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Why Leaf-Spine Architecture and How to Design It?

Why Leaf-Spine Architecture and How to Design It?

With the demand for higher bandwidth, faster speeds and optimal applications, equipment architecture trends have changed from traditional 3-tier architecture model to leaf-spine architecture in existing data centers. 3-tier architecture vs. leaf-spine architecture: which is better? What should be considered when deploying one network architecture over the other?

3-Tier Architecture vs. Leaf-Spine Architecture

The traditional 3-tier architecture model is designed for use in general networks, usually segmented into pods which constrained the location of devices such as virtual servers. 3-tier architecture consists of core switches, aggregation/distribution switches and access switches. These devices are interconnected by pathways for redundancy which can create loops in the network. As part of the design, a protocol (Spanning Tree) that prevents looped paths is implemented. However, doing so deactivates all but the primary route. A backup path is then only brought up and utilized when the active path experiences an outage. The 3-tier architecture comes with a number of disadvantages, such as higher latency and higher energy requirements.

3-tier architectureLeaf-spine architecture, also called fat-tree architecture, is one of the emerging switch architectures that are quickly replacing traditional solutions. It features multiple connections between interconnection switches (spine switches) and access switches (leaf switches) to support high-performance computer clustering. The role of the leaf is to provide connectivity to the endpoints in the network, including compute servers and storage devices or any other networking endpoints—physical or virtual, while the role of the spine is to provide interconnectivity between the leafs. The requirements applying to the leaf-spine topology include the following:

  • Each leaf connects to all spines in the network.
  • The spines are not interconnected with each other.
  • The leafs are not interconnected with each other for data-plane purposes. (The leafs may be interconnected for control-plane operations such as forming a server-facing vLAG.)

leaf-spine architecture

Compared to the traditional 3-tier architecture, the leaf-spine architecture design drastically simplifies cabling needs, especially when looking at fiber optic connectivity. But some design considerations for leaf-spine architecture should be considered, which will be introduce in the next part.

Design Considerations for Leaf-Spine Architecture

Oversubscription Ratios — Oversubscription is the ratio of contention should all devices send traffic at the same time. It can be measured in a north/south direction (traffic entering/leavign a data center) as well as east/west (traffic between devices in the data center). Current modern network designs have oversubscription ratios of 3:1 or less, which is measured as the ration of downlink ports (to servers/storage) to uplink ports (to spine switches). For example, a 64-port leaf switch should equate to 48 ports down to 16 ports up.

Leaf and Spine Scale — As the endpoints in the network connect only to the leaf switches, the number of leaf switches in the network depends on the number of interfaces required to connect all the endpoints. The port count requirement should also account for multihomed endpoints. Because each leaf switch connects to all spines, the port density on the spine switch determines the maximum number of leaf switches in the topology. A higher oversubscription ratio at the leafs reduces the leaf scale requirements, as well.

The number of spine switches in the network is governed by a combination of the throughput required between the leaf switches, the number of redundant/ECMP paths between the leafs, and the port density in the spine switches. Higher throughput in the uplinks from the leaf switches to the spine switches can be achieved by increasing the number of spine switches or bundling the uplinks together in port-channel interfaces between the leafs and the spines.

10G/40G/100G Uplinks From Leaf to Spine — For a leaf-spine network, the uplinks from leaf to spine are typically 10G or 40G and can migrate over time from a starting point of 10G (N x 10G) to become 40G (N x 40G). An ideal scenario always has the uplinks operating at a faster speed than downlinks, in order to ensure there isn’t any blocking due to micro-bursts of one host bursting at line-rate.

Layer 2 or Layer 3 — Two-tier leaf-spine networks can be built at either layer 2 (VLAN everywhere) or layer 3 (subnets). Layer 2 designs allow the most flexibility allowing VLANs to span everywhere and MAC addresses to migrate anywhere. Layer 3 designs provide the fastest convergence times and the largest scale with fan-out with ECMP (equal cost multi pathing) supporting up to 32 or more active spine switches.


It is important to understand the 2-tier leaf-spine architecture as it offer unique benefits over the traditional 3-tier architecture model. With easily adaptable configurations and design, leaf-spine has improved the IT department’s management of oversubscription and scalability. Deploying leaf-spine network architecture and buying high-performance data center switches are imperative for data center managers as leaf-spine environment allows data centers to thrive while accomplishing all needs and wants of the business.

Proper Cabling Solutions for PoE Network

Proper Cabling Solutions for PoE Network

Ethernet cables

By running power and data transmission over a single Ethernet cable, PoE (Power over Ethernet) has found success across a variety of applications such as IP surveillance cameras, IP phones and wireless access points. However, without the right cabling and network design in place, PoE can encounter cable heating and connectivity issues that may adversely affect performance. So in this post, some cabling recommendations for PoE will be listed for your reference.

working principle of PoE switch

Issues Affect PoE Performance

Heat generation in cable bundles is one of the biggest issues that affect PoE performance. When power is added to balanced twisted-pair cabling, the copper conductors generate heat and temperatures rise. High temperatures will lead to higher insertion loss, and in turn shorter permissible cable lengths. It can also increase bit error rates, and create higher power costs due to more power dissipated in the cabling.

Cabling Recommendations for PoE

Some cabling recommendations for PoE are suggested to help lower cabling temperature.

Use Higher Category Cabling

Higher category-rated cable typically means larger gauge sizes, and as power currents increase, these larger conductors will perform better than smaller cable. Generally, higher category cabling will be necessary to minimize temperature increases while supporting PDs that require more power.

Reduce the Number of Cables per Bundle

If cables are bundled or closely grouped with other cables, cables near the center of the bundle have difficulty radiating heat out into the environment. Therefore, the cables in the middle of the bundle heat up more than those toward the outer layers of the bundle. Separating large cable bundles into smaller bundles or avoiding tight bundles will reduce temperature rise.

Design Pathways to Support Airflow

Enclosed conduit can contribute to heat issues. When possible, using ventilated cable trays would get better airflow. Open mesh cable trays and ladder racks will improve heat dissipation and create more opportunities for loosely grouping cables instead of tight bundling.

Cat 5e vs. Cat 6a: Which Is Better for PoE Cabling?

The type of cabling selected can make a big difference in terms of how heat inside the cable is managed, and how it impacts performance. Typically, Cat 5e and Cat 6a cable can be used to support PoE devices. But it’s better to use Cat 6a for PoE cabling.

With larger-gauge diameter, Cat 6a can reduce resistance and keep power waste to a minimum as it has a lower temperature increase compared to smaller-gauge Cat 5e. This better performance will provide additional flexibility, including larger bundle sizes, closed installation conditions and higher ambient temperatures. For instance, when comparing 23-gauge and 24-gauge cabling, there is a large variance in how power is handled. As much as 20% of the power through the cable can get “lost” in a 24-gauge Cat 5e cable, leading to inefficiency. In addition, less power is dissipated in a 23-gauge Cat 6a cable, which means that more of the power being transferred through the cable is actually being used, improving energy efficiency and lowering operating costs.

FS PoE Switches & Ethernet Cables Solution

FS offers fully managed PoE Gigabit switches, which delivers robust performance and intelligent switching for growing networks. Available with 8, 24, or 48 PoE Gigabit Ethernet ports, the model details of our PoE switches are listed below. Among them, the PS130-8 and PS400-24 are PoE switches, while PS650-48, PS250-8 and PS650-24 are PoE+ switches. Reliable & economical, our PoE switches are ideal for SME networks and can expand your network much more easily than ever.

FS PoE switches specification

Besides PoE, we also have various types of Ethernet cables including Cat 6a, Cat 6, Cat 5e and Cat 7 Ethernet patch cables. Most of them are in large stock and multiple cable colors are available. For more details, please visit

Difference Between Passive and Active Twinax Cable Assembly

Difference Between Passive and Active Twinax Cable Assembly

Optical fiber cabling had gone through rapid development over recent years and maintained its leading role in telecom field. While twinax cable still remained a good way to access the networking industry trends over the last three decades and presented the highest longevity among all media. Twinax cable (see in following Figure) is a type of cable similar to coaxial cable that has two inner conductors instead of one. And owing to its cost efficiency, it is commonly used in short-range high-speed differential signaling applications. Currently there is a twinax cable which comes in either passive or active copper cable. So what is the difference between them? Today’s passage will provide a satisfying solution to you.

Twinax cable

Describing Passive and Active Twinax Cable
A passive twinax cable carries a signal over short lengths (5m or under) of copper with no additional components to boost signal. While an active copper cable contains electrical components in the connectors that boost signal levels. This makes active copper cables a little more expensive than passive copper cables; however, they can connect the Converged Network Adapter (CNA) to a top-of-the-rack switch over longer distances than passive copper cables.

Why Implement Active Over Passive and Vice Versa?
Length and signal strength are always two important factors you should look into when requiring a cable for an application. Typically, we can see passive twinax cables being used between the server and the Top of Rack (ToR) switch. The upside in this configuration is that the passive twinax cabling connection is much cheaper than the cost of an optical link. The downside is that you are limited in distance and there’s also some cable interoperability issue you’ll need to deal with. Passive twinax cables are rated for ranges up to 5m and provide a good working solutions at a great cost.

When the distance between connection points exceeds 5m, it is highly recommended to use active cables to ensure signal is transferred all the way through. The downside is that they are more expensive and use more power. The upside is that you don’t have to worry about distance (up to 300 meters) and, perhaps more importantly, you don’t have to worry about which vendor’s cable you use and the signal is improved and gives peace of mind by creating a trustworthy connection. In regards to active versus passive twinax cables, it depends on what you are connecting together.

QSFP+ Copper Cables—A Cost-effective Application of  Twinax Cable
QSFP+ direct attach copper cable assemblies offer a highly cost-effective way to establish a 40 Gigabit link between QSFP+ ports of QSFP+ switches within racks and across adjacent racks. QSFP+ Copper Cable is an extension of the established interface system SFP+ that is mainly used in short distance. 40G QSFP+ to 4SFP+ copper breakout cable and QSFP to QSFP copper direct attach cable are the two common types of 40G QSFP+ Copper cables.

QSFP to 4SFP+ copper breakout cables are suitable for very short distances and offer a very cost-effective way to connect within racks and across adjacent racks. Take QSFP-4SFP10G-CU1M as an example, this breakout cable connects a 40G QSFP port and four 10G SFP+ ports of Cisco switches and operates at a link length of 1m. While a QSFP+ to QSFP+ passive copper cable consists of a cable assembly that connects directly into two QSFP+ modules, one at each end of the cable. This cable use integrated duplex serial data links for bidirectional communication and is designed for data rates up to 40Gbps. There are various QSFP+ to QSFP+ passive copper cables branded by famous brands, like Cisco, HP, Juniper, Brocade, etc. The following picture shows a Cisco QSFP-H40G-CU3M Compatible QSFP+ to QSFP+ passive copper cable.

Cisco QSFP-H40G-CU3M

There isn’t a truly visual way to tell the difference between active and passive twinax cables. Therefore when you are requiring a twinax cables, please follow the instructions that I have listed above or you should ask your vendors for expertise suggestion. Fiberstore offers a large variety of SFP+ Twinax cables and QSFP+ cables that are well tested and compatible with major brand. If you have any inquiry of our products, please feel free to contact us.

Cost Difference Between MMF and SMF Optics

Cost Difference Between MMF and SMF Optics

Whether to deploy single-mode fiber (SMF) or multimode fiber (MMF) optics is still a hot topic in the industry. From 1 Gigabit Ethernet to 100 Gigabit Ethernet, people look forward to receiving more bandwidth, higher speed in a more cost-effective cabling manner. The cost, as one of the key considerations, is highly concerned by many data center operators.

SMF vs MMF: Optics Cost Comparison

Optics for SMF vs MMF are usually a bit more expensive. Take FS.COM for example, the following table lists the list prices of Cisco compatible SMF optics and MMF optics from 1Gbps to 100Gbps. With the increase of speeds, the gap between the optics price is great.

1Gbps Optics
Multimode SFP Cisco GLC-SX-MMD Compatible 1000BASE-SX SFP 850nm 550m DOM Transceiver US$ 6.00
Single-mode SFP Cisco GLC-LH-SMD Compatible 1000BASE-LX/LH SFP 1310nm 10km DOM Transceiver US$ 7.00
SMF SFP vs MMF SFP LH vs SX US$ 1.00
10Gbps Optics
Multimode SFP+ Cisco SFP-10G-SR Compatible 10GBASE-SR SFP+ 850nm 300m DOM Transceiver US$ 16.00
Single-mode SFP+ Cisco SFP-10G-LR Compatible 10GBASE-LR SFP+ 1310nm 10km DOM Transceiver US$ 34.00
SMF SFP+ vs MMF SFP+ LR vs SR US$ 18.00
40Gbps Optics
Multimode QSFP+ Cisco QSFP-40G-SR4 Compatible 40GBASE-SR4 QSFP+ 850nm 150m MTP/MPO DOM Transceiver US$ 55.00
Single-mode QSFP+ Cisco QSFP-40G-LR4 Compatible 40GBASE-LR4 and OTU3 QSFP+ 1310nm 10km LC DOM Transceiver US$ 340.00
SMF QSFP+ vs MMF QSFP+ LR4 vs SR4 US$ 285.00
100Gbps Optics
Multimode QSFP28 QSFP28 Cisco QSFP-100G-SR4-S Compatible 100GBASE-SR4 850nm 100m Transceiver US$ 400.00
Single-mode QSFP28 QSFP28 Cisco QSFP-100G-LR4-S 100GBASE-LR4 1310nm 10km Compatible Transceiver US$ 2800.00
SMF QSFP28 vs MMF QSFP28 LR4 vs SR4 US$ 2400.00
Note: 100G PSM4 transceiver for up to 500m costs US$ 750.00 (SR4 vs PSM4: US$ 350.00). 100G CWDM4 transceiver for up to 2km costs US$ 1350.00 (SR4 vs CWDM4: US$ 950.00).


SMF vs MMF: Cable Cost Comparison

In general, for fiber type itself, SMF is a bit expensive than MMF. But for 40/100G cabling, the cable price mainly depends on what optics you use. For instance, if you use SMF optics with LC interface and MMF optics with MTP interface, that would be different. Because the LC terminated cables are much cheaper than the OM3/OM4 MMF MTP cables. However, MMF is for short reach while SMF is usually for long reach. Thus, the total cable cost also depends on the actual transmission distance.

SMF vs MMF: Author’s Opinion

For any links outside of a data center or a server room, it is usually recommended to deploy SMF, due to having more bandwidth and further distances. And for short reach interconnection or small data centers, MMF is preferred by many users due to its lower cost. In my opinion, SMF may be better in 1G and 10G applications or for newly-built data center with the consideration of future proof. Because the costs of optics and cables are close enough between SMF and MMF. And for higher speed, such as 100G, single-mode optics like PSM4 and CWDM4 are ideal for use for large data center. Certain larger data center operators have chosen to forgo MMF altogether. Microsoft, for one, has said it will deploy SMF exclusively in its data centers. Actually, no matter mmf or smf, what the best is the suitable. So, SMF vs MMF, which is your choice?

How to Choose QSFP28 Optics for 100GbE Deployment

How to Choose QSFP28 Optics for 100GbE Deployment

As we know, QSFP28 is considered to be the mainstream form factor in today’s 100GbE optics market. There are many products with QSFP28 form factor include QSFP28 DAC (Direct Attach Copper) cables, QSFP28 AOCs (Active Optical Cables) and QSFP28 transceivers with various interface options, like SR4, LR4, PSM4, CWDM4, etc. Among these QSFP28 products, which one is the best for your data center 100GbE deployment? The selection of the most suitable 100G QSFP28 product depends on a few factors. The most basic factors may be the transmission distance you may want to reach and cable types you plan to use.


Copper DAC Used Inside Racks: 1-5 m

QSFP28 passive DAC products include QSFP28 to QSFP28 DACs and QSFP28 to 4x SFP28 DACs are ideal to use for reaches within 5 m, providing a very cost-effective I/O solution for 100GbE connectivity. In stead of discrete components, QSFP28 DACs offers a complete cable assembly for short distance connectivity in a cost-effective manner. If your 100GbE deployment is within 5m intra racks, the QSFP28 DAC is ideal for you.

Multimode Fiber Use Between Switches: 5-100 m

For 100GbE cabling with multimode fiber between switches, there are two options. One is to use QSFP28 AOC which is the best fit for 3-20 meters. And the other option is to use QSFP28 SR4 transceiver with 12-fiber MTP OM3/OM4 cable. QSFP28 SR4 with 12-fiber OM4 MTP fiber cable can support reaches up to 100 m. If you want to use multimode fiber for connectivity, you could choose either QSFP28 AOC or QSFP28 SR4 transceiver with 12-fiber MTP cable. This depends on your reaches and cost consideration.

Single-Mode Fiber Use Between Switches: >100 m-2 km

Reaches over 100 m but less than 2 km are usually called mid-reaches. For most large data center operators, a 100GbE solution which can satisfy the mid-reach connectivity in a cost-effective way is their sweet-spot. PSM4 QSFP28 and CWDM4/CLR4 QSFP28 (view difference between CWDM4 & CLR4) transceivers which can respectively support up to 500 m over 12-fiber MTP single-mode fiber cable and 2 km over duplex LC single-mode fiber cable are recommended here. For more details about PSM4 and CWDM4 in 100G Ethernet, you can visit the previous post by clicking here.

Single-Mode Fiber for Long Span: ≤10km

For very long span 100GbE deployment, such as connectivity between two building, mostly up to 10 km, QSFP28-100G-LR4 with duplex LC single-mode fiber cable is the preferred option defined by IEEE. But now, cost of QSFP28 LR4 is still very high.


After reading the above content, do you know what is best for you now? FS.COM offers a full series of QSFP28 products and professional service for you which helps you with your worry on 100G QSFP28 optics selection. For more information, please visit or contact