How to Troubleshoot Blocking of Non-Loop Ports on an RSTP-enabled NIS Switch?

Keywords:

NIS,RSTP
1.Issue Description
There is a large single-ring network consisting of 27 NIS switches and one NBS switch. The topology is as follows:

SW1 — SW 2 — SW 3 to SW 27 — SW 28 — SW 1
The SW1 model is NBS3100-8GT2SFP. It is configured with an RSTP priority of 4096,serving as the root bridge.
At first, RSTP runs normally. The Alternate port of SW 15 is set to a blocking state to eliminate any loops, enabling normal communication among all switches.

However, after the link between SW 4 and SW 5 is disconnected, SW 5, SW 6, SW 7, and SW8 lose their connections to the ring network.

2. Device Model and Firmware

Device Type	Device Model	Firmware  Version
Reyee  switch	NIS3100-8GT4SFP-HP	Reyee OS 2.280.0.1709

3. Cause Analysis

• The Max Age configuration isincorrect.
4. Solution

After the link between SW 4 and SW 5 is disconnected, SW 1 to SW 4 and SW 9 to SW 28 function normally, but SW 5 to SW 8 are disconnected. It is preliminarily suspected that SW 9 is faulty. However, upon logging into SW 9 and checking the RSTP information, we find that port 10, which connects SW 9 to SW 8, is a designated port, and is in a blocking state.

It is suspected that the RSTP calculation diameter does not support 28 devices.Upon checking the switch configuration, we find that the Max Age value is set to 20 seconds.

The working mechanism of Max Age in RSTP is as follows:
Message Age indicates the total time required for a Bridge Protocol Data Unit (BPDU) totravel from the root bridge to the current switch, including transmission delays. When the BPDU is sent by the root bridge, the Message Age is 0. In practice,the Message Age increases by 1 each time the BPDU passes through a switch. If the Message Age exceeds the configured Max Age, the non-root bridge discards the BPDU.
After a non-root bridge receives a BPDU, it matches the Message Age in the BPDU with its Max Age. If the Message Age is smaller than or equal to the Max Age, the non-root bridge forwards the BPDU. However, if the Message Age exceeds the Max Age, the BPDU is aged out. In this case, the non-root bridge discards the BPDU, and concludes that the network diameter is too large, leading to a failure to connect to the root bridge.
Example:
Assume that a BPDU is generated on switch A, sent to switch B, and then forwarded from switch B to switch C. The Message Age increases by 1 second each time the BPDU passes from switch A to switch B and then from switch B to switch C. When the BPDU reaches switch C, the Message Age is 2 seconds.
In this case, the BPDU is sent from the root bridge SW 1 to SW 28, passing through 20 hops before reaching SW 9. On port 10 of SW 9 (the port connected to SW 8),packet obtaining results show that the Message Age is equal to the Max Agevalue, that is, 20.

When SW 8 receives the BPDU, the Message Age changes to 21, exceeding the Max Age of 20. As a result, SW 8 discards the BPDU, suggesting that it does not receive the BPDU sent by the root bridge.
Change the Max Age on the root bridge to the number of devices on the largest ringminus 1. In this case, since the total number of devices in the ring is 28, theMax Age is set to 27 (28 - 1 = 27). After the Max Age is changed, SW 5 to SW 8restore communication with other switches on the ring network.