So once upon a short time back I worked on the mobile core (I still insist thats the best team to work with - ever). The interface between the sgsn and the GGSN is called the gn interface. (I promise to do more posts on the mobile core).
Now our GGSN's were actually a couple of cisco MWAM blades hosting several ggsn's each.
We used DNS for load balancing traffic from the sgsn to the ggsn. Here's a brief of how it works:
during session setup also known as pdp context activation, the sgsn is supposed to set up a tunnel (gtp tunnel) with a ggsn for each session. Now the user uses an 'apn' like 'safaricom' which defines the service a customer is allowed to access. each ggsn is configured to allow a specific apn access, the sgsn looks at the apn, checks a dns server, resolves the apn to a ggsn's ip address and creates the tunnel.
DNS is therefore used to decide which ggsn to channel the request to. Multiple DNS servers means you get to load balance the traffic among the ggsn's - round robin. It was not perfect, but it was cheaper than trying to get loadbalancers in.
If you'd like to know more about how the mobile core works, how its all put together drop a comment ....the design considerations are definately way more than trying to slap together a document for a pure IP core....
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