Cisco Cisco IOS Software Release 12.2(14)ZA

When configuring the network shared by IOS SLB and the GGSNs, keep the following considerations 
in mind:

Specify static routes (using ip route commands) and real server IP addresses (using real commands) 
such that the Layer 2 information is correct and unambiguous.

Choose subnets carefully, using one of the following methods:

Do not overlap virtual template address subnets.

Specify next hop addresses to real servers, not to interfaces on those servers.

IOS SLB assigns all PDP context creates from a specific IMSI to the same GGSN.

IOS SLB supports both GTP Version 0 (GTP v0) and GTP Version 1 (GTP v1). Support for GTP 
enables IOS SLB to become “GTP aware,” extending IOS SLB’s knowledge into Layer 5.

IOS SLB supports two types of GPRS load balancing:

GPRS load balancing without GTP cause code inspection enabled is recommended for Cisco GGSNs. It 
has the following characteristics:

Can operate in dispatched mode or in directed server NAT mode, but not in directed client NAT 
mode. In dispatched mode, the GGSNs must be Layer 2-adjacent to the IOS SLB device.

Does not support stateful backup. See the 

 for more 

information.

Delivers tunnel creation messages destined to the virtual GGSN IP address to one of the real 
GGSNs, using the weighted round robin load-balancing algorithm. See the 

 for more information about this algorithm.

Requires DFP in order to account for secondary PDP contexts in GTP v1.

GPRS load balancing with GTP cause code inspection enabled allows IOS SLB to monitor all 
PDP context signaling flows to and from GGSN server farms. This enables IOS SLB to monitor GTP 
failure cause codes, detecting system-level problems in both Cisco and non-Cisco GGSNs.

 lists the PDP create response cause codes and the corresponding actions taken by IOS SLB.

Request Accepted

Establish session

No Resource Available

Fail current real, reassign session, drop the response

All dynamic addresses are occupied

Fail current real, reassign session, drop the response

No memory is available

Fail current real, reassign session, drop the response

System Failure

Fail current real, reassign session, drop the response

Missing or Unknown APN

Forward the response