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Eduard Florinescu
Asked: 2014-11-13 04:53:18 +0800 CST

Can a faulty switch connected to the core switch block an entire network?

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I encountered the following situation: The network I administer was blocked, and this was traced down to a specific rack, after I reseted both switches, everything worked.

I supposed it was the core switch so I replaced the core switch. It happened again and this time I reset the secondary switch which practically connects all the stations in that building to the network and it work. The topography simplified is like in the image below.

Can this happen, can a faulty secondary switch block an entire network, and how can I detect when there are similar problems and I have two cascaded switches, which one is the culprit?

enter image description here

networking

1 Answers

  1. Best Answer

    This can happen when you have an issue with Spanning Tree Protocol. Depending on the model of switch, the specifics of how to mitigate this may vary. Basically, when you have switches connected to each other, they dynamically figure out how to send datagrams through each other to reach remote segments. When the switches start up they send messages to each other the gist of which is "I can see you, you can see me, if you can see anyone else let me know" and then changes the state of the port where a switch is connected from "this is a place where I can reach an endpoint" to "this is a place where I can reach another switch." Part of this process is that one switch declares itself the "root bridge" - this is so that when the switches are voting on how to make connections, it can prevent looping (if it sees a packet come back to itself it knows there is a loop and will then prevent traffic through that port). If the secondary switch did that, then also confused the primary one this could result in what you are seeing. This can also happen if you have two connections between the two switches (doesn't look like it in your diagram, but worth checking). This can be done, you'd just need to account for it in your switch configuration.

    Anyway, basically STP is designed to Just Work, but in reality there are things that you might want to do to prevent this sort of thing from happening. I'd recommend taking a look at articles like these for more details on what you might need to do here to sort things out:

    http://www.enterprisenetworkingplanet.com/netsp/article.php/3580966/Networking-101-Understanding-Spanning-Tree.htm

    http://www.networkworld.com/article/2223757/cisco-subnet/9-common-spanning-tree-mistakes.html

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