Configuration BPDUs are sent every 2 seconds from the Root bridge to the other bridges in the network, by default. The above message indicates that a configuration BPDU was not received by a non-designated port on a switch for 20 seconds (assuming a default value of max_age) and the port becomes a designated port to restore connectivity on that segment. [dec]/[chars] designates the port and vlan [dec] is the VLAN number.
The error message means switches are losing BPDUs somewhere along the path from the root switch. Basically, a port that expects BPDUs waits for a max age before the port considers the designated bridge to be lost. If the root bridge is too far away from some bridges of the network, this issue can occur. This issue affects convergence of the spanning tree.
The max age timer controls the maximum length of time a bridge port saves its configuration BPDU information. This is 20 seconds by default and can be tuned to be between six and 40 seconds. The max age takes into account that the switch at the periphery of the network should not time out the root information under stable condition (that is, if the root is still alive). There is an additional time related parameter that is contained in each configuration BPDU, known as the message age. The message age is not a fixed value. It contains the length of time since the BPDU was first originated by the root bridge. The root bridge will send all of its BPDUs with a message age value of zero, and all subsequent switches will add one to this value. When a new configuration BPDU is received that is equal or better than the recorded information on the port, all of the BPDU information is stored, and the age timer begins to run, starting at the message age received in that configuration BPDU. If this age timer is reaching max age before another BPDU is received to refresh the timer, the information is aged out for that port. When a BPDU propagates from the root bridge toward the leaves of the tree, the age field increments each time the BPDU goes though a bridge. Eventually, the bridge discards the BPDU when the age field goes beyond maximum age.
Note:- An IEEE bridge is not concerned about its local configuration of the timers value. It will consider the value of the timers contained in the BPDU that it is receiving. Effectively, that means only a timer configured on the root bridge of the STP is important. So, even if it is not required to configure the same timer value in the entire network, it is at least mandatory to configure any timer changes on the root bridge and on the backup root bridge.
An occasional message of this type is acceptable; however, if multiple error messages occur within a short time for a specific VLAN, a fault is indicated. This message is per VLAN, hence multiple messages for different VLANs are normal.
Issue the show spantree statistics command on ports that you suspect and pay attention to the Max age expiry count to track the number of times the max age expired. Each time this event occur, the counter increments. When the value is not 0, it indicates that the designated bridge for this LAN is unstable or has a problem with the transmission of BPDUs.
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