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VLAN - cable between two different vlans

Daniël van den Berg

8/1/24, 7:16 AM

I've got two "Aruba instant on" switches, and am practicing working with VLANS. I'm only configuring switch A, and pinging switch B to make sure I have a connection.

My PC is hooked up to port 1 of A. Port 1 of B is hooked up to port 2 of A.

All ports except for port 47 and 48 are left as default, port 47 and 48 of A are configured to be in VLAN 2, as untagged. I've disabled ingress filtering on all ports of A.

From what I understand, port 47 and port 48 should act as if they're a different dumb L2 switch.

I plug port 2 of A into port 47 of A, and port 1 of B to port 48 of A. In my head, all I'm doing is introducing an extra L2 switch into the cable from A to B. Why does my PC lose connectivity to B?

1 + 4

switch

vlan

aruba

SelfishCrawler

8/1/24, 8:03 AM

all ports of managed switch are bridged and not dumb. If you want traffic to pass through this ports, you should set them as "tagged" for your VLAN

Daniël van den Berg

8/1/24, 10:22 AM

@SelfishCrawler tagging is only required when trunking afaik. I'm talking about untagged packages, and am using a physical cable as "bridge" between the two VLANS. So either I massively misunderstood what you mean, or it's not applicable.

Daniël van den Berg

8/1/24, 10:28 AM

@SelfishCrawler and are they all bridged? Aren't they only bridged within their own VLAN?

SelfishCrawler

8/1/24, 4:32 PM

From your description, I am not able to fully answer your question, since I can't test it on your exact switch model. Normally, you connect switches through uplink ports, and make them tagged for appropriate vlans. For managed switches, you should also assign an IP on your VLAN 2, since modern switches have a dedicated IP on each virtual interface, that's what can cause your problems also. Share more information for additional help. A good scheme of connection may help

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Jan

8/1/24, 8:03 AM

Switch A Port 2 (VLAN 1) <-> Switch A Port 47 (VLAN 2)
Switch A Port 48 (VLAN 2) <-> Switch B Port 1 (VLAN 1)

Let's assume following IPs

PC 192.168.0.1
Switch A 192.168.0.2 (bound to VLAN 1)
Switch B 192.168.0.3 (bound to VLAN 1)

In order to reach Switch B with its IP 192.168.0.3, your client sends out an ARP request into the network. The ARP request being a broadcast is limited to the subnet the client is in. Now you might think that all three devices are in the same /24 subnet and should be in the same broadcast domain.

However, the switch does not know that. For the switch, the broadcast came in from VLAN 1 and will only get sent out to ports with VLAN 1 on them. That's why VLAN<->Subnet should be a 1:1 mapping, otherwise you introduce weirdness into your network which may also differ depending on the hardware you are using.

To connect two switches with VLANs together, the ports between those switches must be Trunk ports (Cisco jargon for tagged ports).

For VLAN 1 and VLAN 2 to be able to talk to each other (assuming the correct setup of different subnets), a layer 3 instance (either router or a layer 3 switch) is required.

I assume this question was of theoretical nature? If not, can you explain why you want to simulate an extra layer 2 switch?

+ 4

Daniël van den Berg

8/1/24, 10:19 AM

So switch A receives ARP req on port 1. Sends it out on port 2. That cable is connected to port 47, so receives it on 47. Sends it out again on port 48, so switch B receives it. Right? Therefore your second paragraph is not applicable, right? It's indeed theoretical in that this setup won't see production, but practical in that I do physically have this setup laying here.

Jan

8/1/24, 10:51 AM

Yeah I get what you mean, I don't have a definitive answer to that, it might have to do with the mac address table on the switch itself even? I remember a Cisco Meraki router giving us trouble because we connected two ports (with one VLAN each) of the router to two ports on a switch. But the Cisco Meraki presented the same MAC address to the switch on both VLANs, causing network issues. Maybe its that or some other builtin mechanism of the switch

Esa Jokinen

8/1/24, 10:59 AM

Also examine the logs of the switch. Some switches have mechanisms to detect a cable (usually accidentally) connecting two ports on the same switch, and blocks the port.

Daniël van den Berg

8/6/24, 7:23 AM

It's a so called "L2+" switch. Every port has the same MAC, so I'm assuming that's the problem.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: VLAN - cable between two different vlans

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