The time displayed by the Time since last TC received field in the dis stp command output varies on different devices. NE40E-1 is the primary root, and NE40E-2 is a secondary root. The time displayed on the two devices varies by about 34 seconds. The customer is worried that this may cause service malfunctions.
Discover that the Eth-Trunk ports connecting the two NE40E devices operate in the STP mode. When the root bridge receives TC packets, it sends a TC BPDU to each non-root bridge during every hello time until the max age plus the forward delay elapses.
In the MSTP mode, after the root bridge receives TC packets, it starts a hello time twice the max age for sending TC BPDUs. Therefore, if the Eth-Trunk ports operate in the MSTP mode, the Time since last TC received displayed on the two NE40 routers will be close.
After receiving TC packets, the root bridge sends a TC BPDU to each non-root bridge during every hello time until the max age plus the forward delay elapses. In the current network, the hello time is 2 s, the max age is 20 s, and the forward delay is 15 s. Therefore, after the root bridge receives TC packets, it sends a TC BPDU every 2 seconds until 35 seconds (20 s + 15 s) later.
Why is port Eth-Trunk 1 in the STP mode?
The reason is that when a port on an upstream switch running MSTP connects a downstream switch running STP, the port automatically migrates to the STP-compatible mode. However, after the downstream switch running STP is replaced by a switch running MSTP, the port cannot automatically migrate back to the MSTP mode. By then, you can perform a mCheck operation to force the port to migrate to the MSTP mode.