Given the diagram below, does it make sense to trunk between the L2 access switches? Otherwise, all VLAN 10 and VLAN 20 broadcast domain traffic has to go through the L3 distribution switch.

Thick blue lines are trunks from L2 AS to L3 DS. Red dashed lines are proposed trunks between L2 AS.

enter image description here

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 17 '17 at 3:41

Spanning tree will make sure that all traffic from one switch to another switch only has a single path, and that is through the root switch, which really needs to be the distribution switch. The best that you could hope for is that your dashed red lines end up being redundant paths that kick in only when the link to the distribution switch fails.

STP guarantees a loop-free path. The way it does that is to create a single path that is most direct to the root switch. All other paths are placed in blocking. If you had multiple paths that allowed layer-2 loops, then your network would crash with broadcast storms.

A best practice is to only allow a VLAN on a single switch. You can have multiple VLANs on a switch, but they should not extend to other access switches. The helps eliminate the possibility of STP problems. It is no longer necessary to have everything connected to a layer-2 network. We live in a layer-3 world, and routing has become cheap, and it introduces security points and eliminates STP problems. It used to be, "Switch where you can, route where you must," but that is no longer the case.

  • "A best practice is to only allow a VLAN on a single switch. You can have multiple VLANs on a switch, but they should not extend to other access switches." Given a scenario where you have a few switches in a warehouse that are trunked over fiber back to your main equipment room, you would have more than one management VLAN? At that warehouse there are 4 VLANS - 3 of which are shared with the main office: Management, Printers, and Users. – Xevious Mar 4 '17 at 16:12
  • The point is that you do not really need to share user VLANs across the network. A user on one network can print to a printer on a different network. Traffic can be routed at layer-3, rather than only switched at layer-2. You could have a single device management VLAN to all your switches. It is unlikely that you have any interfaces on the device management VLAN (just logical device interfaces) that could accidentally be connected together to form an STP loop, and there really will not be much traffic on a device management VLAN. – Ron Maupin Mar 4 '17 at 16:25

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