SFP Ports – Difference Between SFP/SFP+ Ports and Unified Ports

I have used a variety of SFP modules (including Ciena, Finisar, Cisco, Fiberstore.com...) in a Zyxel MGS-3712F without any issues.

What you are running into isn't a trunking problem. This behavior is what STP does in order to prevent loops. STP will create a loop-free layer-2 by sending BPDUs to determine the possible paths to the root bridge, and it selects one path and blocks the others.

If your switch stack is a true stack (appears to be a single switch), you should be able to use a LAG to fool STP into treating the two links as if they were one link. This will also balance traffic flows across the two links, and switch to the remaining link if one fails.

See the Reference Manual, page 117:

Link Aggregation Groups

Link aggregation groups (LAGs), which are also known as port channels, allow you to combine multiple full-duplex Ethernet links into a single logical link. Network devices treat the aggregation as if it were a single link, which increases fault tolerance and provides load sharing. You assign the LAG VLAN membership after you create a LAG. The LAG by default becomes a member of the default management VLAN (that is, VLAN 1).

A LAG interface can be either static or dynamic, but not both. All members of a LAG must participate in the same protocols. A static port channel interface does not require a partner system to be able to aggregate its member ports.

Static LAGs are supported. When a port is added to a LAG as a static member, it neither transmits nor receives LAGPDUs. The switch supports 26 LAGs. The LAGs menu contains the links described in the following sections.

• LAG Configuration on page 117
• LAG Membership on page 119
• LACP Configuration on page 120
• LACP Port Configuration on page 120