Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 179)

Brocade Virtual ADX Server Load Balancing Guide 163
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Layer 4 health checks
4
Table 14 describes the Layer 4 health check types performance and its description.
Performing Layer 4 UDP keepalive health checks
for the DNS port
You can configure the Brocade Virtual ADX to perform Layer 4 UDP keepalive health checks for the
DNS port (port 53).
To do this globally for the DNS port on all real servers, enter the following commands:
Virtual ADX(config)#server port dns
Virtual ADX(config-port-dns)#udp l4-check-only
NOTE
The l4-check-only command does not apply to the RADIUS protocol.
By default, the Brocade Virtual ADX performs a Layer 4 TCP health check whenever the DNS port on
a real server is brought up.
To configure the Brocade Virtual ADX to perform a Layer 4 UDP health check on the DNS port
whenever it is brought up, add the no tcp keepalive enable command to the DNS port profile as in
the following example:
Virtual ADX(config)#server port dns
Virtual ADX(config-port-dns)#no tcp keepalive enable
TABLE 14 Summary of Layer 4 health checks
Type When performed Description
TCP
•
When you bind a TCP application
port on a real server to a TCP
application port on a virtual server
•
At regular intervals, if keepalive is
enabled for the port and the port
does not have a Layer 7 health
check
The Brocade Virtual ADX attempts to engage in a normal
three-way TCP handshake with the port on the real server:
•
The Brocade Virtual ADX sends a TCP SYN packet to the
port on the real server.
•
The Brocade Virtual ADX expects the real server to
respond with a SYN ACK.
•
If the Brocade Virtual ADX receives the SYN ACK, the
Brocade Virtual ADX sends a TCP RESET, satisfied that
the TCP port is alive.
UDP
•
When you bind a UDP application
port on a real server to a UDP
application port on a virtual server
•
At regular intervals, if keepalive is
enabled for the port and the port
does not have a Layer 7 health
check
The Brocade Virtual ADX sends a UDP packet with garbage
(meaningless) data to the UDP port.
•
If the server responds with an ICMP “Port Unreachable”
message, the Brocade Virtual ADX concludes that the
port is not alive.
•
If the server does not respond at all, the Brocade
Virtual ADX assumes that the port is alive and received
the garbage data. Since UDP is a connectionless
protocol, the Brocade Virtual ADX and other clients do
not expect replies to data sent to a UDP port. Thus, lack
of a response is a good outcome.