sundance-communications.com/forum Forums VOIP Phones & IP PBX Networking Only 1 Switch with Spanning Tree protocol do you need to stop a broadcast storm?

Hello.

I have a question that maybe someone here can help me out with.

I understand that a broadcast storm is when you have your switches configured with multiple connections to other switches in a way that would create multiple routes to a single location as well as a Loop between switches.

When a packet does not have a destination, the switch will send the packet out on all ports and the packet gets redirected from the other switches back to the switch on another port. This will create a large storm of the same packet traveling in a loop.

My understanding is that spanning tree protocol will prevent this from happening.

My question is if you have these 3 switches setup in a loop, does only 1 out of these 3 switches has to have spanning tree protocol or all the switches or does every switch(all 3) have to have spanning tree procotol to prevent the broadcasting storm?

Here could be a situation:

1. Switch A is connected to switch B
2. Switch B is Connected to switch C
3. Switch C is connected to Switch A

Or also a situation where 2 switches have 2 cables between each other?

Therefore creating a looping effect.

Does only 1 of the switches need spanning tree protocol in this situation? For example, lets say switch A ?

Thanks.

In order for STP to work you would need 3 switches that support STP (802.1-D). To explain it very simply the switches need to talk to each other to discuss things such as what switch will become the "Root Bridge" and to decide what paths will be put into what mode. i.e blocking, forwarding, etc. That is a very simple explanation.

So all the switches involved in the loop need STP or all the switches in the entire network(including the ones not in the loop)

Lets say in an example where you have 2 connections going from your downstairs switch to your upstairs switch creating a loop. Would only your 2 switches which are connected with the 2 connections need STP or all the switches in the entire network(both downstairs and upstairs departmental switches and desktop switches,etc..)


Also, is STP something that just happens if your switch supports it or is it something you have to activate and configure to work?

Thanks for your help.

Typically STP is enabled by default on most swtiches that support it. You would have to reference that documentation for the specific equipment that you are working with. There may be instances where a small amount of configuration may be neccessary when using switches from different vendors in the same lan. In small networks STP is generally pretty easy to work with and configure and in most cases the only thing you would need to do is run some commands just to verify it is indeed on and working. In your scenario only the 2 switches would need STP support. Any other switches involved would not need STP as long as there is no possibility of a switching loop being created.

I've got nothing to add, but I think it's great that such a complex topic is being discussed... got a little tired of seeing either no new messages, or requests for opinions....

Excellent. Next week we will discuss RSTP

As for the 3 switches all 3 will need STP enabled in order for them to function without a loop.

The two connections that go to the switch downstairs you can either run STP on it or you may bond the 2 connections to have 1 large pipe going to that switch. Depends mainly on what you need the 2 connections for and what speed is needed. If you chose the STP option you main put the 2 connections on different switches the will allow for failover.

Another thing to think about is to check the switch manual for the STP settings for failover. How often the root switch updates and how long it would take for a complete failover.

As mentioned earlier RSTP may be better for you depending on your needs.

So much could be said about this topic. As someone stated earlier STP is a IEEE 802.1d standard. This states to go from blocking to forwarding takes around 45 seconds. If you turn port-fast one on your switchport the switch transitions directly from blocking to forwarding. When I studied from my CCNP I knew what every timer was but I still remember BLOCKING, LISTENING, LEARNING, FORWARDING, & DISABLED. Also remember the route bridge sends out BPDUs (BRIDGE PROTOCOL DATA UNITS) at Layer 2. It calculates paths based on media speed, it assigns a 19 cost to ethernet for example. Lower costs are considered preferred paths. I have included an example of a sh spanning-tree command on a switch I configured in my data center to be the root bridge. Notice each VLAN has a spanning-tree instance (PVST Per-VLAN Spanning TREE).

STP-3750-48#sh spanning-tree

VLAN0001
Spanning tree enabled protocol ieee
Root ID Priority 8193
Address 0018.bab8.fe80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8193 (priority 8192 sys-id-ext 1)
Address 0018.bab8.fe80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 19 128.1 P2p
Gi1/0/48 Desg FWD 4 128.48 P2p
Gi1/0/49 Desg FWD 4 128.49 P2p
Gi1/0/50 Desg FWD 4 128.50 P2p
Gi1/0/51 Desg FWD 4 128.51 P2p
Gi1/0/52 Desg FWD 4 128.52 P2p


VLAN0100
Spanning tree enabled protocol ieee
Root ID Priority 8292
Address 0018.bab8.fe80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8292 (priority 8192 sys-id-ext 100)
Address 0018.bab8.fe80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 19 128.1 P2p
Gi1/0/48 Desg FWD 4 128.48 P2p
Gi1/0/49 Desg FWD 4 128.49 P2p
Gi1/0/50 Desg FWD 4 128.50 P2p
Gi1/0/51 Desg FWD 4 128.51 P2p
Gi1/0/52 Desg FWD 4 128.52 P2p


VLAN0121
Spanning tree enabled protocol ieee
Root ID Priority 8313
Address 0018.bab8.fe80
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

Bridge ID Priority 8313 (priority 8192 sys-id-ext 121)
Address 0018.bab8.fe80
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300

Interface Role Sts Cost Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Gi1/0/1 Desg FWD 19 128.1 P2p
Gi1/0/48 Desg FWD 4 128.48 P2p
Gi1/0/49 Desg FWD 4 128.49 P2p
Gi1/0/50 Desg FWD 4 128.50 P2p
Gi1/0/51 Desg FWD 4 128.51 P2p
Gi1/0/52 Desg FWD 4 128.52 P2p