Author Topic: Should this VLAN setup work or my equipment failed?  (Read 2960 times)

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Offline Rick Law

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Should this VLAN setup work or my equipment failed?
« on: July 27, 2021, 10:12:16 pm »
Should this work?

I have a 24 ports 3com managed switch I acquired from a company closure sale.  The FANs failed so other failures is possible.

- The default is all ports on VLAN1 untagged.  They all worked.  The switch supports 802.1Q, I just left them as default untagged.
- I assigned 6 ports (that was functioning well in VLAN1) as VLAN2 untagged, intended as an isolated environment for equipment setup.  That worked - I do have that 6 ports communicating with each other but isolated from VLAN1.
- What doesn't work is when I patch cable VLAN2 to VLAN1 (using VLAN2 like a separate switch/hub joined to my main VLAN1 network via a patch cable)

I was expecting joining VLAN2 to VLAN1 via patch cable would work, but it doesn't.  Am I understanding VLAN wrong or could that be equipment failure?

Thanks for your input...
 

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Re: Should this VLAN setup work or my equipment failed?
« Reply #1 on: July 28, 2021, 03:29:11 am »
Sounds like the switch has some anti bridging feature to prevent two network domains accidentally being joined together.
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Offline Rick Law

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Re: Should this VLAN setup work or my equipment failed?
« Reply #2 on: July 28, 2021, 08:11:11 pm »
Sounds like the switch has some anti bridging feature to prevent two network domains accidentally being joined together.

Thanks for your input, I was thinking it may be that as well...  But I was not sure if it may be I misunderstood tagging and I should be using tagging some manner.  After reading the CISCO's tutorial on tagging, I was still at a lost, so I just wanted a second opinion.  Thanks for giving me that, I appreciated it.
 

Online nfmax

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Re: Should this VLAN setup work or my equipment failed?
« Reply #3 on: July 28, 2021, 08:43:36 pm »
Your two VLANs are two different networks. To route traffic between them you need a router, not a switch. This could be a PC with two network interfaces, set up to route traffic between them.
 

Offline Rick Law

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Re: Should this VLAN setup work or my equipment failed?
« Reply #4 on: July 28, 2021, 11:52:55 pm »
Your two VLANs are two different networks. To route traffic between them you need a router, not a switch. This could be a PC with two network interfaces, set up to route traffic between them.

Actually it is kind of half way...  I generally don't like doing firmware/BIOS upgrade or machine setup while it is exposed to the outside.  I would do it on a side-switch without connection to the main-network.  With a only s laptop and the device being setup on that switch, I can do things in a safer environment.  When ready, I may patch it into the main network for final test, then unplug and re-plug into the main network for deployment.

This latest project is a bit different.  I am trying to determine how big a speed-penalty by using a pair of MoCA modem to connect to my main-network.  So I am swapping connections in all sorts of ways - all within the same network but with and without the MoCA connection.  The physically separated side switch was doing the job.  I was considering replacing the side-switch with VLAN2 so I don't need another physical switch taking up space (and electrical outlet spots) thus giving me more flexibility.  Too bad that is not to be.

 

Offline fordem

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Re: Should this VLAN setup work or my equipment failed?
« Reply #5 on: September 06, 2021, 12:10:14 pm »
I was expecting joining VLAN2 to VLAN1 via patch cable would work, but it doesn't.  Am I understanding VLAN wrong or could that be equipment failure?

It should work - you need to track down why it doesn't, and I would start with ip addressing - are you using ip addresses in the same logical network?

A switch with two static VLANs should behave exactly like two physical switches.
 

Offline Rick Law

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Re: Should this VLAN setup work or my equipment failed?
« Reply #6 on: September 07, 2021, 03:32:56 am »
I was expecting joining VLAN2 to VLAN1 via patch cable would work, but it doesn't.  Am I understanding VLAN wrong or could that be equipment failure?

It should work - you need to track down why it doesn't, and I would start with ip addressing - are you using ip addresses in the same logical network?

A switch with two static VLANs should behave exactly like two physical switches.

I too think it should work.  On this switch, VLAN1 is the default for all ports.    VLAN2 is created by reassigning ports from default VLAN1 to VLAN2.

I am mostly of on the same subnet -- all typical connections are one subnet. A couple of boxes on VLAN1 need to talk to each other and they are on VLAN1 with a separate subnet.  These two boxes doesn't talk to the other machines on the network.  They just need to ride on the same network cables.

VLAN1 and VLAN2 do act like two separate physical switches - except when I put a patch cable from VLAN2 to VLAN1, then VLAN2 stops working.

I have in mind trying to create VLAN4 and shift VLAN 1,2 and 3 up by one, so my main network is on VLAN2 and nothing is on the default VLAN1.  I'll see if I can patch into VLAN3 (test) to VLAN2 (main) and see if that functions - that is the intend, have a side "switch" that only get patched into main network as needed.

I have not had the time to try that yet...
 

Offline fordem

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Re: Should this VLAN setup work or my equipment failed?
« Reply #7 on: September 07, 2021, 01:06:14 pm »
VLAN1 and VLAN2 do act like two separate physical switches - except when I put a patch cable from VLAN2 to VLAN1, then VLAN2 stops working.

What do you mean by "VLAN2 stops working"?

If you have two devices communicating with each other on VLAN2, do they stop communicating when a patch cable is plugged in?  Presumably you are using tcp/ip - if you are pinging host #1 from host #2, do the pings stop when the patch cord is plugged in?
 

Offline mansaxel

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Re: Should this VLAN setup work or my equipment failed?
« Reply #8 on: September 07, 2021, 01:34:02 pm »
Most switches do not like their ports connected like that. Do not do it.

You need a router. A real one, not a NAT toy. It it probably sensible to build a 802.1q trunk to the router from the switch.

You need an IP network per VLAN, and assign the router addresses on these two networks.

Then, computers on those two networks need to get IP addresses on the corresponding network, and also a routing entry that points to where the other network is. If the router is responsible for connectivity to other networks as well, like the Internet, it's probably sufficient to point the default gateway to the router. Do keep in mind that there needs to be routes back to the network too.

Offline MarginallyStable

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Re: Should this VLAN setup work or my equipment failed?
« Reply #9 on: September 07, 2021, 06:00:13 pm »
May need to disable loop-back storm detection for the ports. Some switches detect this as a loop back and disable the port (even though it technically isn't)
 

Offline fordem

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Re: Should this VLAN setup work or my equipment failed?
« Reply #10 on: September 07, 2021, 07:21:47 pm »
Most switches do not like their ports connected like that. Do not do it.

As long as the ports are in separate VLANs (or broadcast domains), there's no reason why this cannot be done, if you do it with ports in the same broadcast domain, you'll end up with a broadcast storm.

Quote
You need a router. A real one, not a NAT toy. It it probably sensible to build a 802.1q trunk to the router from the switch.

You need an IP network per VLAN, and assign the router addresses on these two networks.

Then, computers on those two networks need to get IP addresses on the corresponding network, and also a routing entry that points to where the other network is. If the router is responsible for connectivity to other networks as well, like the Internet, it's probably sufficient to point the default gateway to the router. Do keep in mind that there needs to be routes back to the network too.

This is only required if you wish to have separate networks on the VLANs and also communication between the VLANs - there are different reasons to VLAN (or segment) a network, and there are also occasions when you need to have all the VLANs (segments) on the same ip network - it all depends on what the network "designer" is trying to achieve.
 

Offline fordem

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Re: Should this VLAN setup work or my equipment failed?
« Reply #11 on: September 07, 2021, 07:27:37 pm »
Should this work?

I have a 24 ports 3com managed switch I acquired from a company closure sale.  The FANs failed so other failures is possible.

- The default is all ports on VLAN1 untagged.  They all worked.  The switch supports 802.1Q, I just left them as default untagged.
- I assigned 6 ports (that was functioning well in VLAN1) as VLAN2 untagged, intended as an isolated environment for equipment setup.  That worked - I do have that 6 ports communicating with each other but isolated from VLAN1.
- What doesn't work is when I patch cable VLAN2 to VLAN1 (using VLAN2 like a separate switch/hub joined to my main VLAN1 network via a patch cable)

I was expecting joining VLAN2 to VLAN1 via patch cable would work, but it doesn't.  Am I understanding VLAN wrong or could that be equipment failure?

Thanks for your input...

Did you remove these six ports from VLAN1 or are they still members?  If they are, your problem may be caused by a broadcast loop, you can usually see this on the port activity lights.
 

Offline mansaxel

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Re: Should this VLAN setup work or my equipment failed?
« Reply #12 on: September 07, 2021, 07:45:59 pm »
Most switches do not like their ports connected like that. Do not do it.

As long as the ports are in separate VLANs (or broadcast domains), there's no reason why this cannot be done, if you do it with ports in the same broadcast domain, you'll end up with a broadcast storm.

Yes, if 802.1q VLANs were so separate as they seem to the innocent bystander. But they aren't, not always, and the FIB in the switch isn't always as simple as it's made out to be.

I've been employed building networks the last 20+ years, and all the people I met who've tried this trick come from it burned: Looping back to the same switch is universally frowned upon. Exception being routed traffic that goes up in a .1q trunk to to a router-on-a-stick and comes back with the source MAC of the router interface, on another VLAN. That's just inefficient, not dangerous, per se.

It's like building a PSU without proper social distancing between primary and secondary; it might work, but the margin for error is practically gone.

Quote
You need a router. A real one, not a NAT toy. It it probably sensible to build a 802.1q trunk to the router from the switch.

You need an IP network per VLAN, and assign the router addresses on these two networks.

Then, computers on those two networks need to get IP addresses on the corresponding network, and also a routing entry that points to where the other network is. If the router is responsible for connectivity to other networks as well, like the Internet, it's probably sufficient to point the default gateway to the router. Do keep in mind that there needs to be routes back to the network too.

This is only required if you wish to have separate networks on the VLANs and also communication between the VLANs - there are different reasons to VLAN (or segment) a network, and there are also occasions when you need to have all the VLANs (segments) on the same ip network - it all depends on what the network "designer" is trying to achieve.

Having the same IP network on different VLANs is IMNSHO counterproductive and violates "rule of least surprise". If someone tried that trick in my network, stern words would be uttered.

I fully appreciate the desire to isolate parts of a network. It is something I do, a lot, but I also make certain that I have an IP address plan and routing set up that makes lack of reachability a policy decision, not a design consequence.

Anyway, we're getting way off on a tangent here. This is not what the OP was trying to achieve, I guess.

There's a "Why do you want this?" question that's not getting asked.

Dear thread starter: Why?

Offline Rick Law

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Re: Should this VLAN setup work or my equipment failed?
« Reply #13 on: September 07, 2021, 10:07:28 pm »
VLAN1 and VLAN2 do act like two separate physical switches - except when I put a patch cable from VLAN2 to VLAN1, then VLAN2 stops working.

What do you mean by "VLAN2 stops working"?

If you have two devices communicating with each other on VLAN2, do they stop communicating when a patch cable is plugged in?  Presumably you are using tcp/ip - if you are pinging host #1 from host #2, do the pings stop when the patch cord is plugged in?

VLAN2 pc's can not get to VLAN1.  As to if PC's on VLAN2 can or cannot communicate between themselves, my notes are too mixed up -- I have to retest that to be sure. 

Should this work?

I have a 24 ports 3com managed switch I acquired from a company closure sale.  The FANs failed so other failures is possible.

- The default is all ports on VLAN1 untagged.  They all worked.  The switch supports 802.1Q, I just left them as default untagged.
- I assigned 6 ports (that was functioning well in VLAN1) as VLAN2 untagged, intended as an isolated environment for equipment setup.  That worked - I do have that 6 ports communicating with each other but isolated from VLAN1.
- What doesn't work is when I patch cable VLAN2 to VLAN1 (using VLAN2 like a separate switch/hub joined to my main VLAN1 network via a patch cable)

I was expecting joining VLAN2 to VLAN1 via patch cable would work, but it doesn't.  Am I understanding VLAN wrong or could that be equipment failure?

Thanks for your input...

Did you remove these six ports from VLAN1 or are they still members?  If they are, your problem may be caused by a broadcast loop, you can usually see this on the port activity lights.

They are removed from VLAN1.    The way this switch works is first to create the VLAN, then move the port(s) to it.  So the removal is a certainty.
 

Offline fordem

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Re: Should this VLAN setup work or my equipment failed?
« Reply #14 on: September 07, 2021, 10:38:08 pm »
They are removed from VLAN1.    The way this switch works is first to create the VLAN, then move the port(s) to it.  So the removal is a certainty.

You might want to recheck this, it's been a long time since I've worked with a 3com switch, but on most switches it IS possible to have ports be members of multiple VLANs, creating the VLAN and adding the ports are steps one & two, removing the ports from their original VLAN may need to be done separately.
 

Offline fordem

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Re: Should this VLAN setup work or my equipment failed?
« Reply #15 on: September 07, 2021, 11:12:06 pm »
I've been employed building networks the last 20+ years, and all the people I met who've tried this trick come from it burned

As you've chosen to start with this, I'll pick up where you left off - we're in 2021 now, so that would put you as starting around the turn of the century, I've got another 10 years or so on you, and that is just the building network side of things, if I remember correctly the first time I connected two computer systems to one another would have been I believe in 1981 - so do forgive me if I'm not impressed - by the way - you can no longer make that statement, you've now met someone who hasn't been burned.

Quote
Having the same IP network on different VLANs is IMNSHO counterproductive and violates "rule of least surprise". If someone tried that trick in my network, stern words would be uttered.

It's one of the easier ways to do a "multi-tenancy" connection and keep your tenants isolated - maybe you have no need for it, but, as I said, how you do what you do is dictated by what the network is needed to do - and in case you haven't realised it, with multi-tenancy, the "lack of reachability" IS the intent and not a "happy accident".

 

Offline mansaxel

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Re: Should this VLAN setup work or my equipment failed?
« Reply #16 on: September 08, 2021, 07:16:56 am »
I've been employed building networks the last 20+ years, and all the people I met who've tried this trick come from it burned

As you've chosen to start with this, I'll pick up where you left off - we're in 2021 now, so that would put you as starting around the turn of the century, I've got another 10 years or so on you, and that is just the building network side of things, if I remember correctly the first time I connected two computer systems to one another would have been I believe in 1981 - so do forgive me if I'm not impressed - by the way - you can no longer make that statement, you've now met someone who hasn't been burned.
Ok, you win in years. Fine. And you've better luck than I, too. Congratulations.
Quote
Having the same IP network on different VLANs is IMNSHO counterproductive and violates "rule of least surprise". If someone tried that trick in my network, stern words would be uttered.

It's one of the easier ways to do a "multi-tenancy" connection and keep your tenants isolated - maybe you have no need for it, but, as I said, how you do what you do is dictated by what the network is needed to do - and in case you haven't realised it, with multi-tenancy, the "lack of reachability" IS the intent and not a "happy accident".
I see. Where I come from, we build networks to connect things together. Sometimes, we conclude that parts of the network probably should not talk to each other, or, which is crucial, should have partial, regulated connectivity. For this, we have routing and firewalling. Sensible, controllable L3 technologies.  L2 networks, TBH, are failure domains. They need to be kept small, and local.

If the OP wants separation, get another switch. They cost next to nothing. Then, if needed, get a patch cable, connect the switches, and start climbing the spanning tree...

Online Ranayna

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Re: Should this VLAN setup work or my equipment failed?
« Reply #17 on: September 10, 2021, 12:23:33 pm »
Which 3com switch do you have?
I remember 3com from now almost 15 years ago before they were bough by HP.
And i have absolutly shitty memories about those things. They were *extremely* bug-ridden. If it is a comware switch i might be able to assist somewhat. If it's older i would suggest scrapping it.

Generally, assuming your model is routing capable, you should set up a route on the switch connecting the two different VLANs.
Connecting them together with cables can only work if the VLANs are untagged on each port you want to connect (and don't forget to set the pvid). Also, as long as you have any VLANs in common on both ports, even if it is just the default VLAN 1, you will trigger a loop.

Assuming it's a comware switch, if you can post the current config, i might be able to take a look.
 

Offline Rick Law

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Re: Should this VLAN setup work or my equipment failed?
« Reply #18 on: September 10, 2021, 08:35:33 pm »
Re-edited this entire post:  Initially I clicked the "submit" by mistake earlier.

I know this switch is > 10 years old, but not sure exactly how old.  The management webpage logo just say 3com.  If it is ComWare, there is nothing on those few pages with the word "comware".

According to the management webpage:
Type: 3Com Switch 3824
Software version: 1.10
Hardware version: R01

--------
This switch has the Broadcast storm control feature and is turned on.  All ports are untagged.

The motive for my OP is not so much to make it work, but to understand.  I was disturbed by it not working when I thought it should.  So I wanted to validate my understanding more than I want to make it work.  It appears all agreed it should work except if this switch has some hidden features/bugs . 

This weekend (after some needed sleep), I am going to add another VLAN, promote every port up by 1 so nothing is on the default VLAN1.  My main network will be VLAN2 instead of VLAN1, and my test would be the newly added VLAN.  May be it is patching to the default VLAN1 that it objects to.  We'll see...
« Last Edit: September 10, 2021, 09:00:22 pm by Rick Law »
 

Online Ranayna

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Re: Should this VLAN setup work or my equipment failed?
« Reply #19 on: September 10, 2021, 09:21:45 pm »
Sorry, comware is the type of operating system running on the switch. When 3com was aquired by HP, their OS was called comware to differentiate it from HPs existing switches. Comware is still in development to this day (but likely not long anymore) and i am familiar with it.

But that model pre-dates comware by quite a bit. According to the getting started guide i found, this thing is now almost 20 years old. It must have been one of the first gigabit switches.

I'm sorry, while i have worked with the command line of these a bit when i started my apprenticeship, i do not remember much about those.

Moving away from the default VLAN is a good idea for what you want to do. You might also try experimenting with turning off the broadcast storm protection. Should the connection cause a storm that is quickly obvious, since the port LEDs will go bonkers. Be carefult though, if the switch is conencted to the rest ob you home network, a broadcast storm might affect it.

 

Offline ve7xen

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Re: Should this VLAN setup work or my equipment failed?
« Reply #20 on: September 11, 2021, 12:21:16 am »
Most likely STP is enabled by default on this switch, and it doesn't like having two ports connected. Not sure about this old 3Coms, but it's pretty typical to enable STP by default, and often the lowest common denominator mode which doesn't support VLAN at all, so doing this will put one of the ports into BLOCKING mode. The switch will periodically generate frames (STP BPDUs) out all ports, and if it receives any of its own BPDUs back, it will block that port to protect against loops. You can poke around in the spanning tree stuff and maybe confirm this or disable it. Bridging two VLANs like this should 'work', but 'work' may not mean what you think it means.

But the larger point to make here is that connecting the two VLANs with the cable literally defeats the entire point of the VLANs. It makes the two VLANs the same broadcast domain, the splitting of which is the purpose of VLANs in the first place. The only practical effect it has vs. just putting everything on the same VLAN is that it forces traffic over the slow Ethernet link instead of the switch backplane, and consumes (at least) twice the TCAM in the switch, but there will be no practical difference to how traffic is distributed to hosts.

You should be using different IP subnets on the two VLANs, and put a router between them, not just bridge them (which also will only 'work' if you use the same IP subnet on both VLANs). If you're doing this for 'security' then that router should probably be a 'firewall' that won't just blindly route any packet. You might be able to consolidate that role with your Internet router and trunk both VLANs from the switch to that router, and handle both inter-VLAN routing as well as your gateway to the Internet on the same box.
« Last Edit: September 11, 2021, 12:23:38 am by ve7xen »
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Online Ranayna

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Re: Should this VLAN setup work or my equipment failed?
« Reply #21 on: September 11, 2021, 08:08:15 am »
I suspect, and that even looks logical if you assume that no other equipment is at hand: He seems to look for a simple way to detach a complete piece of his network by just unplugging a single cable.
That seems to be a valid usecase in my opinion, under the assumption that this switch is all he has, and the network really is small.

 

Online Cerebus

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Re: Should this VLAN setup work or my equipment failed?
« Reply #22 on: September 11, 2021, 03:09:16 pm »
It's almost certainly spanning tree.

Spanning tree is designed to detect interconnected switch topology by sending out probe packets. If they go out from one switch and hit another switch that provides both switches with some information about the topology. That switch in turn will transmit spanning tree packets that contain information indicating that it has seen a spanning tree packet from the first switch. If those probe packets ultimately come back to the originating switch it tells it that there is a loop somewhere. There's an algorithm that the switches all run that (usually) allows the the whole interconnected switch fabric to converge on a topology that does not have loops (by disabling ports aka putting them into BLOCKING mode).

In this case, the switch will occasionally send out spanning tree packets on (say) VLAN1, tagged with its own switch/bridge ID. It will see these same packets come back in on a port on VLAN2, will examine the sending bridge id, go "But I sent that, I shouldn't be receiving it" conclude that there's a loop (which is the whole point of spanning tree) and block the port that the spanning tree packet came in on. From the point of view of the switch, this is no different in loop detection terms to two ports on the switch in the same VLAN (or no VLAN at all) being interconnected. In that circumstance if it didn't block one of the ports the traffic would come in one port, be sent out the other, come back in the first, out the second again and so on.

So there's no fault, it's working as intended.
Anybody got a syringe I can use to squeeze the magic smoke back into this?
 

Online magic

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Re: Should this VLAN setup work or my equipment failed?
« Reply #23 on: September 13, 2021, 09:25:26 am »
I wasn't aware that STP doesn't distinguish between VLANs, but on second thought it makes perfect sense: otherwise, you could bridge two VLANs using OP's method on two different switches and there would be a loop until one of the switches disables the port.

Another vote for "disable STP".
 

Offline mansaxel

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Re: Should this VLAN setup work or my equipment failed?
« Reply #24 on: September 13, 2021, 09:35:53 am »
I wasn't aware that STP doesn't distinguish between VLANs, but on second thought it makes perfect sense: otherwise, you could bridge two VLANs using OP's method on two different switches and there would be a loop until one of the switches disables the port.

Another vote for "disable STP".

I'd rephrase that to "reform STP".  (And to elaborate, since the ports aren't 802.1q trunks, how can you tell which VLAN they're on, as long as you don't listen to LLDP or similar? Of course the switch must react to its own BPDUen and block!)

STP functionality is necessary once you go over 2 switches in your network. Anyone who thinks different either routes all their packets on level 3 or is dangerously unqualified to operate networks. Of course classic STP as Radia Perlman invented it is not what you need today, but barring configuration, that's what you'll get if you fail to understand and correctly configure the evolved spanning tree systems that came after it.

I like BPDUen. They enable Order.
« Last Edit: September 13, 2021, 09:38:07 am by mansaxel »
 


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