STP Question

Unanswered Question
Jul 3rd, 2008

my knowlege is the Designated Port in STP is used for forwarding frame for a subnet. for example ports PC's are connected to a vlan on a switch are Designated because they are actively forwarding frames and they are edge ports. can some correct me if i am wrong. since root port can only receives hello from the root bridge, they cannot send out hellos right?

Also the Root port only receives hello from the root and once the switch updates the hello when a convergence happens, they send out the hello through the Designated port not the Root port. right?

I have this problem too.
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Jon Marshall Thu, 07/03/2008 - 09:57

Franco

Pretty much yes to what you said in your first paragraph. Designated ports are ports that forward traffic onto the local subnet or to put it another way forward traffic away from the root bridge.

Root ports receive BPDU's from the root bridge and forward them on via the designated port.

However there is a BPDU that is sent out on the root port and this is when there is a topology change within the network. The switch that has the topology change sends a TCN (Topology Change Notification) out of it's root port and this is forwarded out of each switches root port until it reaches the root bridge.

The root bridge then sends out BPDU with the TC (Topology Change) bit set and as you say these are received on the root port and forwarded out the designated port.

Jon

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