Access layer design

refer to

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



STP Logical interfaces limitation

For Cisco 6500 series switches:

1,  HSRP should be limited to 500 per each aggregation switch

2, RSTP has logic interface limitation as 10000 while MTP has limitation as 50000. number of Logical interfaces = number of vlans * number of trunk port (etherchannel ports count individually) + no trunk port interfaces; Verify with “show spanningtree summary total”

The maximum logical interfaces for Per VLAN Spanning Tree Plus (PVST+) is 1800 for each module and 13,000 total for the switch. The show spanning-tree summary totals command displays the number of logical interfaces in the STP Active column.

The only way around this is to run Multiple Spanning Tree (MST) versus PVST, which has different limits:

  • PVST+ 13,000 total 1,800*/slot
  • RPVST+ 10,000 total 1,800*/slot
  • MST 50,000 total 6,000*/slot

Otherwise, pruning unnecessary VLANs from trunks is the best way to reduce the number of logical interfaces on a module or switch. But, regardless of STP mode, 10 Mbps, 10/100 Mbps, and 100 Mbps switching modules support a maximum of 1,200 logical interfaces per module.

For Nexus 7000

  • PVST+ RSTP 13,000 total, No per I/O module limit
  • MST 75000 total; No per I/O module limit