Spanning-Tree Cost Calculation

[lagapidis (Lazarus Agapidis)]

Hello Sathish

What Andrew is stating here is that the cost of the last link that was traversed by a BPDU is added by the receiving switch. Let me clarify using the following diagram:

• A BPDU will be sent from the root bridge (SW1) to SW2. When the root bridge sends this BPDU, it will not add any cost, but will simply send it.
• When SW2 receives this BPDU, it will add the cost of the link between SW1 and SW2. The receiving switch adds the cost.
• As it prepares to forward the BPDU to SW4, that added cost of the link is included in the BPDU, and it sends it along its way.
• SW4 receives the BPDU with only the cost of the link between SW1 and SW2 included. SW4 will add the cost of the link between SW2 and SW4.

This is what Andrew means in his post. Does that make sense?

I hope this has been helpful!

Laz

[sathish84be (Sathish Murugesan)]

Hello @lagapidis

you stated here that to increase the port priority for Gi0/2 in order this become root port . if i am correct, this supposed to be done in the upstream switch , right?

[lagapidis (Lazarus Agapidis)]

Hello Sathish

Just to let our other readers know, the quote you are referring to is this one:

@lagapidis

If you want to have G0/2 go into the forwarding state, and G0/1 to be blocked, then you must increase the priority value of the G0/2 port. That sounds counterintuitive. Specifically:

• the higher the priority value, the more likely a port will go into the forwarding state
• if the priority value is the same, the lower port number will be chosen first

The specific scenario that the original poster was describing was two switches connected to each other with two links. One of those switches would become the root bridge, while the other would be a non-root bridge. One of the two ports on the non-root bridge would be set to be the root port and the other would be blocked.

The configuration that I am suggesting in my post should be applied to the non-root switch. Does that make sense?

I hope this has been helpful!

Laz

[ranjeetmallipeddi9 (Ranjeet Mallipeddi)]

Hi Rene,

w.r.t to the third image I have the following questions:

1. SW2 has a root port and for SW4 the root port is the interface that connects SW2 and Sw4. What if this interface is blocked (made alternate or non designated port) on SW2 ? How does a switch decide to block an interface or keep it in forwarding state ? is there any information on the BPDU frame that decides this in a CST ? while I do get the concept that the designated and alternate ports are chosen I dont understand what happens in situations like this.

Please let me know how a switch resolves this.

Thanks

[lagapidis (Lazarus Agapidis)]

Hello Ranjeet

This is the diagram you are talking about:

Now you’re asking what happens if the port on SW2 becomes blocked, correct? But I’m not sure which port you mean, the one indicated with the red arrow or with the blue arrow? Let’s look at both!

First of all, the port indicated by the red arrow has become a root port. This is because it is directly connected to the root switch, which is SW1. This port will never become blocked as long as SW1 is the root bridge. The only thing that can happen is if this port fails (i.e. the cable is cut or removed) or if it is shut down. If that happens, the whole topology will reconverge, connecting to the root bridge via the 10 Mbit link, and the root ports based on the BPDUs that are being received via SW1–>SW3–>SW4.

Now about the port with the blue arrow, what if that becomes a blocked port? If that is the case, then SW4 will receive the appropriate BPDUs from all neighbor switches, and will determine that the least cost path to the root will be via its link to SW3.

@ranjeetmallipeddi9

How does a switch decide to block an interface or keep it in forwarding state ? is there any information on the BPDU frame that decides this in a CST ?

The only information in the BPDU is the cost, and based on that, the root, designated, and alternate/blocked ports are determined. Take a look at this NetworkLessons note that further describes how the blocked port is determined based on the cost to the root bridge.

I hope this has been helpful!

Laz