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I have a Cisco 2960-CX switch mounted in a Mobile CT (hospital owned). This "trailer" moves between my 7 campuses as needed. Is there a way for the switch to obtain an IP address dynamically, over a trunk link, connecting to a different management network/vlan# at each campus?

I can only seem to get it to work if I preconfigure the switch with the proper vlan #. I'd like for the local vlan # not to matter. I've tried using the "native vlan" option on the trunk, but I believe this only affects inbound packets.

My backup plan would be to preconfigure the switch with a local vlan # for EACH campus, and add all of them to the trunk ahead of time, using the "ip address dhcp" in each.

The reason I need the "trunk" is I need to pass 3 vlans out to this switch. One for a PC/Printer/access point, another for VoIP and a third for the CT (Layer 2 extended from the firewall).

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  • It has to be configured for the correct VLAN number or use the native VLAN for the management. – Ron Maupin Oct 8 '20 at 12:54
  • Having dealt with "other" vendor hardware in the past, I know on HP Procurves I can "tag" and "untag" on the same port. I wasn't sure if there was a equivalent command set for Cisco – alsterb Oct 8 '20 at 13:05
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    If the site is tagging the VLAN, then it is not the native VLAN, and you must tag on the switch. The native VLAN is the untagged VLAN, and you are only allowed one untagged (native) VLAN on a trunk, regardless of the vendor or switch model. – Ron Maupin Oct 8 '20 at 13:13
  • Is "native vlan" only on inbound packets or will it strip outbound packets with that tag # on it? – alsterb Oct 8 '20 at 13:30
  • The native VLAN is the VLAN that is not tagged on a trunk. The tags separate the VLANs on a trunk, and one VLAN can be untagged and still be separate from the other VLANs. A Native VLAN will not have tags on the frames, either in or out. – Ron Maupin Oct 8 '20 at 13:32
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Is there a way for the switch to obtain an IP address dynamically, over a trunk link, connecting to a different management network/vlan# at each campus?

You cannot automatically adapt a specific management network VLAN ID on the switch to some external network. However, you can configure multiple management networks with their respective IP address (SVI) on the 2960. Each SVI can in turn be configured by DHCP or statically.

Those VLANs not configured/working on the currently used downlink simply won't work - static addresses can't be connected to, DHCP discoveries time out. Of course, your switch will use a different management IP address, depending on the current location/trunk.

Just make sure that your management networks VLAN IDs don't collide with other VLANs you've got.

A completely different approach would be to use inner and outer VLAN tags ("QinQ" from 802.1ad) to allow you to use a seperate VLAN layer for management. That would require (at least) the uplinkes switch to support QinQ and of course, an appropriate configuration.

My preferred solution would be somewhat different: I'd seriously try to harmonize the potential downlinks to use the same VLAN config throughout, regardless of location.

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Just to clarify, the native VLAN is untagged. Any untagged traffic arriving on the port belongs to that VLAN. Any traffic for that VLAN sent by the switch will have no tag. If the port receives a tagged frame for the native VLAN it will remove the tag -- depending on the firmware version, and as long as it's a trunk port.

The Cisco UI will only allow one untagged VLAN per port. (but the hardware can do it.) Other vendors may allow the logically impossible multiple untagged VLANs per port. (don't ask!)

Also, keep in mind CDP/LLDP can cause a port to be err-disabled if the port configuration does not agree with the link partner.

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