The Topology Change (TC) detection and propagation mechanisms used in RSTP are different from those used in traditional STP. It is important to understand the differences between these two methods of TC detection and propagation.
IEEE 802.1D Topology Changes
Although described in detail in the previous chapter, this section summarizes the traditional Spanning Tree Protocol TC process. In traditional STP, when a port moves to the Forwarding or Blocking states, the switch originates a TCN BPDU. This TCN BPDU is sent out of the Root Port toward the Root Bridge.
The TCN BPDU is relayed until it reaches the Root Bridge. The Root Bridge then sets the TC flag in BPDUs that are then propagated to the rest of the STP domain, causing the MAC tables of domain switches to age out in 15 seconds.
IEEE 802.1w Topology Changes
In RSTP, a TC is initiated only when a non-edge port transitions to the Forwarding state. This differs from 802.1D where the transition of edge ports into the Forwarding state resulted in the origination and generation of BPDUs throughout the STP domain, even though the transition of such ports had no direct consequence to the STP domain as a whole.
Another significant difference between 802.1D and 802.1w is that RSTP no longer uses the specific TCN or TCA BPDUs, unless a legacy bridge needs to be notified. Interoperability between traditional STP and RSTP will be described later in this chapter. Instead, RSTP simply sends out BPDUs that have the TC bit set.
In RSTP, BPDUs with the TC bit set are sent out by the initiator and not just by the Root Bridge any more. The BPDUs are then propagated throughout the active topology by all neighboring switches. Figure 4-13 below shows an RSTP BPDU with the TC bit set:
Fig. 4-13. RSTP BPDU with the TC Bit Set
When a switch running RSTP detects a topology change, it immediately sends BPDUs out of all non-edge ports with the TC bit set and begins the TC While timer, which is set to 2 times the Hello Time interval (4 seconds). The switch will continue to send these BPDUs out of these ports for the duration of the TC While timer. This is because no ACK is used in RSTP, as would be the case in traditional STP. In other words, RSTP BPDUs never have the TCA bit set. To prevent loops, the switch also flushes out MAC addresses that are known out of the ports on which the BPDUs are being sent.
When another switch running RSTP receives a BPDU with the TC bit set, it clears the MAC addresses learned on all its ports, except the one that receives the topology change. In addition, the switch also starts the TC While timer and sends BPDUs with TC set on all its designated ports and Root Port. This is continued throughout the entire RSTP domain.