Explanation/Reference:
Section: Enterprise Network Design Explanation
Explanation:
Loop-free inverted U designs support all service module installations, have all uplinks active, and support virtual LAN (VLAN) extensions. A service module is a piece of hardware that extends the functionality of a Cisco device, for example, the Secure Sockets Layer (SSL) Service Module for Catalyst 6500 series switches and Cisco 7600 series routers performs the majority of the CPU-intensive SSL processing so that the switch's processor or router's processor is not burdened by large numbers of SSL connections. 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 Spanning Tree Protocol (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.
Loop-free U designs do not support VLAN extensions, have all uplinks active, and support all service module implementations. Loop-free U designs offer 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 VLAN beyond an individual access layer pair would create a loop. 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.
Flex Link designs have a single active uplink, support VLAN extensions and all service modules, and disable STP by default. 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 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.
Reference:
Cisco: Data Center Access Layer Design