We're in the process of installing multiple Dell MX7000 Chassis that uplink at 100Gb to a pair of Dell S5232F-ON Switches. We use VLT to interlink the S5232 Switches. Above the S5232 Switches are currently a pair of Cisco 9500 10Gb switches on the same L2 that aren't LAG'd/Stacked. Is there any way I can uplink both of the S5232 Switches to both of the Cisco Switches for redundancy without creating loops? Eventually we'll only have L3 between the Switch layers and then the problem will go away!
2 Answers
The basic approach is to use redundant links with a spanning tree protocol that blocks bridge loops. RSTP (IEEE 802.1w) is the easiest to configure, just choose a reasonable root bridge with the lowest priority number and a failover root with the second-lowest priority number. However, you can't utilize blocked links in any way, they're just for redundancy. Make sure you configure VLAN trunks accordingly as RSTP is completely VLAN agnostic.
MSTP (IEEE 802.1s) is the next better. It allows you to configure multiple instances with their own root bridge or root ports. Grouping VLANs into those instances allows you to utilize more trunk links otherwise blocked.
Proprietary STP variants like Cisco's RPVST+ require support in each participating device. With a mixed setup you need to be careful where you use it.
Another approach is to use aggregates links. With multiple uplinks that requires a form of stacking or multi-chassis link aggregation (MC-LAG or MLAG) on the side of the uplink switches. There's no open standard for that unfortunately, so you can only stack/aggregate compatible devices.
Yet another way is shortest path bridging (SPB) from IEEE 802.1aq which more or less combines all of the above by using a routing algorithm on the data link layer. Sadly, there's still rather limited support for SPB.
Rapid spanning-tree. You can link from the dell switches to the Cisco ones and make sure the cisco ones have a higher STP root priority (lower number configured) so they will remain the root bridge. Assuming those cisco switches are already linked to each other, the dell switches will see that and one link or the other will be blocked on STP until/unless the other link goes down.
Alternately you can start off with layer 3 configuration or setup the Cisco ones to support VPC or some other multi-chassis LAG design.
