Switch Configuration Tutorial: Spanning Tree Protocol STP prevents loops

There was an article before, introduction if the enterprise only use one switch to the network, there will be a single point of failure, if the switch is broken, at this time the normal work between several modules, the client can not access the server, can not connect the Internet, can not access the printer, that is, a point of failure, The entire network is not functioning properly. So in a typical switched network, we all need to redundant the switch, but the introduction of redundancy will occur a problem, a very serious problem is the network loop. Then the network loop will bring the broadcast storm, the multiple repetition data frame, the MAC address table instability and other factors. Then you can see the network loop problem is really not small, the solution is to use the Spanning Tree protocol STP.

Spanning-tree Protocol: referred to as STP, the protocol can be applied to loop network, through a certain algorithm to achieve path redundancy, while the loop is trimmed to a loop-free tree network, so that packets in the loop network to avoid "immortality."

The fast Spanning Tree protocol RSTP (Rapid spanning tree Procotol) implemented by Ethernet switching is the optimized version of the spanning Trees Protocol, and its "fast" time delay in the root port and the specified port entering forwarding state is greatly shortened under some conditions, This reduces the time required to stabilize the network topology.

MSTP (multiple spanning tree procotol) is an abbreviation for the multiple spanning trees protocol, which is compatible with STP and RSTP.

Let's introduce the STP:STP is created to overcome the problem of transparent bridging in redundant networks, the aim is to avoid and eliminate loops in the network by negotiating a loop-free path to the root Network bridge, which can be achieved by determining where loops exist in the network and by dynamically blocking redundant links. In this way, it ensures that there is only one path to each destination, so no loops are ever generated.

A suboptimal interface in the loop is set in block state to break the loop. Note that only the interface is set to block state and is only a state, rather than actually shutting down the interface. This block interface can also be restored to the forwarding (forwarding) state once there are problems with other links in the network. As shown in the figure:

The basic idea: in this place each switch is called the Network Bridge, so we introduce the time, if talked about the network bridge, knew is refers the switch to be possible. Each switch at the start of the time that they are the root bridge, then to discuss the beginning of the competition who is the root, we define a game rules, between the bridge transfer special message BPDU (bridge procotol data Unit) Network Bridges Protocol data units, Contains enough information to do the following: from all bridges in the network, select one as the root Network Bridge, and then calculate the bridge to the root Network Bridge shortest path. And the root bridge is sent once every 2 seconds BPDU. The BPDU data frame contains a lot of content, as shown in the figure:

We only care about the Network Bridge ID (including two-byte priority and 6-byte MAC address) here.

The way to select the root bridge is to compare the bridge ID of each switch, which is the axle ID, in the form of: