Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 220)

204 Brocade Virtual ADX Server Load Balancing Guide
53-1003247-01
Element health checks
4
Changing the health-check type
For TCP application ports, you can change the health-check type between Layer 4 and Layer 7. By
default, the Brocade Virtual ADX performs a Layer 7 health check in the following cases:
• The port is one of the following ports well-known to the Brocade Virtual ADX:
- FTP – port 21. (Ports 20 and 21 both are FTP ports but on the Brocade Virtual ADX, the
name “FTP” corresponds to port 21.)
- HTTP – port 80
- IMAP4 – port 143
- LDAP – port 389
- MMS – port 1755
- NNTP – port 119
- PNM – port 7070
- POP3 – port 110
- RTSP – port 554
- SMTP – port 25
- SSL – port 443
- TELNET – port 23
• The port is not well-known to the Brocade Virtual ADX but you used the protocol command to
specify the protocol of one of the well-known ports. By specifying the protocol, you configure
the Brocade Virtual ADX to use the protocol’s Layer 7 health-check method for the port.
If the TCP port is not one of the ports above or you did not specify a Layer 7 health-check method
(using the protocol command), the Brocade Virtual ADX uses the Layer 4 health check for TCP.
NOTE
Changing the health-check type for UDP application ports has no effect. If the application port is
RADIUS (1812) or DNS (53) or uses the health-check method of one of these ports, the Brocade
Virtual ADX uses a Layer 7 health check. Otherwise, the Brocade Virtual ADX uses the Layer 4 health
check for UDP.
The Layer 7 health-check methods differ depending on the application:
• TCP – 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 – 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. Because UDP is a connectionless protocol, the Brocade
Virtual ADX and other clients do not expect replies to data sent to a UDP port. Therefore,
lack of a response is a good outcome.