Monitor Types | F5 BIG-IP LTM Monitor Types

Monitor Types

Types of monitoring:-

1. Address check :- IP address – Node

2. Service check :- IP- Port

3. Content check :- IP:Port & Check data retuned

4. Interactive check :- Interactive with servers and multiple commands and multiple responses.

Address check :- the simplest type of test is the address check one type of address check is internet control message protocol (or ICMP). It is used to ping an IP address and listen for a response from the node.

An address check only the tests the node and does not tell BIG-IP any thing about how a service is performing i.e pool members.

If there is no response from the node within the monitors set timeout period, the node is marked unavailable. This will also cause the pool member using this IP address to be marked down.

 Service Check :- Service check test if the server is listening on a port for the IP address. When such monitors are associated with a pool member, they determine the availability of a service.

For a TCP service check, a TCP connection is opened. If there is a positive response from the server, then the TCP connection is closed. This positive response is considered a successful check.

If there is no positive response before the timeout period then the member is marked down and unavailable to pool.

Service check only test whether the server is listening on a port and does not provide any insight into the quality of content that might be returned. So remember, it is important to understand what is being checked and whether that is enough to determine upon down state approximately.

Content check:- Content check go beyond testing whether a node is responding or a member is listening. They also test whether the server is responding with correct content.

After the TCP connection is established a command is sent. In the example, an HTTPGET/ command is issued. Each servers response is checked against the receiver string.

for example, if the receive string is defined as the text ‘server’ and the response contains the text ‘server’, the text is constructed successful and the pool member is marked up.

If the connection fails or the response does not include the receive string, the member is marked down and unavailable for client requests.

It is important to pick an appropriate receive string for your content check. As shown on this page you might think It is ok to check for the characters “server” but....       

the ‘404-file or directory not found’ web page also contains the text ‘server’. This means the error page also passes the monitor check and the member would be marked up. This is an example of the receive string not being specific enough.

Interactive check:- Services like FTP require interactive checks because information like user name, password, directory and file name are typically required.

BIG-IP opens a TCP connection and then initial commands are sent and examined. Additional commands can be sent and examined before the connection is closed.

If any condition fails, the member is marked down.

Most interactive checks are external monitors LTM calls an external scripts (like shell, Perl etc) to perform one or more tests and aggregate the results.

Monitor check example chart