MPLS

MPLS stands for 'Multi Protocol Label Switching'.

Similar to the Internet, service providers' MPLS networks are shared infrastructure because many customer enterprises connect to and share the same infrastructure to make WAN connections.

However, the label switching in the name of MPLS allows VPNs to be created over the MPLS infrastructure through the use of labels.

Some important terms:

• CE router -> Customer Edge router

• PE router -> Provider Edge router

• P router -> Provider core router

When the PE routers receive frames from the CE routers, they add a label to the frame.

These labels are used to make forwarding decisions within the service provider network, not the destination IP.

The CE routers do not use MPLS, it is only used by the PE/P routers.

When using a Layer 3 MPLS VPN, the CE and PE routers peer using OSPF, for example, to share routing information.

For example, in the diagram below Office A's CE will peer with one PE, and Office B's CE will peer with the other PE.

Office A's CE will learn about Office B's routes via this OSPF peering, and Office B's CE will learn about Office A's routes too.

When using a Layer 2 MPLS VPN, the CE and PE routers do not form peerings.

The service provider network is entirely transparent to the CE routers.

In effect, it is like the two CE routers are directly connected.

• Their WAN interfaces will be in the same subnet.

If a routing protocol is used, the two CE routers will peer directly with each other.

Many different technologies can be used to connect to a service provider's MPLS network for WAN service.