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Understanding STP

Latest reply: Aug 25, 2021 07:16:36 401 3 3 0 0


To  prevent loops from causing broadcast storms and making the MAC address  table unstable, the Spanning Tree Protocol (STP) runs switches when  redundant links are used on the network.

STP is a basic feature of  datacom products including switches, routers, and WLAN products. This  chapter describes STP configurations on Huawei CloudEngine series  switches.

Understanding STP

STP,  defined by IEEE 802.1D, prevents loops on a local area network (LAN).  Switching devices running STP exchange information with one another to  discover loops on the network, and then block certain redundant links to  eliminate loops. If an active link fails, STP activates a running  standby link with the highest priority to ensure network connectivity.

As the scale of LANs continues to grow, STP has become an increasingly important LAN protocol.

Root Bridge, Root Port, and Designated Port

Three elements are involved in trimming a ring network into a tree network: root bridge, root port, and designated port.

  • Root  bridge: There is only one root bridge on the entire STP network. The  root bridge is the logical center, but not necessarily the physical  center, of the network. Typically, the root bridge should have high  performance and be located at a higher layer of the network. When the  network topology changes, the root bridge will be re-elected  accordingly. It is recommended that you manually configure the root  bridge and secondary root bridge. Configure an optimal core switch as  the root bridge to ensure stability of the STP Layer 2 network.  Otherwise, new access devices may trigger STP root bridge switching,  causing short service interruptions.

  • Root  port: The root port on an STP device has the smallest path cost to the  root bridge and is responsible for forwarding data to the root bridge.  Among all STP-capable ports on a device, the port with the smallest root  path cost is a root port. An STP device has only one root port, and  there is no root port on the root bridge.

  • Designated  port: The designated port is located on a designated bridge and  forwards configuration BPDUs to the device or LAN, as illustrated in Figure 1.

    AP1 and AP2 are ports of S1; BP1 and BP2 are ports of S2; CP1 and CP2 are ports of S3.

    • S1 sends configuration BPDUs to S2 through AP1, so S1 is the designated bridge for S2, and AP1 is the designated port on S1.

    • S2  and S3 are connected to the LAN. If S2 forwards configuration BPDUs to  the LAN, S2 is the designated bridge for the LAN, and BP2 is the  designated port on S2.Figure 1-1  Designated bridge and designated port

In addition, a switching  device has a bridge ID (BID) and a port on the switching device has a  port ID (PID). A PID is composed of a port priority and a port number. A  BID is composed of a bridge priority and a bridge MAC address. On an  STP network, the device with the smallest BID acts as the root bridge.

After the root bridge,  root port, and designated ports are selected successfully, a tree  topology is set up on the entire network. When the topology is stable,  only the root port and designated ports forward traffic. The other ports  are in Blocking state; they only receive STP BPDUs and do not forward  user traffic.

BPDU Format

A  BPDU carries the BID, root path cost, and PID. A BPDU is encapsulated  in an Ethernet frame. Its destination MAC address is a multicast MAC  address, 01-80-C2-00-00-00. The Length or Type field specifies the MAC  data length, and is followed by the LLC header and BPDU header. Figure 2 shows the format of an Ethernet frame.

Figure 1-2  Format of an Ethernet frame

Each bridge actively sends  configuration BPDUs during initialization. After the network topology  becomes stable, only the root bridge proactively sends configuration  BPDUs. Other bridges send configuration BPDUs only after receiving  configuration BPDUs from upstream devices. A configuration BPDU is at  least 35 bytes long and includes the parameters such as the BID, root  path cost, and PID. A bridge processes a received configuration BPDU  only if either the sender BID or PID is different from that on the local  bridge receive port. If both fields are the same as those on the  receive port, the bridge discards the configuration BPDU. Therefore, the  bridge does not need to process BPDUs with the same information as the  local port.

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MVE Created Aug 14, 2021 13:44:26

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MVE Author Created Aug 16, 2021 12:11:19

Thanks for sharing.
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Created Aug 25, 2021 07:16:36

Good post. Keep up the good work!
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