Root Ports

[_bmal]

I'm doing a lab on STP. The topology has 2 switches with a redundant up-link between them and 2 servers (one connected to each switch)

Some questions asked were:

What is the function of a root port? So far I've only managed to find that it's the port a switch chooses for its best path to the root switch.

Why can't there be more than one root port for a switch? I think it might be because there can only be one "best" path to the root switch

Why does one switch not have a root port? After issuing the show spanning-tree command on both switches, only one of them has a root port. Why is this?

[balaji.bandi]

here is the good description to understand how spanning tree root election and root port.

https://community.cisco.com/t5/switching/stp-root-port-vs-designated-port/m-p/1517842

[Deepak Kumar · Accepted solution]

Hi,

What is the function of a root port? So far I've only managed to find that it's the port a switch chooses for its best path to the root switch.

Ans: Root Port is a single selected port on a Switch, other than Root Switch, with the least Path Cost to reach the Root Bridge. 

Why can't there be more than one root port for a switch? I think it might be because there can only be one "best" path to the root switch

Ans: If there will two root port then it will root cause of the Layer 2 loop in the network.

Why does one switch not have a root port? After issuing the show spanning-tree command on both switches, only one of them has a root port. Why is this?

Ans: All the ports on a Root Bridge (Root Switch) are Designated Port and there is no Root Port on a Root Bridge (Root Switch). So one switch is not showing any root port.

[paul driver · Accepted solution]

Hello

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@_bmal wrote:

I'm doing a lab on STP. The topology has 2 switches with a redundant up-link between them and 2 servers (one connected to each switch)

Some questions asked were:

What is the function of a root port? So far I've only managed to find that it's the port a switch chooses for its best path to the root switch.

correct 

Why can't there be more than one root port for a switch? I think it might be because there can only be one "best" path to the root switch

It can have root ports all depends  if a switch is acting as the stp root for all vlans or certain vlans and of course what stp mode is running (pvst) as an example 

Why does one switch not have a root port? After issuing the show spanning-tree command on both switches, only one of them has a root port. Why is this?

thats because in your case that switch is the stp root for all vlans but in certain environments a stp root switch can be also be served an stp root so in that case it will also have root ports towards a stp root for the vlans its not a stp root

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