Spanning Tree connections question

Rodzilla G

Although going after my ICND2 cert, I am in desktop support and not actively working with switches and routers other than simulators. My question about STP is......are there particular ports on a switch that are required for a single port, or if you have redundancy....multiple ports for STP to work? In other words, which ports or are there particular ports only that STP must use in order to work? I think that answer is no, because from what I have read that out of the box switches will work and utilize a very basic structure of STP even without configuration.

I don't know if my question makes sense, but any feedback is welcomed.

instant000

You seem to ask more than one question.

1. Whether or not spanning tree runs by default.
2. Does spanning tree depend on particular ports to work.

1. Whether or not spanning tree runs by default.

Um, if the question is whether or not spanning tree runs by default, the answer is "it should".

This should be the default for Cisco switches, I believe PVST+. Let me reference:

Source: Catalyst 3560 Software Configuration Guide, Release 12.2(5SE - Configuring STP* [Cisco Catalyst 3560 Series Switches] - Cisco Systems

2. Does spanning tree depend on particular ports to work.

Not really. Spanning tree depends on BPDUs to work. If the BPDUs don't flow right, you can have all sorts of issues, as spanning tree relies on these to learn the topology and turn off links that should be blocked.

How LAN Switches Work - Cisco Systems

If you want more information on spanning tree, then I would recommend a book by Radia Perlmann:

Interconnections: Bridges, Routers, Switches, and Internetworking Protocols (2nd Edition)


Hope this helps.

Rodzilla G

instant000 wrote: »

You seem to ask more than one question.

1. Whether or not spanning tree runs by default.
2. Does spanning tree depend on particular ports to work.

1. Whether or not spanning tree runs by default.

Um, if the question is whether or not spanning tree runs by default, the answer is "it should".

This should be the default for Cisco switches, I believe PVST+. Let me reference:

Source: Catalyst 3560 Software Configuration Guide, Release 12.2(5SE - Configuring STP* [Cisco Catalyst 3560 Series Switches] - Cisco Systems

2. Does spanning tree depend on particular ports to work.

Not really. Spanning tree depends on BPDUs to work. If the BPDUs don't flow right, you can have all sorts of issues, as spanning tree relies on these to learn the topology and turn off links that should be blocked.

How LAN Switches Work - Cisco Systems

If you want more information on spanning tree, then I would recommend a book by Radia Perlmann:

Interconnections: Bridges, Routers, Switches, and Internetworking Protocols (2nd Edition)


Hope this helps.

Thanks for the insights. Sorry for the misunderstanding. I knew that STP should be running by default, I just didn't know if there were specialized ports, such as a Gigabit port was needed, or a port number range. This will help a lot. Thanks again.

pamccabe

With spanning tree, there is an election process for the root bridge when the switch is powered on. This is done with bpdus that get sent out by the switches. The root switch will put all posts for the vlan in a forwarding state. The other bridges will elect ports to be root or designated ports based on cost to the root bridge. In case of a tie, the lowest port number wins. For non root bridgs, all other parts for that vlan should be put in a blocking state. This eliminates loops. At least that is how I understand it. Someone correct me if I am wrong.

xXErebuS

All ports will participate and send bpdus at first (unless porfast / bpdufilter or bpduguard is used). After root election STP process only designated ports will send BPDUs.

Rodzilla G

xXErebuS wrote: »

All ports will participate and send bpdus at first (unless porfast / bpdufilter or bpduguard is used). After root election STP process only designated ports will send BPDUs.

Ok...would this be done on Core, distribution, or access routers? I guess my reason for asking this is....If I have a 24 port switch and lets say switchport f 0/1 is considered the designated port to the root bridge, but I also have a PC traffic also going through that switchport....would there be any problems? I don't know if my question is making sense or not.

xXErebuS

pamccabe wrote: »

With spanning tree, there is an election process for the root bridge when the switch is powered on. This is done with bpdus that get sent out by the switches. The root switch will put all posts for the vlan in a forwarding state - if this is so why would we need portfast?. The other bridges will elect ports to be root or designated ports based on cost to the root bridge - what if it's neither of these?. In case of a tie, the lowest port number wins. For non root bridgs, all other parts for that vlan should be put in a blocking state - What if the port is an access port?. This eliminates loops. At least that is how I understand it. Someone correct me if I am wrong.

I will try to simplify hopefully without being called out!

I think you are confused about states and roles... each port can have different states and roles i.e. there are not 3 designated ports and 5 root on a switch. Every port has the capability of being these roles/states.

States:
Your switch ports will start in the blocking state and go through the listening, learning, forwarding (or blocking stage in which it still listens for bpdus) states (in that order, there is also a "disabled" state which is pretty much means port is..... well disabled).

Roles:
Valid uplinks to the root bridge are known as root ports - there can only be 1 of these on any switch (should not be any of these on the root switch for the vlan that your switch is the root for). Valid downstream ports (ports that have links connected to another router's root port) are known as designated ports - there can be multiple of these on any switch.

Ports that received BPDUs with inferior cost to the root bridge are known as blocking ports - there can be multiple of these on any switch (should not be any of these on the root switch for the vlan that your switch is the root for)

There are also alternate ports which is a backup of a root port, and backup ports which are backups of designated ports (not common since it would require a shared medium).

Ports in the same vlan that receive no BPDUs are also in the forwarding state since they are not connected to any switches (as long as they have something connected) - these are considered edge ports.

xXErebuS

Rodzilla G wrote: »

Ok...would this be done on Core, distribution, or access routers? I guess my reason for asking this is....If I have a 24 port switch and lets say switchport f 0/1 is considered the designated port to the root bridge, but I also have a PC traffic also going through that switchport....would there be any problems? I don't know if my question is making sense or not.

Typically anything related to layer 2 is going to be done in your access layer.

I hope my explanation makes more sense to you.

Not sure what your asking; if a PC is connected to Fa0/1 and its the path to the root bridge then you have a shared media segment which is a no no....

Fa0/1 would be designated and would be a trunk. In this case if PC traffic on Fa0/2 tries to connect it will go through Fa0/1 fine in this scenario and Fa0/2 will be considered an "edge port"

Rodzilla G

xXErebuS wrote: »

Typically anything related to layer 2 is going to be done in your access layer.

I hope my explanation makes more sense to you.

Not sure what your asking; if a PC is connected to Fa0/1 and its the path to the root bridge then you have a shared media segment which is a no no....

Fa0/1 would be designated and would be a trunk. In this case if PC traffic on Fa0/2 tries to connect it will go through Fa0/1 fine in this scenario.

Bingo, I think that is what I needed to know......That designated port is a TRUNK port. I was just so confused thinking......how can BPDU's and regular data go through the same port without there being problems. Yes, each port is it's own collision domain, but still was confusing me. That makes more sense, having that form a trunk port. Thank you!!!

xXErebuS

Rodzilla G wrote: »

Bingo, I think that is what I needed to know......That designated port is a TRUNK port. I was just so confused thinking......how can BPDU's and regular data go through the same port without there being problems. Yes, each port is it's own collision domain, but still was confusing me. That makes more sense, having that form a trunk port. Thank you!!!

I think your still confused. It doesn't HAVE to be a trunk.... they go through the same port b/c they are on the same link. Just as you may have CDP information going through this link as well.... trunking allows multiple vlans.

Rodzilla G

xXErebuS wrote: »

I think your still confused. It doesn't HAVE to be a trunk.... they go through the same port b/c they are on the same link. Just as you may have CDP information going through this link as well.... trunking allows multiple vlans.

Yes I was. I was sitting at my laptop with Packet tracer open. I started connecting switches to switches, and I thought....this is what I believe is called a point to point or link to link connection from a switch to switch within Spanning tree. So obviously normal end user/PC data won't go through there. And (technically for the test anyway, that would involve a cross over connection). But I had to see it while I was building it for me to understand it better. I apologize for all of the confusion. But I really want to understand it. In order to pass, I must have the knowledge. And this helps. Thanks for everyone's insights.

xXErebuS

Rodzilla G wrote: »

Yes I was. I was sitting at my laptop with Packet tracer open. I started connecting switches to switches, and I thought....this is what I believe is called a point to point or link to link connection from a switch to switch within Spanning tree. So obviously normal end user/PC data won't go through there. And (technically for the test anyway, that would involve a cross over connection). But I had to see it while I was building it for me to understand it better. I apologize for all of the confusion. But I really want to understand it. In order to pass, I must have the knowledge. And this helps. Thanks for everyone's insights.

O but it will go there....

You don't have to have a crossover cable if your ports on BOTH support MDIX

Rodzilla G

xXErebuS wrote: »

O but it will go there....

You don't have to have a crossover cable if your ports on BOTH support MDIX

Yes I understand that.....that is why I stated (technically for the test). This is what is needed for the ICND2 portion.