生成樹學(xué)習(xí)（Spanning tree learning）

1、生成樹學(xué)習(xí)（Spanning tree learning）A,1, enterprises are becoming more and more dependent on the internet. The Internet is the life of many companies. Network outages can cause disastrous losses on business, revenue, and customer trust.A network link, failure of a critical port on a device or switch can ca

2、use network disruption. Therefore, redundant functions must be taken into consideration when designing a network so as to maintain high network availability and eliminate any single point of failure. Redundancy devices can be implemented by installing standby devices and network links in critical ar

3、eas. Sometimes its very expensive to set redundant functions to all links and devices in the network. Network engineers often need to balance the cost of redundancy functions and the requirements for network availability.2, redundancy means two different paths to reach the destination. For example,

4、in real life, there are two roads to a city. There are two bridges to cross the river, or two doors to go out of the room. When one of these paths is blocked, one can choose another. The way to provide redundancy is to connect the switch to multiple links. Redundant links in an exchange network can

5、reduce congestion and provide high availability and load balancing. However, linking switches together can also cause problems. For example, the broadcast characteristics of Ethernet traffic result in switched loops. Broadcast frames are sent continuously in all directions, leading to broadcast stor

6、ms. The broadcast storm will deplete all available bandwidth, resulting in no network connection and disconnection of existing network connections.3, broadcast storm is not the only problem caused by the redundant link in the switching network. Unicast frames can sometimes cause problems, such as mu

7、ltiple frames of transmission and instability of MAC databases.Multiple frame transmission source host to the destination host sends a unicast frame after frame, if the destination MAC address in any connected switch in the MAC table does not exist, then each switch will flood the frame from all por

8、ts. In the network that exists in the loop, the frame may be sent back to the original switch. This process is repeated, resulting in multiple copies of the frame in the network. The end destination host receives many copies of the frame.This situation can cause three problems: bandwidth waste, wast

9、e of CPU time, and repeated transaction flow.The MAC database is unstableSwitches in redundant networks may get erroneous information about the location of the host. When there is a loop, a switch may associate the destination MAC address with two different ports. The result is a frame forwarding er

10、ror or is not forwarded in the best possible way.Two,1, spanning tree protocol (STP) is a mechanism used to disable redundant links in an exchange network. STP provides the redundancy necessary for network stability and reliability, but does not result in switched loops.STP is an open standard proto

11、col that enables the creation of acyclic logical topologies in an exchange environment.STP is relatively perfect and requires little configuration. When the STP switch is first powered on, it checks whether the switched network has loops. If a potential loop is detected, the switch blocks some conne

12、ction ports and retains some other ports to forward the frames.2, STP generates a tree for all switches in the extended star switching network. The switches will constantly check the network to make sure there are no loops and all ports can run as required.STP prevents the switched loop by:Force a p

13、articular interface into a standby or blocked stateKeep other interfaces in forwarding stateWhen the existing forwarding path fails, the network is reconfigured by enabling an appropriate standby pathIn the vocabulary related to STP, the term bridge is generally used to refer to switches. For exampl

14、e, root bridge refers to the main switch or focus in the STP topology. The root bridge uses bridge protocol data unit (BPDU) to communicate with other devices. BPDU is a frame that is multicast to all other switches every 2 seconds. The information contained within BPDU includes:Identity identifier

15、for source switchIdentity identification of the source portSource port overheadValue of aging timerHello timer value3, switch on the switch power supply, each port of the switch will go through four states: blocking, listening, learning and forwarding. The fifth state disable means that the administ

16、rator has closed the switch port.When the switch passes through these States, the LED on the switch will change from flashing orange to stable green. Port through all of these states into the forwarding ready state takes about 50 seconds.When the switch opens, it first enters the blocking state, so

17、that the loop is prevented for the first time. Then change to the listening mode and receive the BPDU from the neighbor switch. When this information is processed, the switch determines which ports can forward frames and do not produce loops.The ports that can forward frames are changed to the learn

18、ing mode, and then into the forwarding mode.The access port does not cause loops in the switching network, and such ends are when connected to hostsThree,1, to run STP, the switches in the network need to determine which switch is the focus of the network. STP uses this focus (also known as root bri

19、dge or root switch) to determine which ports are entering a blocking state and which ports are in the forwarding state. The root bridge sends BPDU to all other switches that contain network topology information. Thus, in the event of a failure, the network can be reconfigured according to this infor

20、mation.There is only one root bridge in each network, which is elected according to the bridge ID (BID). The bridge priority value plus the MAC address makes up the BID.The default priority of the bridge is 32768. If the MAC address of a switch is AA-11-BB-22-CC-33, then the BID of the switch is: 32

21、768: AA-11-BB-22-CC-33. The port will always enter the forwarding state. A relay port may cause a network loop to move forward or blocking.2, the root bridge is determined by the lowest BID value. Since switches usually use the same default priority value, the lowest MAC address switches become root

22、 bridges.When each switch is switched on, it assumes that it is a root bridge and sends BPDU that contains its own BID. For example, if the root ID of the S2 notification is lower than S1, then S1 stops telling its root ID and accepts the root S2 of ID. S2 then becomes the root bridge.STP divides po

23、rts into three types: root ports, designated ports, and blocked ports.Root portThe port that provides the minimum overhead path to the root bridge is the root port. The switch calculates the minimum overhead path by using the bandwidth overhead of each link required to reach the root bridge.Specifie

24、d portThe specified port forwards traffic to the root bridge, but the path to which it is connected is not the least overhead path.Blocked portBlocked ports do not forward traffic.3, before configuring the STP, network technicians need to plan and evaluate the network and select the switches that be

25、st fit the root of the spanning tree. If the MAC addresss lowest switch is used as the root switch, it may not be able to achieve the best forward effect.The switches located at the center are best suited as root bridges. If the root switch is not in the central location, then the blocking port outs

26、ide the network may cause traffic to reach the destination through a longer routing path.To specify the root bridge, you need to configure the lowest priority value for the BID of the selected switch. The bridge priority command can be used to configure its priority. The priority range is 0 to 65535

27、, but the value is incremented by 4096 increments. The default value is 32768.The priority setting command is:S3 (config) #bridge priority 4096The command that restores the priority to the default value is:S3 (config) #no bridge priorityFour1, after determining the root bridge, root port, designated

28、 port, and blocking port, STP sends the BPDU to the switched network every 2 seconds. STP listens to these BPDU all the time to make sure there are no link failures or new loops appear.If a link fault occurs, the STP will be recalculated by the following:Change some blocking ports to forwarding port

29、sChange some forwarding ports to blocked portsA new STP tree is formed to keep the network running in a acyclic stateSTP is not immediately effective. When the link is disconnected, the STP detects the fault and then recalculate the best path in the network. This calculation and conversion process t

30、akes about 30 to 50 seconds of each switch. During calculation, the port that is performing recalculation does not transmit user data.Some user applications also expire during recalculation, which can lead to productivity degradation and loss of revenue. Frequent STP re calculation will affect the n

31、ormal operation of the network.2 、 suppose the switch port connects to a high capacity enterprise server. If the port is recalculated due to STP, the server will stop service for 50 seconds. Business losses resulting from this will be difficult to estimate.In a stable network, there is very little S

32、TP recalculation. In an unstable network, it is important to check the stability and configuration changes of the switches. A common cause of frequent STP recalculation is the power or power supply problem of the switch. A problematic power source will cause the device to restart unexpectedly.3, by

33、implementing some enhancements to STP, we can minimize the downtime caused by STP recalculation.PortFastThe STP PortFast enables the access port to immediately enter the forwarding state, bypassing listening and learning status.If PortFast is used for access ports connected to a single workstation o

34、r server, these devices can be immediately connected to the network without waiting for STP to converge.4, there are some useful commands that can be used to test the operation of a spanning tree.Show ID, Spanning-tree display the root bridge ID and port stateShow spanning-tree Summary display port

35、state informationShow spanning-tree root - shows the status and configuration of the root bridgeShow spanning-tree Detail display detailed information portShow spanning-tree Interface display STP interface status and configurationShow spanning-tree display Blockedports blocked ports5, when IEEE makes the original 802.1D spanning tree protocol (STP), the recovery time between 1 and 2 minutes is considered acceptable. Now, third - layer switching and advanced routing protocols