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The Time since last TC received Field in the dis stp Command Output Shows Different Time Values

Publication Date:  2012-07-27 Views:  40 Downloads:  0
Issue Description
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. 
Alarm Information
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Handling Process
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. 
 
Root Cause
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. 
 
Suggestions
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