How many labels for VPN in MPLS ?
For those who has good amount of knowledge in MPLS already may know the answer. Or if you have taken my CCDE course before, this question is basic for you.
But understanding this fundamental piece of knowledge is key to understand MPLS Applications. MPLS Applications mean use cases with MPLS.
With MPLS , VPNs can be provisioned as either Layer 2 or Layer 3.
In the same network, we see both Layer 2 and Layer 3 VPNs together most of the time (Especially in the Service Provider networks)
In fact, on the same MPLS PE device , Service Providers deploy both Layer 2 and Layer 3 VPN together.
Layer 2 VPN with MPLS transport can be deployed without Pseudowire (PW). As of 2018, EVPN (Ethernet VPNs are getting more popular as pseudowire-less Layer 2 VPN mechanism (In fact EVPN can provide MPLS Layer 3 VPN as well as of 2018).
If for the MPLS Layer 2 VPNs, PW will be provisioned (Martini and Kompella methods) for the MPLS Layer 2 VPN, then 2 MPLS labels are necessary.
One for transport MPLS label which is used for the reachability between the PE devices. Another for the service (PW label) label , also called is VC (Virtual Circuit) label.
PE devices have to reach each other and this is achieved with transport label , which is also known as tunnel or topmost label.
In the label stack , this label is found at the top , MPLS P device does its operation based on the transport label. They don’t know the PW label at all. This provides scalability in MPLS Layer 2 VPN design.
For the MPLS Layer 3 VPN, two label is necessary. One for transport , as in the case of MPLS Layer 2 VPN, one for service label , which is mostly known as VPN label.
I explained the roles of each device types (/PE , P , CE ) in this article.
If you have any question regarding MPLS , please ask them in the comment box below.