STP - RSTP BPDU Flags and their usage
In the Rapid Spanning Tree Protocol (RSTP) version of STP, the flag fields found within the BPDUs have specific roles. Unlike the conventional 802.1D BPDUs, which use only bits 0 and 7 of the 1-byte flags field (with bits 2 to 6 unused), RSTP redefines these bits. In 802.1D, bit 0 is the Topology Change (TC) flag, and bit 7 is the Topology Change Acknowledgment (TCA) flag. However, in RSTP, bit 0 is used for TCA, and bit 7 is used for TC.
The TCA flag in RSTP acknowledges the receipt of a BPDU with the TC flag set. This mechanism helps prevent unnecessary flooding of BPDUs across the network. Although the TCA flag might not frequently be set to 1 when using RSTP, it still plays a crucial role in maintaining network stability by acknowledging topology changes.
Links:
https://forum.networklessons.com/t/rapid-spanning-tree-rstp/1138/162?u=lagapidis
https://networklessons.com/spanning-tree/rapid-spanning-tree-rstp