Cisco Cisco ASR 5000
Topology-based Gateway Selection
▀ How It Works
▄ SGSN Administration Guide, StarOS Release 18
If the ‘topology' option is configured (part of
gw-selection
configuration - see Configuring Topology-based GW
Selection for S4-SGSN) and the hostname for the existing S-GW has the "topoff" prefix, then the co-located S-GW/P-
GW node will be selected, if available.
GW node will be selected, if available.
Intra RAU, New SGSN RAU, Intra SRNS, New SRNS, IRAT
Assuming ‘topology’ option is configured (see Configuring Topology-based GW Selection for S4-SGSN) then for all of
these procedures selection of the S-GW node will be based on the available P-GW. Therefore, the SGSN will do DNS
with RAI-FQDN to get the list of S-GW hostnames and apply topology matching to determine the hostname of an
available P-GW.
these procedures selection of the S-GW node will be based on the available P-GW. Therefore, the SGSN will do DNS
with RAI-FQDN to get the list of S-GW hostnames and apply topology matching to determine the hostname of an
available P-GW.
Before performing the topology matching, the SGSN checks to determine if the existing S-GW address is available in
the DNS result. If the S-GW address is listed as available, then the SGSN continues with the S-GW. If the S-GW
address is not listed as available in the query results, then the SGSN looks for an S-GW that is co-located or a
topologically-closer to the available P-GW.
the DNS result. If the S-GW address is listed as available, then the SGSN continues with the S-GW. If the S-GW
address is not listed as available in the query results, then the SGSN looks for an S-GW that is co-located or a
topologically-closer to the available P-GW.
We must also consider how the P-GW hostname is selected when multiple PDN connections are available. Currently,
the SGSN selects the first available valid P-GW hostname from the list of PDN connections. For in-bound roamers, the
PDN connection belongs to the home network P-GW will not be used for topology matching.
the SGSN selects the first available valid P-GW hostname from the list of PDN connections. For in-bound roamers, the
PDN connection belongs to the home network P-GW will not be used for topology matching.
Limitations
Topology matching is not applicable for inbound roamers with home routed PDN connections, as the hostnames
are under different operator's administrative control.
Topology-based GW selection may not be applicable if the P-GW and/or the S-GW address is locally configured
or if the static P-GW address is received from the HSS (because the hostname/canonical node name would not
be available for topology matching).
be available for topology matching).
Standards Compliance
This feature complies with the following standards:
TS 23.060 version 10
TS 29.303 version 10
TS 29.274 version 10