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Amateur network admin here. I'm trying to create a link via VLAN between a VOIP PBX and an internet router. It's a large installation, so there are 4 managed switches between the two endpoints. I created a VLAN on each switch (3 HPs, 1 Cisco) and set a port on each end switch as untagged for that VLAN, plugging the PBX and the router into those two ports respectively. And of course, I marked the traffic on each trunk as untagged for the same VLAN.

What I want to know is this: there is another switch on the network that doesn't have this VLAN configured but still manages to ping[1] the router (which has its own specific internal IP). How is this possible? Is it passing by the management VLAN to get to the switch where the router is plugged in?

The only rational explanation I can think of is that a switch, when it pings, talks directly to the other switches and then they each ping individually on all their VLANs.

[1] : when say "it pings" I mean that I cannot on the command line to the switch and use it's internal ping command.

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I would tell the switch used the Trunk to send the ping, didn't tagged the ping packet, and the remote switch received the ping via the Trunk.

You could do a simple packet capture to confirm my theroy with a mirror port on the Trunk.

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