Solved: VLAN Trunking over backup link

martens72 · ‎01-19-2011

We have two 3560 acting as layer 2 access switches. Both are uplinked to a root 3750 switch stack. I am wondering if a backup link can be implemented between the two access switches, for purposes of failover. I included a diagram, because although I am sure STP will block the port, I am not sure of behavior if an uplink fails.

We run VTP transparent mode, and no SVI on the access switches - the vlans are trunked to root. In the diagram, if Trunk A goes down, will Switch B forward VLAN 10 traffic (from Trunk C) to root, even though it has no ports assigned to vlan 10, and no vlan database entry for 10?

Hopefully this is very straightforward question

Jon Marshall · ‎01-19-2011

martens72 wrote:

We have two 3560 acting as layer 2 access switches. Both are uplinked to a root 3750 switch stack. I am wondering if a backup link can be implemented between the two access switches, for purposes of failover. I included a diagram, because although I am sure STP will block the port, I am not sure of behavior if an uplink fails.

We run VTP transparent mode, and no SVI on the access switches - the vlans are trunked to root. In the diagram, if Trunk A goes down, will Switch B forward VLAN 10 traffic (from Trunk C) to root, even though it has no ports assigned to vlan 10, and no vlan database entry for 10?

Hopefully this is very straightforward question

Michael

If the 3750 is a switch stack then you can run etherchannel from each 3560 and make sure the physical connections in the etherchannel are spread across the switch stack members. This would give you the redundancy you need without having to interconnect the 3560 switches and would be a better solution because all members of the etherchannel would be forwarding ie. no STP blocking any of the individual links.

If you still wanted to interconnect your 3560 switches then you would need each 3560 to have both vlans on it. And you would also need to make sure that STP blocked the interconnect and not one of the direct links or else you could overload one of the uplinks. I would only do this if -

1) the trunk links currently are single connections

and

2) you literally only have one spare fibre connection

otherwise i would as previously suggested use etherchannels from the 3560s to the 3750 switch stack.

Jon

View solution in original post

Jon Marshall · ‎01-19-2011

martens72 wrote:

We have two 3560 acting as layer 2 access switches. Both are uplinked to a root 3750 switch stack. I am wondering if a backup link can be implemented between the two access switches, for purposes of failover. I included a diagram, because although I am sure STP will block the port, I am not sure of behavior if an uplink fails.

We run VTP transparent mode, and no SVI on the access switches - the vlans are trunked to root. In the diagram, if Trunk A goes down, will Switch B forward VLAN 10 traffic (from Trunk C) to root, even though it has no ports assigned to vlan 10, and no vlan database entry for 10?

Hopefully this is very straightforward question

Michael

If the 3750 is a switch stack then you can run etherchannel from each 3560 and make sure the physical connections in the etherchannel are spread across the switch stack members. This would give you the redundancy you need without having to interconnect the 3560 switches and would be a better solution because all members of the etherchannel would be forwarding ie. no STP blocking any of the individual links.

If you still wanted to interconnect your 3560 switches then you would need each 3560 to have both vlans on it. And you would also need to make sure that STP blocked the interconnect and not one of the direct links or else you could overload one of the uplinks. I would only do this if -

1) the trunk links currently are single connections

and

2) you literally only have one spare fibre connection

otherwise i would as previously suggested use etherchannels from the 3560s to the 3750 switch stack.

Jon

martens72 · ‎01-19-2011

thanks for the quick reply Jon. This is a temporary setup, and the thought was to add additional redundancy as overkill. I had a feeling the vlan would stop at Switch B.

Just for sake of knowledge, would this setup work if VTP were enabled, and the access switches were clients, with root as server?

Also, would an SVI with no Ip address on Switch B serve to inform the switch of vlan 10? Or does a physical port have to be actually assigned to the vlan?

Thanks in advance

Jon Marshall · ‎01-19-2011

martens72 wrote:

thanks for the quick reply Jon. This is a temporary setup, and the thought was to add additional redundancy as overkill. I had a feeling the vlan would stop at Switch B.

Just for sake of knowledge, would this setup work if VTP were enabled, and the access switches were clients, with root as server?

Also, would an SVI with no Ip address on Switch B serve to inform the switch of vlan 10? Or does a physical port have to be actually assigned to the vlan?

Thanks in advance

Michael

Yes, with VTP server/client it would work because all vlans would be on all switches.

You don't need an SVI on the switch nor do you need an access port on the switch in that vlan. You simply need that vlan in the vlan database. If it is there then a trunk link allowing that vlan would be fine.

Jon