User's Manual 386 Document #: LTRT-68822
Mediant 2000
SAS Redundancy mode: If users cannot be distinguished (i.e., associated to a
specific SAS gateway), then the SAS Redundancy feature is used to configure SAS
Cascading. This mode routes the call in a loop fashion, from one SAS gateway to the
next, until the user is located. Each SAS gateway serves as the redundant SAS
gateway (“redundant SAS proxy server”) for the previous SAS gateway (in a one-way
direction). For example, if a user calls a user that is not registered on the same SAS
gateway, the call is routed to the second SAS gateway, and if not located, it is sent to
the third SAS gateway. If the called user is not located on the third (or last) SAS
gateway, it is then routed back to the initial SAS gateway, which then routes the call to
the default gateway (i.e., to the PSTN).
Each SAS gateway adds its IP address to the SIP via header in the INVITE message
before sending it to the next (“redundant”) SAS gateway. If the SAS gateway receives
an INVITE and its IP address appears in the SIP via header, it sends it to the default
gateway (and not to the next SAS gateway), as defined by the SASDefaultGatewayIP
parameter. Therefore, this mode of operation prevents looping between SAS
gateways when a user is not located on any of the SAS gateways.
The figure below illustrates an example of a SAS Cascading call flow when configured
using the SAS Redundancy feature. In this example, a call is initiated from a SAS
Gateway (A) user to a user that is not located on any SAS gateway. The call is
subsequently routed to the PSTN.
Figure 29-2: SAS Cascading Using SAS Redundancy Mode - Example
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