Cisco Cisco Packet Data Gateway (PDG) Manual De Manutenção

Generally Available    06-30-2010

SGSN Features in Release 8.1

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The NPU FastPath feature is proprietary and only available on the ST40 SGSN systems. 
The purpose of this type of internal direct tunnel is to optimize resource usage and reduce 
latency when processing GTP-U packets. Incoming traffic passes through the switch fabric 
and the routing headers are changed to re-route traffic from the incoming network 
processing unit (NPU) of the ingress PSC directly to the outgoing NPU of the egress PSC. 
This means that intervening NPUs and CPUs are by-passed. This provides the SGSN with 
router-like latency and increased node signaling capacity.

Figure 1-5 SGSN NPU FastPath

Fast path is established when both ends of a tunnel are available. Two fast path flows are 
established, one for the uplink and one for the downlink direction for a given PDP context.

If FastPath cannot be established, the NPU forwards the GTP-U packets to a CPU for 
processing and they are processed like all other packets.

FastPath can not be established for subscriber PDP sessions if:

●

Traffic Policing is enabled

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Subscriber Monitoring is enabled

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Lawful Intercept (LI) is enabled

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IP Source Violation Checks are enabled

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GTP-v0 tunnel is esatlished with a GGSN

GTP-C/RANAP

GTP-U