STP Question

bcall64

I need a little bit of clarification regarding STP cost to the root port. I have 3 switches and they are all connected. A is connected to B, B is connected to C, and C is connected to A. A has been elected as the root bridge and C is the designated bridge because it has the lowest cost to the root bridge.

My question is for Switch B. Let's say I have a PC connected to Switch B and it's trying to send traffic to a device connected to Switch A. Which route would it take? I assume it would just send it directly to Switch A because it has a directly connected root port which would have the lowest cost being directly connected.


I guess my confusion is understanding the role of the designated bridge. Can someone clarify? I hope my mspaint image is sufficient.

bcall64

Anyone?

networker050184

Have a little patience my friend. People here help when they can find time. Give it more than an hour.


The designated bridge is the bridge on a segment that has the lowest cost to the root. In other words the switch 'closest' to the root of the tree.

lordy

Sure, Traffic flows from B and it's connected host directly to A and hosts on it. How could it be different?

prtech

As networker said there is a designated bridge per segment. Since B has two segments connected to it, B sees two designated bridges: A and C. The port pointing to the designated bridge becomes the root port. Since you can only have one root port per switch for loop prevention, the port with the lowest cost to reach the root bridge will stay as root while the other is blocked.

Zartanasaurus

The root port is the lowest cost path to reach the root. This is not necessarily the port directly connected to the root since you could set the port cost to 2000.
The designated bridge is the bridge on a segment advertising the lowest cost to the root. Depending on topology, the other end of the designated port will be either a RP or a blocked port. That's where the loops are prevented. If you took away the link between B & C, you wouldn't have to worry about loops. B & C have to decide which one of them is going to put their port into blocking, to prevent the loop. The other bridge, C, becomes designated for that segment and it continues to send BPDUs which is how B knows to keep its port blocked.

bcall64

Zartanasaurus wrote: »

The root port is the lowest cost path to reach the root. This is not necessarily the port directly connected to the root since you could set the port cost to 2000.
The designated bridge is the bridge on a segment advertising the lowest cost to the root. Depending on topology, the other end of the designated port will be either a RP or a blocked port. That's where the loops are prevented. If you took away the link between B & C, you wouldn't have to worry about loops. B & C have to decide which one of them is going to put their port into blocking, to prevent the loop. The other bridge, C, becomes designated for that segment and it continues to send BPDUs which is how B knows to keep its port blocked.

I think I get it now. So any devices attached to C will send out of the RP for C and any devices from Switch B will send out of the RP for B. The part I don't get is if Fa0/2 is in blocking mode what is the function of the DP on C. Does a blocking port send out but just doesn't accept any traffic? If that's the case then does all the traffic just go to the DP on Switch C and then to A? I'm almost there with STP. It's always been a weak point for me.

Zartanasaurus

bcall64 wrote: »

The part I don't get is if Fa0/2 is in blocking mode what is the function of the DP on C.

I explained this in my last sentence.

Does a blocking port send out but just doesn't accept any traffic? If that's the case then does all the traffic just go to the DP on Switch C and then to A? I'm almost there with STP. It's always been a weak point for me.

To be technically accurate a blocked port does accept some traffic. BPDUs obviously. CDP. Probably a few other things. For the purposes of transit traffic, a blocked port doesn't send or receive traffic. So to understand the process, put a server on Switch B and a host on Switch C. If the host wants to send packets to the server, how do switches know where to forward traffic? And how/where would Switch C learn about the server on Switch B?

sratakhin

The packet may also take another path depending on the interfaces costs. It doesn't matter in your case, but if you had gigabit links between B-C and C-A, the packet will be sent to switch C instead.

bcall64

Zartanasaurus wrote: »

I explained this in my last sentence.


To be technically accurate a blocked port does accept some traffic. BPDUs obviously. CDP. Probably a few other things. For the purposes of transit traffic, a blocked port doesn't send or receive traffic. So to understand the process, put a server on Switch B and a host on Switch C. If the host wants to send packets to the server, how do switches know where to forward traffic? And how/where would Switch C learn about the server on Switch B?

I spent about an hour running through different scenarios in packet tracer and going over some practice questions and I finally understand. STP port states are determined by a couple different items which are MAC Address and Cost to the root switch. All ports on the root switch are designated ports. Once you determine which switch has the lowest priority and mac address you just need to determine the lowest cost for all the different paths it can take and then you know what state all the ports are in. If two switches have the same costs then the designated switch will be the one with the lower MAC address. Once you have that you're golden. Thanks for everyone's help.