STP root port

Hi! I have a little question about stp.

If you have a topology in which a switch is connected to root bridge directly and also on a another side trough several other switches and the link trough other switches is much faster (the cost to root bridge is smaller than the one on a port directly connected), what will be the root port on this switch? The one that is directly connected to root bridge or the one with smaller cost value? Thank you!

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Comments

danb83

I am studying STP / RSTP at the moment.

The lowest cost path always wins!

The physically shortest path is not neccesarily the best.

newman555

I also think that lowest cost wins, but I found somewhere that directly connected ports to root bridge become root ports, so that's why I was uncertain...

DevilWAH

newman555 wrote: »

I also think that lowest cost wins, but I found somewhere that directly connected ports to root bridge become root ports, so that's why I was uncertain...

all ports on the root switch become designated ports.

however the second switch will assing the better link as the root port.

each switch in STP only deals with its own loacl ports, it does not worry about the stat of the port at the other end.

So in this case the second switch would set its root port to the one with the lowest path cost back to the root switch. This would leave the port connected directly back to the root directly as nither a degisnated port or a root port, so it will be set to a blocking state.

So even though the other end of the link is up on the root switch, the link as a whole is effectivly down.

hexem

Remember the cost is incremented as it hit's a switch other than the root, so the root starts with port cost of 0, and as it moves futher away from the root switch it's incremented.

hexem

There's only ever one root port for 802.1d, and that will be the lowest cost port, if there's a tie the lowest numbered port is chosen.

CiskHo

DevilWAH wrote: »

all ports on the root switch become designated ports.
<snip>
So in this case the root switch would set its root port to the lowest path cost.

Seems like incorrect wording to me. How could a root switch whose ports are all "designated" have a "root" port? Root switches do not have root ports, only designated ports. May want to rephrase "root" switch to "nonroot" switch or "switch directly connected to the root switch".

Just trying to make sure there is no confusion.

DevilWAH

CiskHo wrote: »

Seems like incorrect wording to me. How could a root switch whose ports are all "designated" have a "root" port? Root switches do not have root ports, only designated ports. May want to rephrase "root" switch to "nonroot" switch or "switch directly connected to the root switch".

Just trying to make sure there is no confusion.

oppppppppppps

please re-read my post and check I have it right now

quite a few miss types in there... Sorry about that people, can't help me writing such nonsense...

mgeorge

newman555 wrote: »

I also think that lowest cost wins, but I found somewhere that directly connected ports to root bridge become root ports, so that's why I was uncertain...

Typically the directly connected ports from a switch to a root bridge in an STP domain will be elected as the root as it only has a single link to account for the cost.

For example you have a 3 switch topology, SW1 is connected to SW2 and SW3 then SW2 and SW3 are directly connected.

SW1 is the root and sw2 is connected to sw1 via a 1Gbps link which gets a STP cost of 20000, the only way you're going to beat that cost is if SW2 has a 10Gbps link to SW3 and SW3 has a 10Gbps link back to the root SW1 which in turn would total 4000

You can however manually specify port cost in a transit path to influence the root port election.