17.3.1. Switched networks

Switched Ethernets have become affordable and extremely popular in the last few years, with configurations ranging from enterprise-class switching networks with hundreds of ports, to the small 8- and 16-port Fast Ethernet switched networks used in small businesses. Switched Ethernets are commonly found in configurations that use a high-bandwidth interface into the server (such as Gigabit Ethernet) and a switching hub that distributes the single fast network into a large number of slower branches (such as Fast Ethernet ports). This topology isolates a client's traffic to the server from the other clients on the network, since each client is on a different branch of the network. This reduces the collision rate, allowing each client to utilize higher bandwidth when communicating to the server.

Although switched networks alleviate the impact of collisions, you still have to watch for "impedance mismatches" between an excessive number of client network segments and only a few server segments. A typical problem in a switched network environment occurs when an excessive number of NFS clients capable of saturating their own network segments overload the server's "narrow" network segment.

Consider the case where 100 NFS clients and a single NFS server are all connected to a switched Fast Ethernet. The server and each of its clients have their own 100 Mbit/sec port on the switch. In this configuration, the server can easily become bandwidth starved when multiple concurrent requests from the NFS clients arrive over its single network segment. To address this problem, you should provide multiple network interfaces to the server, each connected to its own 100 Mb/sec port on the switch. You can either turn on IP interface groups on the server, such that the server can have more than one IP address on the same subnet, or use the outbound networks for multiplexing out the NFS read replies. The clients should use all of the hosts' IP addresses in order for the inbound requests to arrive over the various network interfaces. You can configure BIND round-robin[52] if you don't want to hardcode the destination addresses. You can alternatively enable interface trunking on the server to use the multiple network interfaces as a single IP address avoiding the need to mess with IP addressing and client naming conventions. Trunking also offers a measure of fault tolerance, since the trunked interface keeps working even if one of the network interfaces fails. Finally, trunking scales as you add more network interfaces to the server, providing additional network bandwidth. Many switches provide a combination of Fast Ethernet and Gigabit Ethernet channels as well. They can also support the aggregation of these channels to provide high bandwidth to either data center servers or to the backbone network.

[52]When BIND's round-robin feature is enabled, the order of the server's addresses returned is shifted on each query to the name server. This allows a different address to be used by each client's request.

Heavily used NFS servers will benefit from their own "fast" branch, but try to keep NFS clients and servers logically close in the network topology. Try to minimize the number of switches and routers that traffic must cross. A good rule of thumb is to try to keep 80% of the traffic within the network and only 20% of the traffic from accessing the backbone.