Technical data

8 Brocade ServerIron ADX Advanced Server Load Balancing Guide
53-1003441-01
SIP SLB and call persistence
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Sample deployment topologies
ServerIron ADX switches offer application-aware advanced intelligence for SIP server load
balancing. The following sections describe some SIP server load balancing scenarios.
SIP server load balancing with DSR mode
Figure 3 shows an SIP server farm built around ServerIron ADX application switches.
FIGURE 3 SIP server farm with DSR mode
Figure 3 demonstrates a typical use case in which the ServerIron ADX application switch provides
Call-ID based server persistence for UDP SIP traffic. The Call-ID attribute that uniquely identifies a
SIP call is used to maintain session persistence. Due to the unique call flow requirements of SIP,
most SIP implementations require you to enable Direct Server Return (DSR) mode on the
ServerIron ADX switch.
Because User1's SIP phone does not know the location of User2's SIP phone, it initiates a new SIP
session by sending an INVITE request to the SIP proxy server. It also generates a unique identifier
(Call-ID) for the call. Because the SIP proxy server used by User1's SIP phone is actually the virtual
IP address hosted on the ServerIron ADX switch, the ServerIron ADX switch receives the INVITE
request and, using a server selection mechanism, identifies the best available SIP server for this
INVITE. The ServerIron ADX uses the Call-ID attribute value to select one of the SIP servers in either
stateless or stateful mode. For all SIP transactions within a dialog that use the same Call-ID, the
ServerIron ADX selects the same SIP server. A new INVITE message with a different Call-ID is again
subjected to server load balancing and may be forwarded to a different SIP server.