In the world of networking, choices often boil down to a trade-off between redundancy and complexity. One such debate revolves around using single-homed versus dual-homed Fabric Extenders (FEXes).
Understanding FEX Homing
A FEX is an extension of a parent switch, usually a Nexus series switch, and functions as a remote line card managed by the parent switch. In a single-homed setup, each FEX is connected to a single parent switch. The advantage of this approach lies in its simplicity, but it comes with a potential drawback: if the parent switch fails, the FEX will shut down its interfaces. However, servers equipped with multiple Network Interface Cards (NICs) using link aggregation (LAG) or teaming (either static or Link Aggregation Control Protocol – LACP) can counter this vulnerability. By allowing the servers themselves to handle redundancy, this configuration removes hidden complexity inherent in a dual-homed setup.
On the contrary, dual-homed FEXes, or Active-Active (AA) FEXes, connect to two parent switches. This offers better redundancy – in theory. If one parent switch fails, the other can continue to provide service. But this redundancy comes at a price: complexity and potential problems.
Control Plane and Data Plane in FEX Setups
In a single-homed FEX setup, the control plane – responsible for managing the FEX, including exchanging management and configuration data with the parent switch – has only one parent switch to communicate with, simplifying its tasks. The data plane, responsible for forwarding packets through the network, operates without significant changes, whether in a single-homed or dual-homed setup.
The challenge comes with dual-homed FEXes. Here, the control plane has to manage communications with two parent switches, adding a level of complexity and potential inconsistencies.
Port Suspension in Single-Homed and Dual-Homed FEX
Port suspension scenarios differ depending on your FEX setup. In a single-homed design, the main scenario where ports would be suspended is if the single parent switch goes offline or experiences a severe error. With no parent switch, the FEX goes offline, and its ports get suspended.
In a dual-homed FEX setup, ports may be suspended if the control planes of the two parent switches get out of sync, causing the FEX to suspend the affected interfaces to prevent potential issues like loops, misrouting, or traffic blackholing.
While dual-homed FEXes appear to provide superior redundancy, their added complexity, potential for control plane inconsistencies, and more numerous scenarios for port suspension lead many network professionals to favor the simplicity and predictability of single-homed FEXes. As with many networking decisions, understanding your environment’s specific needs and tolerances will help you make the right choice.