You have a couple options. 1.  You can simply connect the cables between the switch stack of M6220 and the 5548.  Set up a trunk connection allowing the needed VLANs across.  Spanning tree as long as it is enabled on the switch and not turned off on the specific ports connecting the switches will by default prevent loops and allow the most cost effective link to be active leaving the second connection as a alternate link in a blocking state. 2.  You can set up LAG ( Link Aggregation).  This is where you set up the individual physical ports into a virtual port-channel.  This tells spanning tree to treat the multiple physical ports as a single aggregated link to the other switch.  You will need to have matching port channels on either side of the connection. ( On each switch).  This allows the same speed connection but more throughput.  Allowing more traffic to pass.
I need to connect two powerconnect M6220 to the existent lan infrastracture. The two switch are configured in stack. Which is the best way to uplink the new switch to the other two powerconnect 5548. The old switch are not stacked, and linked among them with two 10GB Ethernet cable (one of the link is disabled).  I need to implement fault tolerant network, maybe i need to configure the spanning tree on the two 10 GB link. Could I use  Spanning tree to configure the uplink between the old and the new, too? There is another better solution? there is another way to prevent network loop?  thanks, Francesco
connect m6220 stacked switches to existent Lan infrastracture