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Troubleshooting Guide

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

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How Do I Set the Source MAC Address Field in STP BPDUs?

How Do I Set the Source MAC Address Field in STP BPDUs?

The STP bridge ID, path cost, and port ID are transmitted through configuration BPDUs. In most cases, BPDUs refer to configuration BPDUs. They are heartbeat packets. After STP is enabled on a port, configuration BPDUs are sent from the port at intervals specified by the hello timer.

A BPDU is encapsulated in an Ethernet frame, as shown in Figure 22-109. The destination MAC address has a fixed multicast MAC address of 01-80-C2-00-00-00. The length field specifies the MAC data length, and is followed by the LLC header. The BPDU header follows the LLC field.

Figure 22-109 Format of an Ethernet frame

Generally, the source MAC address is the bridge MAC address for sending configuration BPDUs, which is calculated based on the STP topology. For the M-LAG configured through V-STP, V-STP virtualizes the M-LAG master and backup devices enabled with STP into one device to perform STP calculation. After M-LAG master/backup negotiation is successful, the backup device uses the bridge MAC address and instance priority that are synchronized from the master device for STP calculation and packet transmission. This ensures STP parameter calculation on the virtualized device.

Figure 22-110 shows the basic M-LAG topology. Assume that SwitchA is the master device and SwitchB is the backup device. In the BPDUs that SwitchB sends to SwitchC and SwitchD, the source MAC address is the bridge MAC address synchronized from SwitchA but not the bridge MAC address obtained through STP calculation.

Figure 22-110 Basic M-LAG topology
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Updated: 2020-01-07

Document ID: EDOC1000060766

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