Cisco Cisco IOS Software Release 12.2(14)ZA

An HTTP probe establishes HTTP connections to real servers, sends HTTP requests to the real 
servers, and verifies the responses. HTTP probes are a simple way to verify connectivity for devices 
being server load-balanced, and for firewalls being firewall load-balanced (even devices on the other 
side of a firewall).

HTTP probes also enable you to monitor applications being server load-balanced. With frequent 
probes, the operation of each application is verified, not just connectivity to the application.

HTTP probes do not support HTTP over Secure Socket Layer (HTTPS). That is, you cannot send an 
HTTP probe to an SSL server.

A ping probe pings real servers. Like HTTP probes, ping probes are a simple way to verify 
connectivity for devices and firewalls being load-balanced.

A TCP probe establishes and removes TCP connections. Use TCP probes to detect failures on TCP 
port 443 (HTTPS).

A custom UDP probe can to support a variety of applications and protocols, including:

RADIUS Accounting/Authorization probes

GTP Echo probes

Connectionless WSP probes

XML-over-UDP probes for CSG user-database load-balancing

Mobile IP RRQ/RRP

A WSP probe simulates requests for wireless content and verifies the retrieved content. Use WSP 
probes to detect failures in the Wireless Application Protocol (WAP) stack on port 9201.

You can configure more than one probe, in any combination of supported types, for each server farm, or 
for each firewall in a firewall farm.

You can also flag a probe as a routed probe, with the following considerations:

Only one instance of a routed probe per server farm can run at any given time.

Outbound packets for a routed probe are routed directly to a specified IP address.

IOS SLB probes use the SA Agent. You might want to specify the amount of memory that the SA Agent 
can use, using the rtr low-memory command. If the amount of available free memory falls below the 
value specified in the rtr low-memory command, then the SA Agent does not allow new operations to 
be configured. Refer to the description of the rtr low-memory command in the Cisco IOS Configuration 
Fundamentals Command Reference, Release 12.2 for more details.

Probes determine the status of each real server in a server farm. All real servers associated with all virtual 
servers tied to that server farm are probed.

If a real server fails for one probe, it is failed for all probes. After the real server recovers, all probes 
must acknowledge its recovery before it is restored to service.

Probes detect firewall failures. All firewalls associated with the firewall farm are probed.

If a firewall fails for one probe, it is failed for all probes. After the firewall recovers, all probes must 
acknowledge its recovery before it is restored to service.

Make sure you configure the HTTP probe to expect status code 401, to eliminate password problems. 
Refer to the description of the expect command in the 

 for 

more details.