Soft-GRE refers to stateless GRE tunneling, whereby the AP forwards GRE encapsulated traffic to the WLAN-GW, and the gateway (GW) reflects the encapsulation in the downstream traffic toward the AP. WLAN-GW does not require any per-AP end-point IP address configuration. The WLAN-GW learns the encapsulation as part of creating the subscriber state on processing the encapsulated control and data traffic. Following are some of the advantages of soft-GRE:
Resources are only consumed on the WLAN-GW if there is one or more active subscriber on the AP. Merely broadcasting an SSID from an AP does not result in any state on the WLAN-GW.
No per-AP tunnel end-point configuration on WLAN-GW. This is important as the AP can get renumbered.
No control protocol to setup and maintain tunnel state on WLAN-GW.
Soft-GRE tunnel termination is performed on dedicated MS-ISAs. MS-ISA provides tunnel encapsulation/decapsulation, bandwidth shaping per tunnel (or per-tunnel per SSID), and anchor point for inter-AP mobility. The ESM function such as per-subscriber anti-spoofing (IP and MAC), filters, hierarchical policing, and lawful intercept are provided on the carrier IOM corresponding to the ISA where the subscriber is anchored.
By default WLAN-GW uses IOM level resiliency which requires dedicated IOMs with only BB ISAs, these are also referred to as WLAN-GW IOMs. In this mode a single ISA failure causes a full backup IOM to take over, independent of the state of other ISAs. This mode is recommended in combination with ESM as it provides guaranteed resource recovery in failure cases. The WLAN-GW also supports MDA level resiliency as indicated in MDA-based redundancy.