Cisco 3850 Stacking – How Many Stack Cables Are Needed to Stack Two 3850 Switches

Went trough the Dell documents and it doesn't explicitly say anything either way about stacking different platforms together. However the complete lack of documentation regarding stacking different devices together sends a pretty clear message that they are only intended to stack with devices of the same model/line.

More generally, most stacking solutions require that the the stacking technologies and the software versions be the same for switches to participate in a stack. I am not sure if you can run the same version of code for the two platforms, but clearly the stacking technologies are different. While there appear to be other differences in the stacking technologies as well, I will simply state that there is no way to connect the 10Gig HDMI stacking ports to the 1Gig RJ45 stacking ports.

You can keep the older 3448P switch and use it, but it will have to be configured and managed separately. The "stacking" ports on the 3448P should be usable 1Gig ports and can t hen be used to connect to your new switches as noted in the Dell documentation for the 34xx switches ("two Copper ports which can be used to forward traffic in a stand-alone device, or as stacking ports when the device is stacked"). Exact configuration of the devices would be entirely dependent on your environment (do you need tagged/trunk links, etc).

Btw, I can't find documentation of stacking for the 5448 (aside from the occasional use of the word "stack" and most of those had to do with dual stack IPv4/IPv6), so I am guessing that you meant to type 5548 which matches your description.

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