Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 346)

330 Brocade Virtual ADX Server Load Balancing Guide
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SIP SLB and call persistence
7
Sample deployment topologies
Virtual 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 42 shows an SIP server farm built around Virtual ADX application switches.
FIGURE 42 SIP server farm with DSR mode
Figure 42 demonstrates a typical use case in which the Virtual 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 Virtual
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 Virtual ADX switch, the Virtual ADX switch receives the INVITE request
and, using a server selection mechanism, identifies the best available SIP server for this INVITE.
The Virtual 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 Virtual 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.