01-24-2012 12:10 AM - edited 03-07-2019 04:30 AM
I was wondering what pros / cons would there be if you have only one VLAN that must be shared between two switches and there is the choice of making that connection a Trunk or and Access port. For this discussion there is only the two switches on the LAN and while there are multiple VLANs only one of them appears on both devices (VLAN 808) with multiple access ports on each. I don't think the type of switch makes a difference but if there is a difference then that would be interesting to hear about.
Switch 1
VLAN 808
VLAN 200
VLAN 100
Switch 2
VLAN 808
VLAN 555
VLAN 777
An obvious answer might be to say "Use a trunk because its between two switches and you always use a trunk between switches" but I am looking for a deeper description on how a Trunk would be of benefit over an Access port in this situation. Does it even really matter which one is used?
Security?
Processing effort involved for each frame?
Bandwidth differences between the two methods?
If this LAN was larger and with more devices i.e. other switches and routers, would that make a difference?
Other considerations?
Thanks.
Patrick
01-24-2012 12:35 AM
Hi Patrick,
If only a single VLAN is to be shared between two switches then there is really no reason to have the interconnection run as trunk (even if locked down to allow only this single VLAN). Functionally, an access port and a trunk port in this situation would behave identically. A trunk with all VLANs allowed would allow frames from all VLANs to be passed to the other switch, however, the opposite switch would drop the frames for which there is no VLAN created on it.
Running inter-switch links as trunks even in simpler topologies is a matter of best practices. First, as these are managed switches, they must have a management VLAN assigned, and it is appropriate for this VLAN to be a separate VLAN not used for any other purposes, especially not for user data traffic. This would immediately neccessitate running a trunk even in your situation, as you would want two VLANs to span the trunk: 808 and this management VLAN. In addition, future network extensions and enlargements may require adding new shared VLAN. If the interconnection was an access mode link, its reconfiguration to trunk would cause a transient network outage - something that may not be possible to do outside a maintenance window.
My two cents on this...
Best regards,
Peter
01-24-2012 09:30 AM
Thank you for taking the time to reply. From what you wrote, besides best practices and future expansion an access port and a trunk with one VLAN is pretty much the same thing. Thanks.
04-19-2013 12:09 PM
VTP packets will only be transmitted over trunk ports but I'm assuming you wouldn't have VTP running between the two switches if you only want one VLAN to pass between them.
04-19-2013 01:14 PM
This is a good point. Thanks for your answer.
04-21-2013 02:04 PM
Hello Patrick,
trunk - frames will be sent with VLAN tag. If only one VLAN allowed, then only frames with allowed vlan tag will be sent
access - frames will be sent untagged
Best Regards
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