Cisco Systems 78-11741-02 Dynamic Feedback Protocol for IOS SLB, Alternate IP Addresses, IPC-138

Configuring Server Load Balancing

IOS SLB Functions and Capabilities

Dynamic Feedback Protocol for IOS SLB

The IOS SLB Dynamic Feedback Protocol (DFP) is a mechanism that allows host agents in load-balanced environments to dynamically report the change in status of the host systems that provide a virtual service. The status reported is a relative weight that specifies the capacity of a host server to perform work.

Alternate IP Addresses

IOS SLB enables you to Telnet to the load-balancing device using an alternate IP address. To do so, use either of the following methods:

•Use any of the interface addresses to Telnet to the load-balancing device.

•Define a secondary IP address to Telnet to the load-balancing device.

This function is similar to that provided by the LocalDirector (LD) Alias command.

Transparent Web Cache Balancing

You can balance transparent Web caches if you know in advance the IP addresses they are serving. In IOS SLB, configure the IP addresses, or some common subset of them, as virtual servers.

Note A Web cache can start its own connections to real sites if pages are not available in its cache. Those connections cannot be load balanced back to the same set of caches. IOS SLB addresses this situation by allowing you to configure “client exclude” statements so that IOS SLB does not load balance connections initiated by the Web caches.

NAT

Cisco IOS Network Address Translation (NAT), RFC 1631, allows unregistered “private” IP addresses to connect to the Internet by translating them into globally registered IP addresses. Cisco IOS NAT also increases network privacy by hiding internal IP addresses from external networks.

IOS SLB can operate in one of two redirection modes:

•Directed mode—The virtual server can be assigned an IP address that is not known to any of the real servers. IOS SLB translates packets exchanged between a client and real server, translating the virtual server IP address to a real server address via NAT.

•Dispatched mode—The virtual server address is known to the real servers; you must configure the virtual server IP address as a loopback address, or secondary IP address, on each real server. IOS SLB redirects packets to the real servers at the media access control (MAC) layer. Because the virtual server IP address is not modified in dispatched mode, the real servers must be Layer 2 adjacent to IOS SLB, or intervening routers might not be able to route to the chosen real server.

The main advantage of dispatched mode is performance. In dispatched mode, the Layer 3 and Layer 4 addresses are not modified, which means IP header checksum adjustment occurs quickly, and checksum adjustment or recalculation for TCP or UDP is not required. Dispatched mode is also simpler than in directed mode because packets for applications with IP addresses in the packet need not be examined and modified.

Cisco IOS IP Configuration Guide

IPC-138