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This chapter introduces LocalDirector and includes the following sections:
Cisco LocalDirector is a network appliance with a secure, real-time, embedded operating system that intelligently load balances IP traffic across multiple servers. LocalDirector optimizes the performance of your site by distributing client requests across a cluster of low-cost servers (called a server farm), dramatically reducing the cost of providing
large-scale Internet services and accelerating user access to those applications.
Ideal for mission-critical applications, LocalDirector allows you to build a highly redundant and fault-tolerant server farm system. Servers are automatically and transparently placed in and out of service, providing fault tolerance. LocalDirector itself is equipped with an optional hot-standby failover mechanism, which builds increased redundancy for the server farm system.
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. Figure 1-1 shows a basic network configuration for LocalDirector.
The load-balancing options of LocalDirector provide a flexible and adaptable method for directing IP traffic. LocalDirector load balances the following types of IP protocols, which can be specified for virtual servers and real servers:
As a load balancer, LocalDirector can be configured to maximize the number of IP connections a server farm can manage. IP traffic is directed to different servers based on service, speed, quantity of connections, or client IP address. The servers can be a collection of heterogeneous hardware platforms and operating systems.
LocalDirector supports the two industry-standard modes of load balancing, as set with the redirection command:
The following are LocalDirector terms and definitions:
The following are LocalDirector features:
LocalDirector protects your network by allowing only specific traffic to pass between virtual and real servers, thus restricting both internal and external access to servers. LocalDirector security features include the following:
LocalDirector concepts covered in this section include the following:
A virtual server presents a single IP address that represents a group of real servers. Service requests to the virtual server are load balanced among the real servers. Real servers are actual host servers with unique IP addresses that provide IP services to the network. The virtual server IP address is published to the user community, but the real server IP addresses can remain unpublished, allowing you to hide actual site implementation details and provide single points of contact for users.
Virtual server addresses can only be accessed from the client side of LocalDirector. Also, clients and the real servers bound to LocalDirector virtual servers cannot be located on the same side of LocalDirector.
When you define virtual and real servers, you can specify the port traffic that runs on them, providing the following benefits:
See the section "Port-Bound Servers" in Chapter 4, "Installing and Configuring LocalDirector," for a configuration example.
This feature allows clients to be load balanced to different real servers according to their source IP address. Client-assigned load balancing is accomplished by extending the concept of a virtual server to include a bind-id. The bind-id is used with the assign command to associate a client IP address with a specific instance of a virtual server.
There are many possible uses of this feature, including:
If a server is not responding to requests or is responding with TCP RSTs, LocalDirector fails the server. There are two cases when a real server responds with a TCP RST:
Values set with the reassign and threshold commands determine if a server is considered failed, and can adjust how quickly a server that is not accepting connections is taken out of service. The default threshold value is 8, and the default reassign value is 3. Each real server can have different threshold and reassign values. A reassign value of 0 prevents the real server from being tested.
The reassign command controls how many times a connection synchronization
(TCP SYN) packet from a requesting client is sent to a nonresponsive server before it is reassigned to another server. The default is three TCP SYN packets. After the third packet receives no response or a TCP RST from the server, the fourth packet is sent to another server.
Each reassign process increments the reassign tally by one. When the tally reaches the threshold value, the server is considered failed. With a default threshold value of 8, the reassign process will happen eight times before the server is considered failed.
To increase the rate at which servers are considered failed, reduce the threshold and reassign values. To keep servers that are refusing connections from being failed by LocalDirector, increase the threshold and reassign values. (For example, a site receiving 400 connections per second may need to increase the threshold value to 30.)
The retry command determines how quickly a server is put in "testing" mode and given another packet after being failed by this process. The retry default is 60 seconds. On the 61st second, a connection from a virtual server is directed to the server to determine if it responds. If that connection receives a response, the server is no longer in the failed state, and is put back in service with the reassign and threshold tallies reset to 0. To increase how quickly a server is given a packet after being failed by LocalDirector, reduce the value of the retry command.
When the autounfail command is enabled (it is by default), LocalDirector brings the server back in service as soon as it responds to an existing connection. The server is brought back in service before waiting for the retry time to pass, and working only with servers that are responding with data.
The data command limits the number of connections sent to a server that is not sending data. When a real server reaches the number of unanswered connections set with the data command, LocalDirector checks whether other servers bound to the virtual server are also at 80 percent of their threshold capacity (DataIn value). If the other servers are close to reaching this value, LocalDirector assumes the site is busy and does not fail the server.
The timeout command sets the number of minutes an idle connection to the server is maintained, preventing incomplete connections from being counted toward LocalDirector load balancing.
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 whether the real server is ready to accept more connections:
An automatic slowstart algorithm is available to help bring new servers up to speed with the weighted or leastconns options of the predictor command.
Previously, a server brought into service under heavy network traffic would be bombarded with connections because it had 0 connections. The effect of too many connections at once would disable servers or seriously decrease their performance.
The slowstart option can be set using the roundrobin or none options of the predictor command. The roundrobin slowstart option load balances 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 switches to either weighted or leastconns, as specified in the configuration.
Slowstart is used when:
LocalDirector 3.2 supports UDP load balancing, including a mix of UDP with TCP control streams. UDP streams are kept active with the timeout command.
You can set the number of minutes a UDP connection is active by using the timeout command. When a UDP packet is received on the virtual server, LocalDirector checks the connection cache, looking for an existing connection. If the connection exists, the connection timer is restarted. If the connection does not exist, LocalDirector selects a real server for the virtual server in the normal way, based on the predictor command option configured for the server, as in a TCP-based connection.
IP precedence allows a value or keyword to be added to the IP header information. This value or keyword determines the priority for processing these packets in the routers. The clear counters command in LocalDirector, and the Cisco Policy Manager Version 1.0 application (for a GUI interface), set the IP precedence value on a virtual server basis. A site can assign each type of service it has to a different virtual server, where that virtual server is assigned a precedence.
Accelerated server load balancing (ASLB) allows LocalDirector to use the faster packet-switching speed of Catalyst 6000 family switches to accelerate load-balanced traffic. The Catalyst 6000 family switch forwards the first packet in a flow through LocalDirector, which selects the real server for the connection. The Catalyst 6000 family switch forwards all subsequent packets directly to and from the real server until the client or the server terminates or resets the connection.
ASLB operates on any LocalDirector model with LocalDirector Version 3.2 software. Dispatched assisted mode has been added to the redirection command to enable ASLB. Two 10/100BaseT (all LocalDirector models) or two 1000BaseTX Ethernet (GigE) ports (LocalDirector 430 only) are required to interface connections with the Catalyst family 6000 switch.
Catalyst 6000 family switches allow users to specify up to 1024 server virtual IP addresses and TCP port pairs for ASLB. For instructions on configuring Catalyst 6000 family switches, see the Catalyst 6000 Family Accelerated Server Load Balancing Installation and Configuration Note.
Posted: Tue Feb 22 15:32:29 PST 2000
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