OSI model second Layer Data Link Layer-STP protocol

The origin of the 1.STP protocol.

In the two layer network, the switch plays an important role, if there is a switch failure will affect the use of the network, in order to avoid the existence of a single point of failure, in the actual two-layer link will be used in the link redundancy, that is, the use of a number of connections between switching devices, that is, multiple switches are connected, This allows other switches to be used, even if one switch fails. This is link redundancy.

Link redundancy will------bring loops.

1.1 The loop affects a----broadcast storm.

Host A sends an ARP message, and when switch B receives it, it sends a copy of the remaining ports except the receiving port. B-->a-->c--->b cycle (outer ring)

Switch a receives a message from switch C and sends the Exchange Price B. B-->c-->a--b (inner ring)

Since the exchange price does not have a TTL value, these messages are always circulating like this, occupying the switch resources.

and Host B will receive 2 messages. Imagine you have a mobile phone shell of a single, come over two parts of the phone shell, or cash on delivery.

1.2 Loop affects two---mac address oscillations.

, host A sends an ARP message. Switch B is the first to learn the MAC address of host A on g0-0-3. Also the Switch B-bar data frame is sent to a--->c---->b and learns from the G0-0-2 port to the same MAC address.

After the switch to learn the MAC address is quasi, (because the office with a laptop, the direct bar to unplug, to another office plug, but also to the Internet. ）

The MAC address will be generated on two ports to jump. Causes the switch to get a MAC address that is unstable.

2. Spanning Tree calculation process

Election of 2.1 switches.

1. Select the bridge (the switch is from the Network Bridge evolved) ID small for the root switch, swa the smallest, selected as the root.

2. Each switch is considered to be a root switch when it is started, and then sends BPDU messages and exchange device-to-bridge IDs on and off the network.

3. Bridge ID Composition 32768 (priority). 00e0-fc16-ee43 (MAC address) where the priority can be modified, the value range is 0-65535 inside 4096*n that is 4,096 times times (n takes 0 and a positive integer)

2.2 Root port of the non-root switch is elected.

1. Select a small cost (the larger the bandwidth, the smaller the overhead), the closer the root switch is.

2. Neighbor Bridge ID is small.

3. Port ID (port number) (upstream ports) Select a small

2.3 Designated Port of the electoral segment

0. This network segment refers to the data link segment, such as the connection between switch B and C is the network segment.

1. Select a small cost (the larger the bandwidth, the smaller the overhead), the closer the root switch is.

2. The adjacent switch bridge ID is small.

3. Port ID (port number) (upstream ports) Select a small

The remaining switch ports are blocked.

3. Role of the switch port

Switch Port Status:

Transitions for each State

After booting from disabled to listenning,listenning state by default for 15 seconds, how to receive the BPDU is told to need to blocking, directly into the blocking state.

Otherwise enter the learning state, the default stay of 15 seconds, if need to block, directly into the blocking state, otherwise enter the forwarding state becomes normal use port.

If blocking is required, go directly to the blocking state.

The root switch sends a BPDU message every two seconds, informing the non-root switch state (whether it is still alive). Non-root switch 20s does not receive the root switch message, it is considered the root switch is dead, re-elect the root switch .

OSI model second Layer Data Link Layer-STP protocol