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When we learn the STP or RSTP,we all learn the election of root bridge, the lowest priority number or MAC address of switch will become the root bridge.So here is my first question:

1.So what can the root bridge do for the ethernet,what is its main function?i know it is important for the network,and all introductions of STP will teach us how to find it,but they don't why do we need to choose the root bridge from all switches?

There is a thing which is familiar with root bridge in the rapid super ring,which is invented by Korenix,that is ,ring-master,but the election of it is opposited to root bridge,the " highest " MAC address of switch can be the ring master,so here is my second question :

2.The same as the root bridge,korenix also tells us how to find the ring master,but they don't tell us why do we need it,what is the main function of ring master?what can the ring master do for the network?

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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
    Dec 23 '21 at 22:37
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The root bridge is the switch that 'anchors' the spanning tree: simplified, all switches evaluate their potential paths towards the root bridge and only the 'best' path is kept active, the other links are blocked. The root port of a switch is chosen by its path cost to the root bridge (lower is better), or the lowest priority value, if configured.

If that root bridge is a random switch somewhere on the edge of your network, the spanning tree may be created so that your core/most important switches cannot talk to each other directly but need to use some other switch(es) in between. That may significantly decrease overall performance and efficiency. Also, if that random root switch goes offline and online again, your spanning tree will need to reconverge each time.

As I've already tried to point out in other answers, Korenix's "Rapid Super Ring" and such are based on standard protocols like RSTP and ERPS. Korenix doesn't really document how exactly their stuff works (apart from the standards they use), so you'd have to ask them.

And as I've also pointed out, Ethernet works best and most effienctly in a tree topology - unless you can use a real meshing protocol like Shortest Path Bridging or perhaps TRILL. In large networks where this really makes sense you should seriously consider a routed instead of a switched approach though - routed networks can support meshing with load balancing and multiple failover much more easily than switched (Ethernet) ones.

Proprietary protocols (or variants) also inherently lock you in to a vendor which might not be too desirable.

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  • so can i say that because the STP has the root bridge,so all switches can have the shortest transmitted path to each other switches? Oct 3 '21 at 11:32
  • @user16266657 That's no exactly how this works. The best path is selected for higher bandwidth and lower number of hops towards the root bridge. All traffic then flows along this spanning tree.
    – Zac67
    Oct 3 '21 at 11:43
  • why would this ring thingy work like STP. It is a ring. Packets can go either clockwise or counterclockwise. If one link fails, ring (may need) to change direction. There is no need for spanning tree here.
    – Effie
    Oct 3 '21 at 11:46
  • Depending on what they really do and what you've set up (which we don't know), STP blocks all but the ports belonging to the spanning tree - no matter the physical topology. Again, don't ask us what Korenix do as they don't document that.
    – Zac67
    Oct 3 '21 at 13:31
  • i guess that depends on what part of RSTP the phrase "are based on standard protocols like RSTP" belongs to
    – Effie
    Oct 4 '21 at 11:11
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1.So what can the root bridge do for the ethernet,what is its main function?i know it is important for the network,and all introductions of STP will teach us how to find it,but they don't why do we need to choose the root bridge from all switches?

A spanning tree has to have a root. Here both terms are like in graphs. A root is a tree node which has no parent node. STP selects this root node as a node with highest bridge id and then constructs shortest path spanning tree rooted at this node. This way one can say that root bridge has a somewhat special role in the topology.

There is no other special feature of the root bridge.

2.The same as the root bridge,korenix also tells us how to find the ring master,but they don't tell us why do we need it,what is the main function of ring master?what can the ring master do for the network?

I can't find any explanation on how redundancy works, but from what i read here ring master has a special role, in that it monitors paths, and presumably decides which direction on the ring the traffic is flowing

Note about root node position

What (R)STP effectively does is to prune a topology (which is a graph) to a tree topology. Nodes can only communicate with each other using tree links. Each path can go either down the tree or first up then down the tree. All other paths in original graph are prunned and not available. For example, a node with highest betweenness centrality could be a good candidate for the root.

Also, in tree routing, nodes higher in the tree are likely to see more traffic. And all paths can potentially go up to the root of the tree. So, you also probably want the root to be more "capable" bridge in the topology. See this question.

Thus position of root note is important with respect to what the resulting calculated paths are and how effective (in comparison to paths in original topology (graph)).

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  • but if root bridge has no any special feature,why should we have the election of it? we can choose any switch as the root bridge without any reason. Oct 3 '21 at 11:25
  • you can just choose any without any reason. STP/RSTP will work. In practice, you probably want the root to be kinda in the middle (again in the graph sense) of the network, so that the resulting tree has efficient paths.
    – Effie
    Oct 3 '21 at 11:28
  • note, that STP/RSTP effectively prunes network topology to a tree. Nodes that are connected by a non-tree link cannot communicate over this link. They will go up and down the tree instead. If the root is not in a good position, this will result in pretty ineffective paths.
    – Effie
    Oct 3 '21 at 11:30
  • so can i say that because the STP/RSTP has the root bridge,so all switches can have the shortest transmitted path to each other switches? Oct 3 '21 at 11:33
  • updated. well, i would go with "no". all switches will have shortest paths in the tree. but these shortest paths in the tree can be much worse that the paths that were available in the original topology. all non-tree links will not be used for traffic, thus many shortest paths cannot be used.
    – Effie
    Oct 3 '21 at 11:39

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