Technical information

June 2012 Dell EqualLogic Configuration Guide v13.2 54
Figure 23 Mixed speed redundant SAN using split interconnect between 1Gb and 10Gb switches
4.6.1.1 Optimizing Rapid Spanning Tree Protocol Behavior
The LAG between the 10Gb switches in Figure 23 creates a loop in the network. Rapid Spanning Tree
Protocol (RSTP) will compensate for this by blocking paths as necessary. The optimal spanning tree
strategy for this network is to prevent RSTP from blocking the inter-switch trunk between the 10Gb
switches, thus causing some 10Gb traffic to traverse the slower 1Gb switches. To accomplish this you
need to be aware of which switch is acting as the root bridge in the spanning tree. For the mixed
speed SAN design shown in Figure 23 the root bridge is 1Gb SW#1. Based on this information you can
assign a link cost to ensure that the desired link configuration is achieved. Note the location of the
RSTP blocked path in Figure 23. We manually assigned a high link cost to one of the 20Gb uplink LAGs
so that it became the RSTP blocked path.
4.6.1.2 You must use the straight uplink pattern when using non-stackable switches
If you are using switches that do not support a stacking mode then you must use the straight
interconnect uplink pattern shown in Figure 24. Note the following design differences between the
split uplink pattern in Figure 23 and the straight uplink pattern in Figure 24:
• A LAG is used to create the connection between 1Gb SW#1 and 1Gb SW#2.
• A high rapid spanning tree link cost is assigned to the 1Gb switch LAG (note the location of the
RSTP blocked path in Figure 24). Doing this prevents 10Gb inter-switch traffic from having to
pass through the 1Gb switch LAG.