With spanning tree, do all the links between switches have to be trunk links?

StonedHitman

I'm just learning spanning tree and atm I don't understand how spanning tree works in conjunction with vlans. Because of that something has really been nagging me, and that is the question in the title. Just going by what I already know or what I think I know, I assume that per vlan spanning tree(pvst) is what you would use for spanning tree with vlans. But then technically, your already dealing with vlans either way(vlan 1) so I guess that blows my theory out of the water.

FloOz

I'm a little confused by what you are asking. Trunks allow interconnected switches to pass vlan traffic between them. Can you connect two switches and set both sides to a particular access vlan? Yes you can, however then only that particular vlan will be passed on that link.

StonedHitman

Yea, sorry I confused you, it's just that I always get distracted by little details like this.

So, from what I understand, if you neglect to configure trunks between switches while your using spanning tree, spanning tree will still work fine, it will just only work on the native vlan. Is this right?

Jon_Cisco

Per vlan spanning tree is going to allow a different root bridge for each vlan.

If you configure it correctly links can be blocked on one vlan and active for another. Making better use of your connections and allowing some load balancing.

Trunks allow a single port to move multiple vlans between the switches. I suppose you could set up a different access port for each vlan you were planning to use. Not something I have ever considered.

I'm no expert so let me know if I got anything wrong!

Good Luck!

StonedHitman

did000 wrote: »

Hmmm, maybe you have some gaps in your vlan understanding. If some VLAN is not passing some link, then the network is broken for him and there is no STP negotiation there. Btw, here is one extremely good explanation of the Spanning-tree. Check the whole chapter: ICND2 | VLAN Operations

I guess I asked the wrong question, what I meant to ask is this, If you just set up your network and you didn't add any vlans but you are using spanning tree, will the trunk links be necessary? Because by default, if you don't configure vlans, all switch ports are still considered to be in one big vlan.

theodoxa

I was wondering something similar. From reading the the CCNP: SWITCH OCG it would appear --

CST (Common Spanning-Tree) uses untagged (802.1Q Native VLAN) frames
PVST (Per-VLAN Spanning-Tree) uses ISL tagged frames
PVST+ (Per-VLAN Spanning-Tree +) can use either ISL tagged or untagged (802.1Q Native VLAN) frames

This would seem to suggest PVST only sends BPDUs over Trunks and specifically ISL Trunks. The others (CST and PVST+) use untagged frames, so it would seem they should be able to work without a trunk. In fact, I know I have seen STP BPDUs while using Wireshark on a laptop connected to a switch. Then again, I also discovered at one point during my CCNA studies that Windows 7 runs Spanning-Tree (I had the wireless and wired interfaces bridged) when my Desktop PC managed to become the Root Bridge, so I wouldn't discount the possibility of it (Windows) also negotiating trunks (DTP).

theodoxa

StonedHitman wrote: »

I guess I asked the wrong question, what I meant to ask is this, If you just set up your network and you didn't add any vlans but you are using spanning tree, will the trunk links be necessary? Because by default, if you don't configure vlans, all switch ports are still considered to be in one big vlan.

The number of VLANs is irrelevant (some implementations run only 1 instance of STP for all VLANs, be they 1 or 1000). Loops exist within a single VLAN.

StonedHitman

theodoxa wrote: »

The number of VLANs is irrelevant (some implementations run only 1 instance of STP for all VLANs, be they 1 or 1000). Loops exist within a single VLAN.

I understand that but what I want to know is that, in a small network in one vlan with spanning tree configured, does it matter whether the links between switches are trunk links or not?

fredrikjj

You can run STP on access ports, but you would probably want to configure the switch to switch links as trunks anyway in case you want to add additional vlans in the future.

Deathmage

StonedHitman wrote: »

I understand that but what I want to know is that, in a small network in one vlan with spanning tree configured, does it matter whether the links between switches are trunk links or not?

To answer your question yes, STP would need to be enabled on the default vlan and also it doesn't matter if the links are trunked or not, if you don't have STP enabled and a loop is created on the interface(s) associated with the default vlan it will brick the network.

Let me give you a real-world example since this has happened to me on a production environment that DIDN'T have STP enabled on the default vlan; basically envision a wall jack with 4 RJ45 ports on them in a accounting department near a printer. Now imagine a curious employee sees a patch cable sitting on the floor that someone took out of there laptop bag in the office and being the helpful person they are they plug in the cable into port 1 and port 2. BAM in a matter of about 2 minutes with a network without STP enabled on the default vlan will brick the entire network on that vlan.

Also if the bricked network is the default vlan it will more than likely brick all of the other vlans that flow over the default vlan. The end result was my entire network killed itself in a matter of minutes because our MDF switch didn't have STP enabled (I inherited the network from my predecessor and well lets just say that's one of many reasons why I'm here now). Bear in mind this was a network with 12 HP Procurve switches and all it took was one loop to brick all of the switches trunked together to create your classic star-network topology.

So needless to say, even if you have a default vlan with no other vlan configured on the switch it's just good practice to enable STP.

Does this answer your question?

StonedHitman

Yes Deathmage, you did answer my question, thank you, and everyone else.

Magic Johnson

Deathmage wrote: »

To answer your question yes, STP would need to be enabled on the default vlan and also it doesn't matter if the links are trunked or not, if you don't have STP enabled and a loop is created on the interface(s) associated with the default vlan it will brick the network.

Let me give you a real-world example since this has happened to me on a production environment that DIDN'T have STP enabled on the default vlan; basically envision a wall jack with 4 RJ45 ports on them in a accounting department near a printer. Now imagine a curious employee sees a patch cable sitting on the floor that someone took out of there laptop bag in the office and being the helpful person they are they plug in the cable into port 1 and port 2. BAM in a matter of about 2 minutes with a network without STP enabled on the default vlan will brick the entire network on that vlan.

Also if the bricked network is the default vlan it will more than likely brick all of the other vlans that flow over the default vlan. The end result was my entire network killed itself in a matter of minutes because our MDF switch didn't have STP enabled (I inherited the network from my predecessor and well lets just say that's one of many reasons why I'm here now). Bear in mind this was a network with 12 HP Procurve switches and all it took was one loop to brick all of the switches trunked together to create your classic star-network topology.

So needless to say, even if you have a default vlan with no other vlan configured on the switch it's just good practice to enable STP.

Does this answer your question?

Ouch! I had similar, we were doing a 60 person office move around, moving desks, departments etc, and one of us managed to plug the phone in to the floor jack, and then the phone in to the floor jack instead of PC. Result? FLASHING LIGHTS EVERYWHERE. ALARMS. RUN. RUN FOR YOUR LIVES. Thankfully it was out of office hours, I'm adamant it wasn't me but it took us 3 hours to find the phone, as we were all doing it and nobody was watching the switches. We came back to Christmas tree.

They weren't managed by us but I really don't know why you wouldn't enable spanning tree in a production environment nowadays, especially as people get more...curious.

Deathmage

Magic Johnson wrote: »

......especially as people get more...curious.

Exactly! - it's one thing to question STP in a test environment while learning it's a whole different scenerio on a live network and it saves tons of headaches.... It's really more annoying when your thinking it's everything on the face of the planet; if you don't know it's a loop it can mimic multiple different things and in-turn cause hours of high-blood pressure.