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Virtual servers for data center LANs: Considerations for service providers

Server virtualization will impact how you design switches for your customer's data center LAN. This tip provides best practices for switch design.

An exciting trend in computer networking is server virtualization, where many instances of operating systems and applications virtually reside on a single physical box. Of course, this means lots of challenges to your switch designs for customer data centers. Here are some of the things you should think about when deploying virtual servers for your customer's data center LAN.

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IP addressing

Many network engineers size IP subnets based on the number of servers that can physically fit in the racks, or based on the number of network interfaces the switch can support. This, of course, becomes a problem when each virtual server requires one or more IP addresses, and it's very difficult to predict how many virtual images your customers can put on their physical servers. You may want to use larger subnets, but you'll still have to weigh this against wasting IP space, particularly if your customer's space has been poorly managed to date.

Virtual switches

Take steps to prevent rogue switches from being connected to networks you manage, because your SLAs can be affected if the users create loops with wires or do any number of other things. Since many virtual server products also include one or more virtual switches inside the server hardware, the steps you take to prevent rogue switches, like configuring Spanning Tree Protocol, become a little more complicated.

For instance, if the virtual server is configured with a switch, and for redundancy, it's connected to two LAN switches, then it really is an Ethernet loop that you have to break. So you need to be careful configuring things like Portfast or other technologies designed to minimize the time it takes a port to come "up" because it's not expecting to see a BPDU. Of course, you also need to test the compatibility between the switch's implementation of STP and the virtual server software's implementation of STP.


Even if your customer's server admins insist they need Gigabit NICs, the average virtual server uses only a tiny fraction of available bandwidth. But when you put a couple dozen of these servers on a Gigabit port, they may actually generate some traffic. While this isn't normally a concern, it may cause you to consider which ports on the switch you use and which types of switches to buy. This is because all ports aren't created equal. If the switch lets you configure how much memory is allocated to packet buffers, you may want to explore this as well, since it could improve performance.

Of course, these precautions may help justify a network upgrade. So if your customer is starting to deploy virtual servers, you may want to first consult with them about how they should prepare their data center.

About the author
Tom Lancaster, CCIE# 8829 CNX# 1105, is a consultant with 15 years of experience in the networking industry. He is co-author of several books on networking, most recently,
CCSP: Secure PIX and Secure VPN Study Guide, published by Sybex.


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