ACE : Understanding Least Connections Predictor

[AJAZ NAWAZ]

Does anyone have [or know where to get] a detailed explanation for 'Least Connections' Predictor when applied on the ACE module.

This question has arisen after seeing one server [out of four in farm] receiving more connections then others. If the algorithm is functioning as expected, at any onetime, the number of active connections will be evenly spread across the total number of servers - is my understanding accurate,or perhaps I am missing something?.

All servers in the farm are identical spec.

http://docwiki.cisco.com/wiki/Cisco_Application_Control_Engine_(ACE)_Module_Troubleshooting_Guide,_Release_A2(x)_--_Troubleshooting_Layer_7_Load_Balancing

• Least connections—Selects the server with the fewest number of active connections based on the server weight. For the least-connections predictor, you can configure a slow-start mechanism to avoid sending a high rate of new connections to servers that you have just put into service.

Another question I have is 'What is the definition of a new connection?'. Is this based on src/dst IP pairs or TCP session IDs, or what?.

thank you

Ajaz

[Christopher Miles]

Hi Ajaz,

With reagrds to a new connection .... The ACE is a flow based device, If the packet is a TCP or a UDP packet, a 5-tuple-flow lookup (source IP address, source port, destination IP address, destination port, and incoming VLAN) we map it to an existing flow, If no flow exists we will need to build a new connection.

Some more information about least conns

Least conns will  instruct the ACE to select the server with the fewest number of connections based on the server weight, typically you would use this predictor for processing light user requests... If even load balancing is required and all servers are identical the default round-robin will give you more even results.

Server weights take effect only when there are open connections to the servers. When there are no sustained connections to any of the servers, the least-connections predictor method behaves the same way as the round-robin method.

chris