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Troubleshooting Guide

CloudEngine 16800, 12800, 12800E, 8800, 7800, 6800, and 5800 Series Switches

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STP Root Bridge Is Preempted When a High-Priority Device Is Added During Network Migration

STP Root Bridge Is Preempted When a High-Priority Device Is Added During Network Migration

This section provides a troubleshooting case that the STP root bridge is preempted when a high-priority device is added during network migration.

Networking

New_core_01 and New_core_02 are two CE12800s, and Old_core_01 and Old_core_02 are two H3C devices. Before migration, the four devices are core devices (VRRP). The STP priority of New_core_01 and Old_core_01 is 0 (highest priority). During migration, old core devices and access device of core devices are moved and used as access devices to be connected to new core devices.

Problem Description

During migration, the connection between the server and NMS is terminated for a short period of time, and then is restored automatically.

Cause Analysis

  1. The STP priority of New_core_01 and Old_core_01 is 0 (highest priority) but Old_core_01 is the H3C device and its MAC address is smaller than that of the Huawei device, so the effective STP priority is higher than that of New_core_01.
  2. AS_15 is dual-homed to two old core devices. When one line is connected to the new core device and the other line is connected to the old core device, two STP root bridges (New_core_01 and Old_core_01) are connected.
  3. The interface connecting New_core_01 and AS_15 is enabled. As a result, New_core_01 and Old_core_01 preempt to be the root bridge, triggering STP convergence. Old_core_01 becomes the STP root bridge due to higher priority (smaller MAC address).
  4. When the second line of AS_15 is also connected to the new core device, the connection between New_core_01 and Old_core_01 is terminated. In this case, New_core_01 becomes the root bridge again, triggering STP convergence.
  5. During STP convergence, traffic forwarding fails because the corresponding interface is in blocking state.

Procedure

Adjust the STP priority of Old_core_01 so that Old_core_01 and New_core_01 do not preempt to be the root bridge and STP convergence is not triggered.

Suggestion

  1. During network migration, do not add the device with the same priority as or higher priority than the priority of the STP root bridge. Before adding such a device, change the priority.
  2. When new devices are connected and configurations of new devices are incomplete, keep the connected interfaces in shutdown state to prevent loops or STP convergence.

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Updated: 2020-01-07

Document ID: EDOC1000060766

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