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Can you use SFP stacking with trunking?

We have two Prosafe S3300-28x switches stacked using two SFP cables. These switches also have two 10Gig ports, which we would like to connect to a separate Prosafe XS708E 10Gig switch.

The idea being that all the 10Gig traffic happens across the 10Gig switch, but it still has access to the rest of the network via the trunked 10Gig connections.

Diagram:

Prosafe S3300-28X -----10Gig trunk 1-----
    |      |                            |
   SFP    SFP                    Prosafe XS708E
    |      |                            |
Prosafe S3300-28X -----10Gig trunk 2-----

Each 1Gig host has a connection to each S3300, and each 10Gig host has a connection to the 10Gig switch as well as two connections to the S3300 stack. Essentially, we are looking for complete redundancy on the 1Gig, not too fussed about the 10Gig, that's more of a nice-ty, but would still like it to work in case one of the S3300's dies.

The problem we have is that it appears as if the 10Gig trunk 2 (when plugged in) causes a loop in the network, and one of the trunks promptly gets disabled. What I want to know is how can I connect these three switches, with a trunk, to be a fail over?

Our thought was that the trunking should handle the looping, but it seems like it doesn't. Is there something else we should be doing instead of trunking?

  • Did either answer actually answer your question? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for and answer. – Ron Maupin Dec 9 '16 at 1:00
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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
| improve this answer | |
  • Yes, I didn't say but the two S3300's are stacked, so adding the LAG you suggested works a treat. – KolonUK Jul 12 '17 at 15:08
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I agree with Ron. The setup that you gave is very similar to what I've created.

I have 3 Dell PowerConnect 5548 switches stacked using HDMI cables, and two Dell N3024 switches, which are in turn stacked using a QSFP cable. The two N3024 switches have a QSFP break out cable in each, which runs off to various other devices, but four of the cables connect back to the Dell PowerConnects with SFP+ cables.

Using LACP, I've bonded the cables together, meaning the switches can use the entire bandwidth of 40Gbps. If a cable is accidentally removed or fails, then the LAG won't break, but of course the bandwidth goes down.

I hope that helps!

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