Local Area Networks are a base for networking in a data center. Complex networks are very common where the data center uses same network for different use cases such as system management, data replication, and data traffic. In complex networks, data congestion is a common problem. The network is logically divided to avoid data congestion and to ensure seamless network traffic. Splitting a network by using different broadcast domains using routers is a common practice to isolate a sub network. Another way to logically split a large network is using VLANs. Broadcast domain works at Layer 3 whereas you can split the network using VLANs at the switch on Layer 2. VLANs are very common these days and provide many benefits, but many users may not understand how VLANs work and where to use them.

IBM SAN Volume Controller (SVC) and the Spectrum Virtualize family of products offer a wide range of features such as iSCSI on 1 Gbps, 10 Gbps, 25 Gbps, Fibre Channel over Ethernet, and many other features that require a complex network. Because of this, SVC supports VLAN.

Storage Networking Industry Association (SNIA) defines VLAN as,
A logical network that behaves as if it is physically separate from other physical and virtual LANs supported by the same switches and/or routers.

When a virtual network is created from an existing physical network, it’s called a Virtual Local Area Network (VLAN). Devices on that virtual network can only communicate with each other and devices which are on the physical network but not part of the virtual network cannot communicate with them. Broadcast packets sent from one entity in a VLAN will only reach other entities that are present in the same VLAN on the same network. Workstations or entities outside the VLAN will not be able to exchange information with workstations in a VLAN and vice-versa. As understood from the description, a VLAN always consists of a subset of devices that are on the physical network. And these devices can include storage, servers, hosts, and switches. VLAN, as the name suggests, is a property of the network and is mainly driven by switches and routers that hold the network as a connection between storage, server and host. This is very similar to the concept of zoning in the Fibre Channel world.

There can be numerous reasons for implementing VLAN on a physical network, most important of which is to segregate traffic to a particular set of devices that actually need it, and to reduce congestion on a large network. When Local Area Networks started to grow bigger and on one side the number of devices or workstations increased and on the other side users desired greater flexibility with faster data rates, routers in the network became a bottleneck and switch vendors started concentrating on VLANs to separate traffic on a virtual level between dedicated devices. This increased speed and availability while decreasing response time between communication to create a feeling as though you were on an isolated LAN. VLAN boosts performance, makes network modifications easier, augments network security, and reduces cost.

There are multiple use cases where VLAN implementations help.

Use Case 1: iSCSI and IP Replication traffic isolation

The primary purpose of introducing VLAN support for iSCSI is to provide traffic isolation. Being on the same physical network, VLANs provide the capability to segregate systems based on specific requirements at a logical level. Figure 1 illustrates a scenario where iSCSI and IP Replication traffic is being isolated through VLAN 100 and VLAN 200 respectively on the same physical network.
Figure 1: iSCSI Traffic Isolation

Above figure shows a network configuration consisting of a server, Ethernet network, and SVC node where iSCSI and IP Replication traffic is being segregated using a VLAN based on the same underlying physical network.

Use Case 2: VLAN on SVC for iSCSI, IP Replication, and backup applications

VLANs can be used for more than just IP Replication scenarios. Various other features like backup can also run on a VLAN on third-party backup software as part of a physical network, where separate VLANs are deployed to manage iSCSI and IP replication traffic on IBM Spectrum Virtualize products.

Use Case 3: Multi-tenancy and QoS

Another important application of VLAN is to facilitate multi-tenancy and Quality of Services. A scenario where multiple hosts can be distributed on the same VLAN performing diversified tasks is applicable here. Similarly, multi-tenancy can also be achieved using a VLAN, where a single instance of the software or feature runs on a server, serving multiple user groups sharing the same view on the software they use.

The examples given above will help you to analyze how VLAN will benefit your environment. This will help you to avoid network congestions and provide smoother business continuity. More details and configuration best practices can be found in the detailed documentation of IBM Spectrum Virtualize and respective RedBooks.

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