- Overview of GPRS
- Planning to Configure the GGSN
- Configuring GGSN GTP Services
- Configuring Charging on the GGSN
- Configuring Network Access to the GGSN
- Configuring PPP Support on the GGSN
- Optimizing GGSN Performance
- Configuring QoS on the GGSN
- Configuring Security on the GGSN
- Configuring DHCP on the GGSN
- Configuring GTP Load Balancing
- Overview of GDM
- Planning to Configure GDM
- Configuring GDM
- Monitoring and Maintaining GDM
Overview of GDM
This chapter provides a brief introduction to the GTP Director Module (GDM) and its implementation in the Cisco IOS software.
This chapter includes the following sections:
•Load Balancing Processing by GDM
Feature Description
As of GGSN Release 3.0, Cisco Systems adds GDM as part of the Gateway GPRS Support Node (GGSN) feature set in the Cisco IOS software. GDM extends some of the benefits that are available on a Cisco Systems GGSN, to GPRS environments where non-Cisco GGSNs are implemented.
These benefits include reducing APN provisioning requirements in the GPRS Public Land Mobile Network (PLMN), while also providing simple, round-robin load balancing for the GGSNs. A network using GDM has the added benefit of the Cisco Systems Hot Standby Router Protocol (HSRP) to support increased network availability using a backup GDM router.
Like the Cisco Systems' GGSN, GDM provides access to multiple destination networks through a virtual APN that is provisioned at the Home Location Register (HLR). GDM's virtual APN support simplifies the maintenance and provisioning issues in the GPRS PLMN significantly. In this one-to-many model, one APN can be provisioned for multiple subscribers, and that one APN can provide access to many real destination networks. By implementing virtual APN support, service providers can add new access points without having to provision the HLR.
Using DNS, GDM also provides round-robin load balancing for those GGSNs that support access to the same destination networks.
To provide increased network availability, a backup GDM router can be configured to automatically switch over and become the primary GDM router using HSRP. The backup GDM router can provide access to the GGSNs if the primary GDM router, or even a critical interface on the primary GDM router, becomes unavailable.
Although GDM is part of the GGSN feature set, it cannot coexist on a router that is also configured as a Cisco Systems GGSN. However, GDM can be used in a mixed environment of Cisco and non-Cisco GGSNs.
GDM does not add any value to an environment that includes only Cisco Systems GGSNs, considering that Cisco Systems GGSNs have alternative and enhanced load balancing solutions, and can natively provide virtual APN support. For more information about load balancing options for a Cisco Systems GGSN, see the "Configuring Load Balancing on the GGSN" chapter. For information about virtual APN support, see the "Configuring Virtual APN Access on the GGSN" section on page 77.
Request Processing by GDM
This section describes how GDM processes create PDP context requests and retries of those requests, and describes several different request processing scenarios. This section includes the following topics:
•Overview of Request Processing by GDM
•Request Processing Using a Virtual APN
Overview of Request Processing by GDM
GDM's role in the GPRS PLMN is to facilitate the processing of create PDP context requests between an SGSN and one or more GGSNs. GDM processes create PDP context requests sent by an SGSN, and forwards them to the appropriate destination GGSN. GDM does not monitor whether or not a create PDP context request has been successful, or if a path has been established between an SGSN and GGSN for a particular tunnel ID (TID).
In the case of an unsuccessful session establishment, GDM continues to receive retry requests from an SGSN. GDM processes the retries of a create PDP context request for a particular TID and forwards those retries to the GGSN to which the original request was sent. However, GDM only processes those retries for a configurable period of time. GDM forwards retries of a create PDP context request to a GGSN for 30 seconds (default), or for the amount of time that you have configured in the gprs gtp-director retry-timeout command.
Once GDM has sent create PDP context requests to a GGSN and has processed any retries, GDM is no longer involved in any other forms of request processing for that PDP context. User authentication for a PDP context request is handled as usual between the GGSN and the authentication, authorization, and accounting (AAA) server.
All of the other signaling request processing occurs directly between the SGSN and the GGSN, over the GTP path established between them. GDM is never part of the GTP path, and data does not flow through GDM. The GTP path remains as usual between the SGSN and a GGSN for a PDP context. For troubleshooting purposes, it is important to note that GDM is never even aware of whether a PDP context has been successfully established with a GGSN.
GDM is not involved in the processing of the following types of requests:
•Echo Requests
•Delete PDP Context Requests
•Update Requests
Request Processing Using a Virtual APN
In the GDM environment using virtual APN support, a virtual APN is used to select the Cisco Systems GDM router. GDM facilitates the processing of the create PDP context request to the real APNs through the appropriate GGSNs. The GGSNs always provide the physical connectivity to the real target network.
To implement virtual APN support using GDM, you need to determine the name(s) of the virtual access point(s) that you want subscribers to use for access to one or more real APNs that are configured on your GGSNs.
Figure 19 shows how GDM supports a create PDP context request from an MS processed through a virtual APN using GDM in its intended router environment where non-Cisco GGSNs are in use.
Note Recall that you can also use GDM in a mixed environment of Cisco and non-Cisco GGSNs, or in an all Cisco GGSN environment. However, for an environment using only Cisco GGSNs, there are alternative and enhanced load balancing solutions and virtual APN support is already available, which makes GDM less worthwhile for that environment.
Figure 19 Virtual APN PDP Context Activation Using GDM and non-Cisco GGSNs
Request Processing Scenarios
GDM processes a PDP context according to the content found in the APN Information Element (IE) and the Protocol Configuration Option (PCO) of the create PDP context request, according to the following scenarios:
•CreatePDPContext (APN=virtual, PCO=ciscouser@real)
In this format the APN IE exists, and the PCO specifies a username@domain (see Figure 19). This format is used to implement virtual APN support through a virtual APN. In this scenario the APN IE designates a virtual access point. The APN IE is used to direct the request to GDM (the SGSN's DNS query for the virtual APN should return the IP address of GDM). GDM uses the domain as the real APN, and performs a DNS query on the domain name to locate the appropriate destination GGSN.
•CreatePDPContext (APN=real, PCO=ciscouser)
In this format the APN IE exists, and the PCO only specifies a username (no domain). In this scenario the APN IE must designate a real access point. The APN IE is used to direct the request to GDM (the SGSN's DNS query for the real APN should return the IP address of GDM). GDM also uses the APN IE (the real APN) to perform a DNS query to locate the appropriate destination GGSN.
•CreatePDPContext (APN=real, PCO=)
In this format, the APN IE exists, and the PCO is null. This format is found when anonymous access is being used. In this scenario the APN IE must designate a real access point. The APN IE is used to direct the request to GDM (the SGSN's DNS query for the real APN should return the IP address of GDM). GDM also uses the APN IE (the real APN) to perform a DNS query to locate the appropriate destination GGSN.
Load Balancing Processing by GDM
GDM supports basic load balancing using the DNS server's ability to return to GDM a list of IP addresses in round-robin fashion for a particular domain name. The DNS server can return a list of up to 8 addresses to GDM for each domain name. GDM always uses the first IP address returned by the DNS server. The IP addresses correspond to the GGSNs available to support the requested real APN.
The name for which GDM performs a DNS query is based upon the content of the APN IE and the PCO IE of the create PDP context request. If the create PDP context request specifies a domain, then GDM queries the DNS server for that domain name. The username and password that the MS requests in the form of "login@domain" is provided in the PCO IE of the create PDP context request. The domain name in the MS request, and for which GDM queries the DNS server, should correspond to the name of the real APNs that are configured on the GGSNs. Those GGSNs provide connectivity to the physical network for that APN.
If the MS does not specify a domain, or the PCO is null, then GDM queries the DNS server for the real APN found in the APN IE.
For load balancing support, you must configure the DNS server for GDM with the IP addresses of all of the GGSNs (up to 8) that support connectivity to the physical network for the domain. You also should verify that the round-robin mechanism is enabled for the DNS server when it returns a list of IP addresses for a domain.
Benefits
GDM provides the following benefits:
•Reduction in HLR provisioning through virtual APN support.
•Sharing of GGSN resources using round-robin load balancing.
•Backup router support for GDM functions using HSRP.