Cisco Cisco IOS Software Release 12.2(14)ZA

IOS SLB supports the DFP Agent Subsystem feature, also called global load balancing, which enables 
client subsystems other than IOS SLB to act as DFP agents. With the DFP Agent Subsystem, you can 
use multiple DFP agents from different client subsystems at the same time.

For more information about the DFP Agent Subsystem, refer to the DFP Agent Subsystem feature 
document for Cisco IOS Release 12.2(14)ZA2.

With IOS SLB Dynamic Feedback Protocol (DFP) support, a DFP manager in a load-balancing 
environment can initiate a TCP connection with a DFP agent. Thereafter, the DFP agent collects status 
information from one or more real host servers, converts the information to relative weights, and reports 
the weights to the DFP manager. The DFP manager factors in the weights when load balancing the real 
servers. In addition to reporting at user-defined intervals, the DFP agent sends an early report if there is 
a sudden change in a real server’s status.

The weights calculated by DFP override the static weights you define using the weight command in 
server farm configuration mode. If DFP is removed from the network, IOS SLB reverts to the static 
weights.

You can define IOS SLB as a DFP manager, as a DFP agent for another DFP manager, or as both at the 
same time. In such a configuration, IOS SLB sends periodic reports to the other DFP manager, which 
uses the information to choose the best server farm for each new connection request. IOS SLB then uses 
the same information to choose the best real server within the chosen server farm.

DFP also supports the use of multiple DFP agents from different client subsystems (such as IOS SLB 
and GPRS) at the same time.

In GPRS load balancing, you can define IOS SLB as a DFP manager and define a DFP agent on each 
GGSN in the server farm. Thereafter, the DFP agent can report the weights of the GGSNs. The DFP 
agents calculate the weight of each GGSN based on CPU utilization, processor memory, and the 
maximum number of Packet Data Protocol (PDP) contexts (mobile sessions) that can be activated for each 
GGSN. As a first approximation, DFP calculates the weight as the number of existing PDP contexts 
divided by the maximum allowed PDP contexts:

(existing PDP contexts)/(maximum PDP contexts)

Maximum PDP contexts are specified using the gprs maximum-pdp-context-allowed command, 
which defaults to 10,000 PDP contexts. If you accept the default value, DFP might calculate a very low 
weight for the GGSN:

(existing PDP contexts)/10000 = Low GGSN weight

Keep this calculation in mind when specifying maximum PDP contexts using the 
gprs maximum-pdp-context-allowed command. For example, Cisco 7200 series routers acting as 
GGSNs are often configured with a maximum of 45,000 PDP contexts.

IOS SLB supports DNS, HTTP, ping, TCP, custom UDP, and WSP probes:

A DNS probe sends domain name resolve requests to real servers, and verifies the returned IP 
addresses.