First part of this discussion: http://en.community.dell.com/techcenter/b/techcenter/archive/2013/01/07/blade-server-topology-options-with-external-top-of-rack-switches-for-equallogic-arrays-part-1.aspx
In the second part of this discussion, I would like to highlight some of the possible blade server switch topologies with ToR using stack, Link Aggregation Group (LAG), or Virtual Link Trunking (VLT) as switch interconnects. I will also highlight whether the topologies are ideal for internal, external arrays, or both.
Possible Stacking Topologies:
With most stacking implementations, switch firmware upgrades require restarting the switches in the stack. In the Dual Stack (top left) configuration, the blade IOM switches are stacked to each other and the ToR switches are stacked to each other. A single LAG inter-connects these 2 stacks. In the Single Stack (top right) configuration, only the ToRs are stacked and each blade IOM has a LAG to the stack. In the second Single Stack (bottom left) configuration, only Blade IOMs are stacked and each ToR has a LAG into the stack.
Note: It is recommended to have the internal arrays in a separate EqualLogic pool from the External arrays. This avoids the two hops needed for internal and external arrays in same pool to communicate. If the SI configuration is good for a particular configuration, it implies that the switch hop count between internal arrays and/or between external arrays in same pool is at the minimum possible value of 1. The minimal hop count between arrays in a pool keeps the latency for inter-array network traffic within a pool to a minimum.
Possible LAG Topologies:
A particular LAG interconnect can be intentionally blocked with Spanning Tree Protocol (STP) by manually setting a high link cost. If another LAG incurs failure, this LAG would become active after STP re-convergence.
Possible VLT Topologies:
A pair of ToR switches can be set up in a VLT domain with the VLT interconnect (VLTI) between them. Blade IOM switches can have a single LAG link that span across the VLT domain switches. The VLT domain appears as a single LAG destination for the blade switches. If the blade IOM switches are stacked as in the configuration on the left, then a single VLT LAG connects the stack to the VLTI domain pair. If the blade IOM switches are not stacked, then each blade IOM has a VLT LAG going into the VLTI domain pair.
In summary, various factors need to be considered in designing a blade server network topology for connecting to external storage. These include switch interconnect technologies and their associated benefits or limitations for high availability, scalability, manageability and performance. The presence of blade chassis internal arrays in addition to external arrays also impacts the design.
My colleagues Clay Cooper and Guy Westbrook are publishing a series of white papers on detailed test studies and best practices on blade connectivity topologies for EqualLogic:
- http://en.community.dell.com/techcenter/storage/w/wiki/4137.dell-poweredge-m1000e-blade-and-equallogic-ps-series-san-design-best-practices-by-sis.aspx
- http://en.community.dell.com/techcenter/storage/w/wiki/3893.san-design-best-practices-for-the-dell-poweredge-m1000e-blade-enclosure-and-equallogic-ps-series-storage-1gbe-by-sis.aspx
My team and our publications: http://en.community.dell.com/techcenter/storage/w/wiki/2631.storage-infrastructure-and-solutions-team.aspx