How it Works
This section briefly describes how the MPLS feature works on UPF.
The VPP forwarder supports all existing functionalities for MPLS packet processing with its own MPLS stack. The VPP MPLS stack is configured with the appropriate FTN (FEC To NHLFE), forwarding information base (FIB) table, and incoming label map (ILM) table. The egress generates the MPLS packet with the correct MPLS header. It also processes the incoming MPLS packet and switches the packet to the appropriate VRF table based on the incoming label.
VPC-SI also supports VPNv6 as described in RFC 4659 – BGP-MPLS IP Virtual Private Network (VPN) Extension for IPv6 VPN.
Deploying UPFs using the N9 Interface
The 5G Core UPF can deploy two UPFs in series through the N9 interface. The N9 interface connects two UPFs between Intermediate I-UPF and UPF Session Anchor. To provide complete mobility with a stable IP anchor across the whole network, the two UPFs are connected depending on the operator network configuration.
Both upstream and downstream traffic between the UPFs is in the form of GTP-U packets. To support MPLS over the N9 interface, the L3 routers present between the two UPFs are capable of advertising BGP routes and labels to the UPFs.
In the upstream direction, I-UPF receives the GTP-U packets from gNodeB and modifies the content of the GTP-U header based on the destination (anchor) UPF. Based on the lookup of the IP header, MPLS encapsulation occurs to encapsulate the GTP-U packet. The MPLS label that needs to be inserted will be received through BGP from the intermediate L3 devices between UPFs. At the anchor UPF, the MPLS header gets decapsulated and transmitted as an IP packet towards the PDN.
In the downstream direction, MPLS encapsulation occurs in the anchor UPF after the IP packet received from PDN is encapsulated by a GTP-U header. This MPLS header is then decapsulated in I-UPF and the corresponding GTP-U packet is then transmitted to gNodeB.
In the following scenarios, UPF moves the N9 interface to a VRF and establishes a VPNv4 or VPNv6 AF BGP neighborship:
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When the N3 and N9 interfaces share the same physical interface.
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When the N3 and N9 interfaces are separated.