[Definition]  

Layer 3 virtual private network (L3VPN) is a network communication service provided by mobile bearer network carriers for wireless network users. The L3VPN technology used by RTN 900 series NEs is based on Border Gateway Protocol (BGP) and Multiprotocol Label Switching (MPLS), forming a BGP/MPLS virtual private network (VPN). BGP is used to advertise VPN routes within a bearer network, and MPLS is used to forward VPN packets within the bearer network. Figure 4-11 shows a basic L3VPN model.

Figure 1-1 L3VPN model

As shown in Figure 4-11, the L3VPN comprises customer edge (CEs), provider edges (PEs) and Provider (P) devices.

l   CEs: In mobile bearer scenarios, CEs are wireless devices (base stations and base station controllers). They are not aware of the existence of L3VPNs and regard the bearer network as a virtual router.

l   PEs: PEs are directly connected to CEs. All VPN processing (route advertisement, L3VPN packet forwarding, and protocol running) occurs on the PEs.

l   P devices: P devices are intermediate devices on the bearer network. They only support the basic MPLS forwarding capability.

 

[Benefits]

The L3VPN feature enables RTN devices to form E2E networks with CX devices. The use of RTN products with the L3VPN feature brings the following benefits: 

l   Base stations can be conveniently re-homed.

l   O&M means are unified and sustainable on RTN+CX networks.

l   Virtual local area network (VLAN) planning is easy.

l   Make networks more adaptable.

 

[Application Scenario]

On a layered L3VPN consisting of RTN and CX devices, base stations access the bear network through cell site gateways (CSGs). Service gateways (SGWs), mobility management entities (MMEs), and radio network controllers (RNCs) access the bearer network through radio service gateways (RSGs). CSGs, aggregation site gateways (ASGs), and RSGs work together to provide L3VPN services and transmit 3G, Long Term Evolution (LTE) S1, and LTE X2 traffic.

In this L3VPN scheme, RTN and CX devices belong to the same Intermediate System to Intermediate System (IS-IS) process and routing domain. To form IS-IS interconnection with CX devices, RTN devices must meet the following requirements:

l   Neighbor relationships can be established.

l   Network link status and traffic engineering (TE) information can be synchronized.

l   A routing table can be generated on the control plane.

To establish dynamic Resource Reservation Protocol (RSVP) tunnels with CX devices, RTN devices must be able to function as the source or sink nodes. To establish Multiprotocol Interior Border Gateway Protocol (MP-IBGP) connections with CX devices, RTN devices must support neighbor relationship establishment and private network route advertisement and maintenance. On the L3VPN, it is easy to deploy MP-IBGP on RTN devices. Neighbor relationships need to be established between ASGs and CSGs. If a CSG is homed to two ASGs, neighbor relationships must be established with the two ASGs separately. Then the CSG will prefer a route from those advertised by the two ASGs.

No CSG node is involved during hierarchy VPN (HVPN) and hierarchy of VPN (HoVPN) deployment.

Description of the L3VPN deployment:

l   ASGs are connected to CSGs and RSGs by tunnels, but no tunnel needs to be established between CSGs and RSGs.

l   ASGs are configured as BGP neighbors of CSGs and RSGs, but no BGP relationship needs to be established between CSGs and RSGs.

l   ASGs function as route reflectors of CSGs and RSGs to reflect routes between CSGs and between CSGs and RSGs.

l   L3VPN services need to be configured on CSGs and RSGs. If HoVPN is deployed on ASGs, L3VPN services also need to be configured on ASGs. If HVPN is deployed on ASGs, L3VPN services do not need to be configured on ASGs.

l   RTN devices are deployed as CSGs and only need to support basic L3VPN service configurations.

l   Dynamic Host Configuration Protocol (DHCP) interaction is executed among the DHCP client, DHCP relay, and DHCP server within the bearer network.

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