How Session Initiation Protocol (SIP) traffic sessions are allocated in SRX High End devices.
All SIP sessions of one particular SIP server are statically allocated to the same Services Processing Unit (SPU) in SRX High End devices. Only one SPU is in use for all SIP sessions of the same SIP server. SRX is unable to allocate SIP traffic of the same SIP server to other SPUs to load-balance the traffic. Hence, in the case of a high capacity SIP server, it is possible that some SIP traffic exceeds the SIP session capacity of one SPU, so SIP traffic is discarded by the SRX.
For Voice over IP Traffic such as SIP/H.323, SRX uses a hash based on the SIP server’s IP to allocate sessions to the SPU. Hence, all SIP sessions of the same SIP server are allocated to the same SPU.
This is the current behavior of SRX to allocate sessions for SIP traffic. Currently, there is no plan to change SIP ALG distribution mechanism from static to dynamic. Due to the static mapping of the SIP server to the SPU, the maximum concurrent SIP calls and new call per second of one SIP server cannot exceed the SIP capacity of one SPU.
If this happens, the only solution is to use multiple SRX devices to protect one SIP server.