Technical data

Brocade ServerIron ADX Advanced Server Load Balancing Guide 9
53-1003441-01
SIP SLB and call persistence
1
The proxy server receives the INVITE request and sends a 100 (Trying) message to User1's SIP
phone. Because the ServerIron ADX switch is configured in DSR mode, the response message that
is sourced from the virtual IP address flows directly to User1's SIP phone, bypassing the ServerIron
ADX. The proxy server then consults the location service and forwards the INVITE request directly to
User2's SIP phone, again bypassing the ServerIron ADX, and is sourced from the proxy server's own
IP address.
NOTE
The proxy server's IP address must be reachable from all SIP clients.
User2's SIP phone receives the INVITE and alerts User2 of an incoming call. User2 replies with a
Ringing message to the proxy server. If User2 answers the call, a 200 OK message is sent to the
proxy server. The proxy server forwards this message to User1's SIP phone. Upon receiving the 200
OK message, User1's SIP phone sends an acknowledgement (ACK) message directly to User2's SIP
phone, bypassing the proxy server. User1 and User2 SIP phones now begin media exchange and,
upon completion, a BYE message closes the call.
Some SIP servers may be configured to use a virtual IP address (VIP) as the source address for all
communications. Figure 4 shows SIP packet flows in this type of configuration.
FIGURE 4 SIP server farm with DSR mode and SIP server using VIP as source address
In this implementation, the SIP proxy server must use the same Call-ID for both legs of
communication (the same Call-ID for message exchange with both SIP clients within a given SIP
dialog). Session persistence and transaction integrity can only be achieved if the proxy server uses
the same Call-ID.