refer to
https://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Data_Center/DC_Infra2_5/DCInfra_6.html
1, Looped triangle
The triangle looped topology is currently the most widely implemented in the enterprise data center. This topology provides a deterministic design that makes it easy to troubleshoot while providing a high level of flexibility
2, looped square
The square-based looped topology is not as common today in the enterprise data center but has recently gained more interest. The square looped topology increases the access layer switch density when compared to a triangle loop topology while retaining the same loop topology characteristics. This becomes particularly important when 10GE uplinks are used. This topology is very similar to the triangle loop topology, with differences in where spanning tree blocking occurs
Spanning tree blocks the link between the access layer switches, with the lowest cost path to root being via the uplinks to the aggregation switches, as shown in Figure 6-9. This allows both uplinks to be active to the aggregation layer switches while providing a backup path in the event of an uplink failure. The backup path can also be a lower bandwidth path because it is used only in a backup situation. This might also permit configurations such as 10GE uplinks with GEC backup.
The possible disadvantages of the square loop design relate to inter-switch link use, because 50 percent of access layer traffic might cross the inter-switch link to reach the default gateway/active service module. There can also be degradation in performance in the event of an uplink failure because, in this case, the oversubscription ratio doubles.
3, Loop free U
4, Loop free invented U
5,flexlinks
