Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
Spanning Tree Protocol (STP) is disabled by default in Flex Link designs. STP prevents switching loops on a network. Switching loops can occur when there is more than one switched path to a destination. The spanning tree algorithm determines the best path through a switched network, and any ports that create redundant paths are blocked. If the best path becomes unavailable, the network topology is recalculated and the port connected to the next best path is unblocked. There are no loops in a Flex Link design, and STP is disabled when a device is configured to participate in a Flex Link. Interface uplinks in this topology are configured in active/standby pairs, and each device can only belong to a single Flex Link pair. In the event of an uplink failure, the standby link becomes active and takes over, thereby offering redundancy when an access layer uplink fails. Possible disadvantages of the Flex Link design include its inability to return to the original state after a failed link is recovered, its increased convergence time over other designs, and its inability to run STP in order to block redundant paths that might be created by inadvertent errors in cabling or configuration.
STP is not disabled by default in loop-free inverted U designs. Loop-free inverted U designs offer redundancy at the aggregation layer, not the access layer? therefore, traffic will black-hole upon failure of an access switch uplink. All uplinks are active with no looping, thus there is no STP blocking by default.
However, STP is still essential so that redundant paths that might be created by any inadvertent errors in cabling or configuration are blocked.
STP is not disabled by default in loop-free U designs. This topology offers a redundant link between access layer switches as well as a redundant link at the aggregation layer. Because of the redundant path in both layers, extending a virtual LAN (VLAN) beyond an individual access layer pair would create a loop?
therefore, loop-free U designs cannot support VLAN extensions. Like loop-free inverted U designs, loop- free U designs also run STP and have issues with traffic being black-holed upon failure of an access switch uplink.
STP is not disabled by default in looped triangle designs. A looped triangle design can provide deterministic convergence in the event of a link failure. In a triangle design, each access layer device has direct paths to redundant aggregation layer devices. The ability to recover from a failed link in this design is granted by redundant physical connections that are blocked by Rapid STP (RSTP) until the primary connection fails. RSTP is an evolution of STP that provides faster convergence. RSTP achieves this by merging the disabled, blocking, and listening states into a single state, called the discarding port state.
With fewer port states to transition through, convergence is faster. A looped triangle topology is currently the most common design in enterprise data centers.
STP is not disabled by default in looped square designs. Like a looped triangle, a looped square design can provide deterministic convergence through redundant connections. However, the difference between the two is that in a looped square the redundant link exists between the access layer devices themselves, whereas in a looped triangle the redundant link exists between the access layer devices and the aggregation layer devices. In a looped square, the connection between the access layer devices is blocked by STP until a primary link failure occurs.
Reference:
Cisco: Data Center Access Layer Design: FlexLinks Access Model