Cisco Cisco IOS Software Release 12.2(18)SXF

If you have configured all-port virtual servers (that is, virtual servers that accept flows destined for all 
ports except GTP ports), flows can be passed to servers for which no application port exists. When the 
servers reject these flows, IOS SLB might fail the servers and remove them from load balancing. This 
situation can also occur in slow-to-respond AAA servers in RADIUS load-balancing environments. To 
prevent this situation, you can disable automatic server failure detection.

If you disable automatic server failure detection using the no faildetect inband command, Cisco 
strongly recommends that you configure one or more probes.

If you specify the no faildetect inband command, the faildetect numconns command is ignored, if 
specified.

When a real server fails and is removed from the list of active servers, it is assigned no new connections 
for a length of time specified by a configurable retry timer. After that timer expires, the server is again 
eligible for new virtual server connections and IOS SLB sends the server the next qualifying connection. 
If the connection is successful, the failed server is placed back on the list of active real servers. If the 
connection is unsuccessful, the server remains out of service and the retry timer is reset. The 
unsuccessful connection must have experienced at least one retry, otherwise the next qualifying 
connection would also be sent to that failed server.

A backup server farm is a server farm that can be used when none of the real servers defined in a primary 
server farm is available to accept new connections. When configuring backup server farms, keep in mind 
the following considerations:

A server farm can act as both primary and backup at the same time.

The same real server cannot be defined in both primary and backup at the same time.

Both primary and backup require the same NAT configuration (none, client, server, or both). In 
addition, if NAT is specified, both server farms must use the same NAT pool.

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(18)SXD.

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.