Solved: STP Path Cost

Rem Markov · ‎01-10-2023

Hey! I'm learning STP but I'm confused on why do we need a path cost to block a port, the traffic between the two switch will be blocked no matter where which side is blocked.

Is the path price is just a metric to block a port ? like there is no real use to it ?

emurray · ‎01-10-2023

It is difficult to make any difference when you only have 3 switches. Think about a topology with many more switches and maybe more than two paths to the root switch. Then in that scenario path cost will make sense. Also, understand that path costs are related to the interface bandwidth so you would want a switch to prefer a 1 Gbps link over a 100 Mbps link.

If you have a topology with many switches and the Root switch is three or four switches away with many different possible paths, the path cost will be cumulative for every path and the lowest path cost will be selected. This will translate into the path of the interfaces with the highest bandwidth will be selected unless the value is changed manually on any of the switches.

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

I hope this helps.

View solution in original post

MHM Cisco World · ‎01-10-2023

since the SW5 have all port DP then it is Root bridge,
why the cost is need ?
if SW6 to SW5 cost is more than Cost of SW6-SW4-SW5 then the second path will win and e0/2 in SW6 will be BLK and e0/1 is FWD.

Rem Markov · ‎01-10-2023

I'm confused more with why does it matter that I Block the eth0/1 on SW6 rather then eth0/3 on SW4, I get that SW6-SW4-SW5 Is the better way then SW4-SW6-SW5, But why does it matter ? anyways no traffic will go from SW6 to SW4

MHM Cisco World · ‎01-10-2023

dont confuse I am out home now when I return I will send you all detail.

MHM Cisco World · ‎01-10-2023

so just to confirm are you need more detail or no ?

Rem Markov · ‎01-10-2023

Sure ! If it is possible of course

MHM Cisco World · ‎01-10-2023

OK, later tonight I will share answer for both
your Q about which port send/receive BPDU (from your last post)
and effect of Cost for this post.

Rem Markov · ‎01-10-2023

Thank you so much !

emurray · ‎01-10-2023

It is difficult to make any difference when you only have 3 switches. Think about a topology with many more switches and maybe more than two paths to the root switch. Then in that scenario path cost will make sense. Also, understand that path costs are related to the interface bandwidth so you would want a switch to prefer a 1 Gbps link over a 100 Mbps link.

If you have a topology with many switches and the Root switch is three or four switches away with many different possible paths, the path cost will be cumulative for every path and the lowest path cost will be selected. This will translate into the path of the interfaces with the highest bandwidth will be selected unless the value is changed manually on any of the switches.

https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

I hope this helps.