecting RP/DP: 1. The lowest RID is the BID of SW 1 (root bridge) 2. Lowest path cost to root bridge 3. Lowest sender bid. 4. Minimum sender port ID When there are two lines directly connected between two switches, this item will be used to select BPDU (Bridge Protocol Data Unit): it will be sent by the root bridge every two seconds. BPDU is a protocol directly encapsulated in Layer 2, and the last encapsulation number of its MAC address is: 00. (0 1: 80: C2: 00: 00: 00) path cost: the cost value sent by the root bridge is 0, and the cost value is added at the entrance of the next switch, but the cost value at the exit remains unchanged. 10 Mbps: 100/ 100 Mbps: 19/ 1 Gbps:4/ 10 Gbps:2sw 1 # show interfaces status SW 1 # Show spanning-tree duplex full-> P2P duplex half-duplex-> shared SW 1 (configuration -if) # Spanning tree (VLAN 1) cost 22 Each VLAN can generate its own spanning tree. By changing the cost value of each VLAN, each VLAN can choose a different root bridge and generate a different spanning tree. Make full use of the link SW1(config-if) # span-treecost 22 to modify the cost values of all Vlan. The port ID consists of priority+port number. Modify the port priority: the default value is 1 28sw1(config-if) # spanning tree port-priority 16. The method of selecting RP/DP for TCP/UDP encapsulation segment of each segment is1. The lowest RID is the bid of SW 1 (root bridge) 2. Minimum path cost to the root bridge 3. Minimum sender bid 4. All ports of the root bridge with the lowest sender port ID are designated ports (DP). Designated port (DP): the lowest path cost to the root bridge (=) The port with the lowest sender's BID BID smaller bid (forwarding traffic) is the non-designated port blocked (the non-designated port will be blocked) BPDU (bridge protocol data). Unit) For all SWs participating in STP, they get the information of other SWs in the network through the exchange of data messages, which is called BPDU. Function of BPDU: 1. Election root bridge 2. Determine the location of redundant path 3. Loop is avoided by blocking specific port 4. Notify the network of topology changes. Monitor the status of spanning tree. The root bridge sends BPDU every 2 seconds. In the initial network, each SW considers itself as the root bridge, and it will send BPDU. By comparing the lowest bids, a root bridge is selected, and only the root bridge sends BPDU. Non-root bridges only forward. TCN (Topology Change Notification) BPDU This BPDU is generated when the software detects a topology change. Protocol ID: 0 Version: 802. 1D (0) Message type: (configured BPDU=0x00/TCN BPDU=0x80) Root port (RP): Minimum path cost to the root bridge (=) Minimum sender bid (=) Minimum sender port ID entry+cost value = outgoing value (for example, 19 port +8 port =27 port refers to the switch) Port ID (PID): 1 (PRI) Lowest path cost to the root bridge lowest sender bid lowest sender port id < bpdu timer > bpdu multicast address: 01:80. Llo time: the interval between continuous transmission of BPDUs by the root bridge (2s) and forwarding time: the time when SW is in the listening and learning state (15s). Spanning tree VLAN1-10 hello-time3 was changed to the interval between transmission of BPDUs VLAN 65. 438+0- 10 forwarding time 13 Modify the forwarding time spanning tree VLAN 1- 10 Maximum age 33 Modify the maximum survival time 1. Maximum age of blockage (BPDU loss detected) = 20s±2. Listen and forward delay = 15s 3. Learning forwarding delay = 15s Listening to BPDU Learning MAC Frame Forwarding Blocking √××××××××× (Electing Root/RP/ DP) Learning √× Forwarding √ √ < STP Topology Change > SW will send TCN: 1. Link failure (forward-> BLK) 2。 The port enters the forwarding state, and SW already owns DP 3. The non-root bridge receives TCN from its DP and forwards it. TCN BPDU only contains three fields: pro id, version and message. That is to say, BPDU with Flags: TC (topology change) TC is set in the first three fields of BPDU, and only TCA (topology change confirmation) of Root: 1 will be generated. Convergence of current networks; 2. Aging time: direct change 2. Aging-time MAC address table to forwarding time < STP and 802. 1Q >. In 802. 1q trunking, SW maintains an STP for each VLAN allowed in the trunking. (PVST) For software that does not support 802. 1Q, all VLAN have an STP. (SSTP) In a switched network, STP is always running. If there is no relay in the link, STP only maintains the information of VLAN 1. Advantages of each Vlan spanning tree: 1. Load the root end based on VLAN; Disadvantages: 1 BPDU runs on the basis of Vlan; Finally, you can't see CCIE here. By 2009, the number of CCIE in China was only over 3 100, and there were over 20,000 in the world. These are experts, and CCIE's e means experts.