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Cisco LocalDirector (see Figure 1-1) is a hardware and software solution with a secure, real-time, embedded operating system that intelligently load balances TCP/IP traffic across multiple servers. Delivering very fast performance by distributing client requests across a cluster of low-cost servers, LocalDirector dramatically reduces the cost of providing large-scale Internet services, and speeds user access to those applications.
LocalDirector serves as a transparent learning bridge to forward data packets between its interfaces. Because of its bridge capability, LocalDirector must not be installed on the network parallel to another bridge.
The load-balancing options of LocalDirector provide a flexible and adaptable method for directing TCP/IP traffic. You can configure LocalDirector to maximize the number of TCP/IP connections a server farm can manage. TCP/IP traffic is directed to different servers based on service, speed, or quantity of connections.
LocalDirector is a high-performance Internet appliance with over 92 Mbps throughput. It supports a combined total of 10,240 virtual IP addresses and real servers. The real servers can be a collection of heterogeneous hardware platforms and operating systems. Quick setup with no network address changes reduces system administration time.
Ideal for mission-critical applications, LocalDirector provides the capability to build a highly redundant and fault-tolerant server system. Servers are automatically and transparently placed in and out of service, providing fault tolerance for servers. LocalDirector itself is equipped with an optional hot-standby failover mechanism, building increased redundancy for the server system. Figure 1-2 shows the front of the LocalDirector.
LocalDirector has these features:
The LocalDirector shipping carton contains the following:
Follow these guidelines to ensure general safety:
The command interpreter provides a command set that emulates Cisco IOS technologies. This command set provides three administrator access modes:
At startup, the console is in unprivileged mode. You can access privileged mode by entering the enable command. LocalDirector then prompts you for a password. When you first configure LocalDirector, a password is not required. Press the Enter key at the prompt. Assign a password to privileged mode with the enable password command. Exit privileged mode by entering the disable command.
Access configuration mode by entering the configure terminal command while in the privileged mode. You can then write your settings to flash memory, diskette, or to your console computer. Exit configuration mode by entering ^Z.
When you enter commands, you can erase characters with the Backspace or Del key. You can erase the previous word with ^W and erase the previous line with ^U.
LocalDirector concepts covered in this section include the following:
LocalDirector serves as a transparent learning bridge to forward data packets between its interfaces. Because of its bridge capability, LocalDirector must not be installed on the network parallel to another bridge. Only use LocalDirector to connect to servers with a single way in or out to the network.
If there is another path from the network to your servers, a bridge loop will be created and LocalDirector will not work properly. The LocalDirector automatically detects a bridge loop and tries to recover. SYSLOG messages will be generated to indicate that there is a bridge loop.
Virtual servers present a single address for a group of real servers and load-balance service requests between the real servers in a site. Real servers are actual host machines with unique IP addresses that provide TCP/IP services to the network. The virtual server IP address is published to the user community, but the real IP addresses can remain unpublished, allowing you to hide actual site implementation details and provide single points of contact for users.
Clients and servers cannot be located on the same side of the LocalDirector. The LocalDirector uses network address translation (NAT) to make it appear as if the client is communicating directly with the real servers. If the client and server are on the same network segment, the response from the server will bypass the LocalDirector and the traffic will not be load balanced. All traffic must pass through the LocalDirector and be bridged to the real servers.
Virtual servers and real servers can also be seen as a "TCP service" instead of just an IP address. When you define virtual and real servers, you can specify the port traffic that will run on the server. These servers are referred to as port-bound servers, and they provide the following benefits:
To ensure that TCP services will continue to run in the event that a server is failed or out-of-service, you can identify an alternative destination for server traffic by specifying a backup. The term "backup" is used to define a hot-standby for a real or virtual server defined on the LocalDirector. The backup can be a virtual or real server, thus it is possible to use the backup command in any combination.
For real servers, a backup is used if the real server is failed or out-of-service. For a virtual server, a backup is used if all real servers (and their backups) bound to the virtual server are failed or out-of-service. If the virtual server itself is out-of-service, a reset message will be sent to the client requesting the connection.
When the server being backed up returns to service, connections are no longer directed to the backup server and they are sent according to the LocalDirector configuration.
When a real server is failed (it does not respond to a predetermined number of connections set by the threshold command), the following process is used to test the real server to see if it is ready to accept more connections:
Previously, a server brought into service under heavy network traffic would be bombarded with connections since it had zero connections. The effect of too many connections at once would disable servers or seriously decrease their performance.
An automatic slowstart algorithm is available to help bring new servers up to speed with the weighted or leastconns predictor options. The slowstart option can be set to roundrobin or none. The roundrobin slowstart option will load balance network connections until network traffic is stable. When the number of connections on all bound real servers is within 80 percent of the desired distribution, the predictor will switch to either weighted or leastconns, as specified in the configuration.
Slowstart is used when:
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