ServerIron ADX Server Load Balancing Guide
Release 12.0.00
June 15, 2009

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Server Load Balancing > Per Server Based Real Server Backup

Per Server Based Real Server Backup
This following sections describe how to configure a backup server definition on a per-server basis:
Overview of Per Server Based Real Server Backup
The current implementation of the backup server requires that all non-backup servers fail prior to directing requests to backup servers. This may not allow for maintaining the same level of performance in the server farm. The ability to maintain same performance level for a given service is critical for many customers.
Per Server Based Real Server Backup allows the backup servers to be associated with the specified primary servers. When a primary server fails, its backup server starts processing the traffic no matter what state the other primary servers are in. This feature works with the current real server back mechanism, by providing additional control of the backup server selection.
Current Backup Scheme
Currently, when a primary server goes down another server is selected among the active primary servers. Until all the primary servers are down, the server is selected from the backup servers. Additionally, the users can configure backup-stay-active to keep the server selection in the backup groups active, even when some primary servers come back up.
Per Server Based Backup Scheme
With this feature, the associated primary and backup servers back up each other, regardless of the state of the other service ports. If a backup server is associated with a primary server, they work as a pair so each can substitute the other when it becomes unavailable.
If the backup-stay-active is configured, the backup server continues to process the traffic even after the primary server comes up again.
EXAMPLE:
Primary servers: A and B
Backup servers: C and D
Backup association: C is backup of A, D is backup of B.
Condition 1: When A goes down and B is alive, the server is selected from C and B.
Condition 2: When both A and B go down, the server is selected from C and D.
Condition 3: if backup-stay-alive is not configured. When B comes up and A stays down alive, the server is selected from C and B.
Condition 4: if backup-stay-alive is configured, when B comes up and A stays down, the server is selected from C and D.
Slow Start of the Backup and the Primary Servers
If the server selection predictor is least connection, the backup server may be overwhelmed by the flood of the new connections when its primary server goes down. The same is true when the primary server goes back up and starts to take over the connections from the backup server. The slow start mechanism will be used whenever the switching of the backup or primary server happens, to give the server the chance to ramp up.
The slow start mechanism of the backup or the primary server will be the same as the one currently being used for the new servers. The slow start parameters is configured on the real server port as it is right now.
NOTE: The slow start is enabled by default.
One Backup Per Primary Port or Server
There will be the following restrictions:
At the real port mode, the primary and backup ports have one-to-one relationship. That is, the primary port can only be backed up by one backup port and the backup port can only back up one primary port.
At the real server mode, the primary and backup servers have one-to-one relationship. That is, the primary server can only be backed up by one backup server and the backup server can only back up one primary server.
The Back Port has the Precedence over the Back Server
When both the port and the server backup are configured, the port configuration takes precedence over the server configuration.
For instance, the following is configured:
Then, the port 8080 of the server C becomes the backup of the port 8080 of the server B, and it's not the backup of the port 8080 of the server A.
Real Server Backup Commands
Server Backup Association
This command is to configure the backup server for a particular primary server, in the real server mode.
Syntax: [no] backup [server-name]
EXAMPLE: 
To configure the real server R2 as the backup of the real server R1.
ServerIron(config)# server real-name R1 10.10.10.10
ServerIron(config-rs-R1)# port http
ServerIron(config-rs-R1)# exit
ServerIron(config)# server real-name R2 10.10.10.20
ServerIron(config-rs-R2)# backup R1
ServerIron(config-rs-R2)# port http
ServerIron(config-rs-R2)# exit
Server Port Backup Association
This command is to configure the backup server port for a particular primary server port, in the real server port mode.
Syntax: [no] port <port-name> backup [server-name] [port-name]
EXAMPLE: 
To configure the http port of the real server R2 as the backup of the http port of the real server R1.
ServerIron(config)# server real-name R1 10.10.10.10
ServerIron(config-rs-R1)# port http
ServerIron(config-rs-R1)# exit
ServerIron(config)# server real-name R2 10.10.10.20
ServerIron(config-rs-R2)# port http
ServerIron(config-rs-R2)# port http backup R1 http
ServerIron(config-rs-R2)# exit
NOTE: When both server backup and server port backup are configured, the server port backup has the precedence over the server backup.
EXAMPLE: 
ServerIron(config)# server real-name R1 10.10.10.10
ServerIron(config-rs-R1)# port http
ServerIron(config-rs-R1)# exit
ServerIron(config)# server real-name R2 10.10.10.20
ServerIron(config-rs-R2)# port http
ServerIron(config-rs-R2)# port http R1 http
ServerIron(config-rs-R2)# exit
ServerIron(config)# server real-name R3 10.10.10.30
ServerIron(config-rs-R2)# backup R2
ServerIron(config-rs-R2)# port http
ServerIron(config-rs-R2)# port http backup R1 http
ServerIron(config-rs-R2)# exit
 
The server R3 will be the backup of R2, while the http port on R3 will be the backup of the http port on R1.
Display the Backup Bindings
This command is to display the binding relationship between the servers and the ports.
Syntax: show server backup-server-port-binding
EXAMPLE: 
ServerIron(config)# server real-name R1 10.10.10.10
ServerIron(config-rs-R1)# port http
ServerIron(config-rs-R1)# exit
ServerIron(config)# server real-name R2 10.10.10.20
ServerIron(config-rs-R2)# backup R1
ServerIron(config-rs-R2)# port http
ServerIron(config-rs-R2)# port http backup R1 http
ServerIron(config-rs-R2)# exit
ServerIron(config)#server show backup-server-port-binding
Server/Port State - 0: disabled, 1:enabled, 2:failed, 3:test, 4:suspect, 5:grace_dn, 6:active
Real Server rs3:(state 6)
Backup Server : rs2(state 6)
Port 80(state 6) <---------- Port rs2:80(state 6)

Server Load Balancing > Per Server Based Real Server Backup

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