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I have a question about the RSTP ethernet swtiches, I have 40 RSTP ethernet switches, they connect with the line topology and root birdge is 23th device on the line. I am using just 23 device for this test:

HelloTime = 2
Remaining Hops = 20
Forward Delay = 15
Max Age = 20

If I connect the topology like that 23th-22th-21th-....2th-PC I couldnt access 3th,4th,5th until 23th if I remove the 1 device on the network, so if I remove the 2th device, we have 20 device so I could access every device. When the topology have more than 20+ device until root bridge I couldnt access. So I connect 23th-22th-21th-...2th-PC again and changed to Max Age to 21 instead of 20, I could access evry device. Is there logical ? What is the real reason for ?

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  • Did any answer help you? if so, you should accept the answer so that the question does not keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer.
    – Ron Maupin ♦
    Commented Dec 23, 2021 at 19:36

2 Answers 2

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I don't quite understand the exact question here, but the reason for the remaining hops is to avoid changes within the network propagating on one side before they've reached to the other. STP BPDUs are handled in software and thus there might be an event which keeps the CPU busy and therefore struggling to re-converge fast enough when host are 20 hops away and potentially even creating short-term loops. IF you really need more than 20 hops you may eventually consider running MSTP, which (from memory) allows for switches to be 255 hops away.

I want to point hot however, that having a daisy chained topology of a layer2 ethernet segment spread across 20+ hops sounds to me like a recipe for disaster.

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  • Thank you for your comment, why the 40 device line topology disaster ? What is the reason for ?
    – mathco
    Commented Jun 10, 2021 at 4:56
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40 chained switches (or rather a ring?) won't ever work with RSTP/MSTP. The main question that springs up in my mind is What use is RSTP in a chain?

RSTP's design limit with default parameters is a depth of seven (maximum distance from root to any other switch), see here for details. With a deeper topology, the tree will likely never converge, keep spinning TCNs, and produce various wandering connectivity issues.

You can try reducing the Hello Time and increasing Max Age, for a slight increase in possible depth but you're basically on your own here...

Generally, you need to rethink your topology.

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  • Thank you for your comment, if I increase the MAx Age, Could I use 20+ device on my network ?
    – mathco
    Commented Jun 10, 2021 at 4:57
  • By the way, I am trying to line topology not ring
    – mathco
    Commented Jun 11, 2021 at 5:18
  • @mathco In a chained topology, STP serves no purpose, so you could simply deactivate it on the chaining ports.
    – Zac67 ♦
    Commented Jun 11, 2021 at 5:30
  • I need to keep it active against any situation in the network , If I only updated Max Age to 40, what happened on my system ?
    – mathco
    Commented Jun 11, 2021 at 5:33

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