Topology change notifications in STP 9802.1d)?

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Jun 24th, 2009
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Please follow the above link and you will find a book that explains the three reasons why a STP switch will issue a TCN:

• A port moves from a forwarding state to a blocking state on the same switch (it probably lost the designated port or the root port)

• A non-root switch receives an update notification on a port from a downstream neighboring switch.

• A switch is forwarding traffic to a segment and another port moves into a forwarding state

How can you explain the third condition? What exactly does this mean?

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davy.timmermans Wed, 06/24/2009 - 10:06
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it looks like an addition to the first reason. I would search for a better book where it's better explained.

For example a link state (up->down or down-->up) of a user port will cause also the sending of a TCN ( if portfast is NOT configured on the user port). This is off course not a desired situation because each link state change of a user (dis)connecting to the switch will cause the fast aging system (15s instead of 300s) to start.

Each TCN will activate the fast aging timer

Francois Tallet Wed, 06/24/2009 - 10:21
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That's an optimization built in STP. If a switch comes up and only has a port forwarding, it is not bridging anything: you need to have two ports forwarding to do anything useful;-) As a result, no TCN is generated when the first port of an STP bridge goes forwarding.

Note that the topology change behavior has change with RSTP and MST. With those, a topology change is only generated on ports going forwarding (not ports going to blocking).



cristip Wed, 06/24/2009 - 13:24
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Hi Francois

You are speaking about the exception but I am more interested about the rule here.

The book says that a TCN is generated when a port goes in forwarding but it doesn't say if the port is on the same switch and connected to the upstream VLAN where the existing port is forwarding traffic to or it is connected to a downstream VLAN (that means a new switch was added there)

Francois Tallet Wed, 06/24/2009 - 13:42
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I'm just commenting on the third rule:

A switch is forwarding traffic to a segment [i.e. it has at least a forwarding port] and another port moves into a forwarding state.

Now, if you want to do some nit-picking, the sentence is not really exact in the way that it should specify that we're talking about a non-root bridge. The TCN is a special BPDU that is only sent on root ports. If you're the root bridge, you don't send a TCN, you just set the TC bit in the regular BPDUs you send out;-)

Here is a link to an paper I wrote a long while ago. Please, have a look to the whole process and let me know if this is clear with that. I am confused by your upstream/downstream terminology.




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