There are many types of devices in the typical data centre that require multihoming (multiple network adapters) to tie in to multiple network segments. As the number of those systems increases, it becomes more and more difficult to provide the network infrastructure (due to the sheer number of Ethernet connections that need to be provided) from the perspective of cost, space, and wire management.
A technology called VLAN trunking, once primarily the domain of network switches, has now trickled down to the rest of the data centre and can help address the multihoming problem. With VLAN trunking, it is now possible for these multihoming devices to be multihoming in function without the need for multiple physical network adapters and the additional infrastructure associated with them.
VLAN trunking allows a single network adapter to behave as X number of virtual network adapters, where X has a theoretical upper limit of 4096, but is typically limited to 1000 network switches. In the case where a single gigabit Ethernet adapter is trunked in place of using multiple FastEthernet adapters, you can achieve higher performance at a lower cost while increasing flexibility. This really is the best of all worlds. I'm going to give you an overview of VLAN trunking and how it can be used.
VLAN trunking requirements
VLAN trunking requires that the network switch, the network adapter, and the drivers for the operating system all support VLAN tagging in order for them to trunk. Almost any enterprise-grade switch made by Cisco, Extreme, Foundry, and others support VLANs (802.1q).
A few examples of this on the smaller scale are Cisco’s 2950 series and Netgear’s FSM726. Most high-end client adapters support VLAN trunking, but one of the most common ones you'll find is the Intel Pro/100 and Pro/1000, which are often included on server manufacture motherboards. For those systems without an integrated Intel adapter, you'll need to purchase a separate Pro/1000 PCI card. Drivers support on the Intel adapters is excellent and covers almost everything from BSD to Linux to Windows client and server operating systems.
Applications of VLAN trunking
Network devices that need multihoming capability and can benefit from VLAN trunking include:
- Routers
- Firewalls
- Transparent proxy servers
- VMware hosts
- Wireless Access Points (WAPs)
Routers can become infinitely more useful once they are trunked into the enterprise switch infrastructure. Once trunked, they become omnipresent and can provide routing services to any subnet in any corner of the enterprise network. This is, in essence, what a routing module in a high-end core or distribution Layer 3 (L3) switch provides.
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