In the STP circuit like bellow, will the Router2 select a root port? or just the Router1 and Router3 select root port?

enter image description here

in a tutorial it says "all the non-root bridge node will select a root-port, its principle is 'towards'(near) the root-bridge".

my understand is the Router2 is also the non-root bridge node. if it will choose a root port, its G0/0 and G0/1 all are 'towards' to the root bridge. then how to choose ?


Ignore the icon, this is Switch constitutionally.


3 Answers 3


Assuming you are using switches and not routers since router dont't use stp because there are built in mechanisms in each routing protocol to prevent Layer 3 topological loops and also a packet has a ttl value, which is decremented on each router hop so it cannot circulate endlessly. Anyhow

As your question is "Will there be a root port selection on Router2". The answer is no since a "Root port will be selected on a non root bridge which is closest to the root bridge in terms of cost". R2 is not closest to the root bridge in terms of cost there fore no root ports will be selected on R2. But root port will be on R1.

On R2 there will be two types of ports assigned. Designated port and alternate port. If R2 was also directly connected to the root bridge then there would be a root port but in this topology there is none.

On R2 the Gig0/0 port will be in blocking state and on R1 Gig0/0 will be in designated state.

enter image description here


Routers don't participate in STP. Switches / bridges do.

Assuming you're using switches in your diagram:

When there is a tie for the root port, ie. two ports have the same path cost to the root bridge, the port priorities are compared. By default, the port priority is the port number and the lower number is chosen - making Gig0/0 the root port on Switch2. You can change the port priorities on the console to make the switch prefer another port.


PVST+ is default for Cisco switches. With that said: On router 2, one interface will be blocked and the other a root port. The deciding factor will be the cost of the path to the root bridge from both interfaces on router 2. I don’t see cost in your topology, so I can not calculate it.

If you change those routers to switches in packet tracer you can see all this in action and you can experiment with settings to study this.

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