Why do I need to set an STP cost value after STP is enabled on an Eth-Trunk


After STP is enabled on an Eth-Trunk, STP cost values change when member interfaces on the Eth-Trunk change. If the Eth-Trunk is the root interface on the STP, the STP protocol will recalculate the topology of the network connected to the Eth-Trunk. If some interfaces on the network connect to terminals such as PCs and are not configured as edge interfaces, services will be interrupted for 30 seconds.

Other related questions:
Whether to enable STP if an AR router is used as a user access device
If an AR is used as a user access device (the WAN interface of the router is connected to the Internet, and the LAN interface is connected to an intranet), you are advised to disable STP to avoid network flapping due to STP convergence.

Do I need to enable STP after an Eth-Trunk is configured on S series switches
You do not need to configure STP after an Eth-Trunk is configured on S series switches (except the S1700).

How do I configure MSTP and VRRP on an S series switch
An S series switch (except the S1700) connected to user terminals cannot forward VRRP packets (because discarding unknown multicast packets is configured, for example), or the links that VRRP packets (heartbeat packets) pass through are faulty or unstable. To address such problems, configure a heartbeat line between two switches to transmit VRRP packets. Loops may occur between the two switches after the heartbeat line is configured. Therefore, you need to configure a loop prevention protocol such as STP or MSTP to prevent loops. For details on how to configure VRRP and MSTP, see topic Example for Configuring MSTP and VRRP in section "Typical Examples of MSTP/RRPP/SEP/VBST" at S1720&S2700&S3700&S5700&S6700&S7700&S9700 Typical Configuration Examples. The configuration method for the S9300 is the same as that for other switch models. When configuring VRRP on a switch separately, STP does not need to be disabled.

How to prevent network flapping caused by changes of Eth-Trunk member interfaces on CE switches
The path cost of Eth-Trunks change when Eth-Trunk member interfaces go Up or Down. In this case, STP recalculates the path cost and spanning tree, causing network topology changes and network flapping. You can run the stp [ process process-id ] [ instance instance-id ] cost cost command in the Eth-Trunk interface view to set the path cost of Eth-Trunks to a fixed value. In this way, the changes of Eth-Trunk member interfaces do not affect the path cost, preventing network topology changes. For example, set the path cost of Eth-Trunk 1 to 200. system-view [~HUAWEI] interface eth-trunk 1 [*HUAWEI-Eth-Trunk1] stp cost 200 [*HUAWEI-Eth-Trunk1] commit

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