Cisco Cisco ASR 5000
Topology-based Gateway Selection
How It Works ▀
SGSN Administration Guide, StarOS Release 18 ▄
How It Works
Selection of a co-located node or a topologically closer node is based on string comparison of canonical node names
included in two or more sets of records received in a DNS S-NAPTR query result.
included in two or more sets of records received in a DNS S-NAPTR query result.
A canonical node name (a multi-labeled substring of the hostname) is a unique name representing a node. For
comparison, the canonical node names are derived from the hostnames received in the DNS records. For co-located
nodes, the canonical node names strings must be exactly same. Each node may have different hostnames assigned to
each supported interface based on service and protocol.
comparison, the canonical node names are derived from the hostnames received in the DNS records. For co-located
nodes, the canonical node names strings must be exactly same. Each node may have different hostnames assigned to
each supported interface based on service and protocol.
According to 3GPP TS 29.303 [4.3], hostnames must adhere to the following format:
<topon|topoff>.<single-label-interface-name>
.
<canonical-node-name>;
for example:
topon.s5-gtp.pgw.dc.central.bang.kar.3gppnetwork.org.
“topon” indicates that the canonical node name can be used for topology match.
“second-label-interface-name” of “s5-gtp” indicates that this hostname belongs to S5 interface supporting GTP
protocol.
“canonical-node-name” is the portion “pgw.dc.central.bang.kar.3gppnetwork.org”
The canonical node name is obtained by stripping off the first two labels.
First Primary Activation - Gn/Gp-SGSN
Topology matching is applicable only for primary activation for the Gn/Gp-SGSN and is based primarily on canonical
node name comparison. Canonical node name for the SGSN must be defined as part of the SGSN Global configuration
(see Configuring Topology-based GW Selection for Gn/Gp-SGSN). The canonical node names for the GGSN and/or the
P-GW are the substring of hostnames received in the DNS results with query using APN-FQDN. Topology-based GW
selection can only be achieved in the Gn/Gp SGSN through S-NAPTR query, which must be enabled as part of the
feature configuration. If the SGSN’s canonical node name is not configured, then GW selection will proceed as though
topology is not enabled.
node name comparison. Canonical node name for the SGSN must be defined as part of the SGSN Global configuration
(see Configuring Topology-based GW Selection for Gn/Gp-SGSN). The canonical node names for the GGSN and/or the
P-GW are the substring of hostnames received in the DNS results with query using APN-FQDN. Topology-based GW
selection can only be achieved in the Gn/Gp SGSN through S-NAPTR query, which must be enabled as part of the
feature configuration. If the SGSN’s canonical node name is not configured, then GW selection will proceed as though
topology is not enabled.
Primary Activation - S4-SGSN
First primary activation involves selection of both the P-GW and S-GW nodes.
If “topology” is configured (see Configuring Topology-based GW Selection for S4-SGSN), then the S4-SGSN shall
apply topology-based selection for the P-GW and the S-GW node selection. If “weight” is configured, then the highest
degree node pair is selected. If CSR fails, then the next highest degree node pair is selected, which maybe a different P-
GW and S-GW node pair than the pair previously selected.
apply topology-based selection for the P-GW and the S-GW node selection. If “weight” is configured, then the highest
degree node pair is selected. If CSR fails, then the next highest degree node pair is selected, which maybe a different P-
GW and S-GW node pair than the pair previously selected.
Primary Activation for Subsequent PDN
For a UE, all PDN connections must use the same S-GW. So, in subsequent PDN connections the S-GW is already
selected. Therefore, topology will be applied to find the closest P-GW to the selected S-GW.
selected. Therefore, topology will be applied to find the closest P-GW to the selected S-GW.