MPLS-Enabled Applications: Emerging Developments and New Technologies

6.8. P2MP LSP HIERARCHY

In the Foundations chapter, we discussed how MPLS has important hierarchy properties to aid scaling. We discussed how in the data plane, the hierarchy is achieved by label stacking, for example in the following scenarios:

• Traffic from multiple VPNs can share the same transport LSP. An inner VPN label is used to distinguish between VPNs and an outer label is used to denote the transport LSP.

• LDP-signaled LSPs can be tunneled through an RSVP-signaled LSP. An inner label denotes the LDP LSP and the outer label denotes the RSVP LSP.

• RSVP-signaled LSPs can be nested inside other RSVP-signaled LSPs using the RSVP LSP hierarchy scheme. An inner label denotes the inner RSVP LSP and an outer label denotes the outer RSVP LSP.

Similar concepts are applicable in the case of P2MP LSPs. Traffic from multiple VPNs can share the same P2MP LSP, with inner labels being used to distinguish traffic from the different VPNs. This is discussed further in Section 6.9.2.

P2MP LSPs can be nested inside other P2MP LSPs in order to reduce the amount of state in the core of the network. Figure 6.10 shows an example. PE1 is the ingress of P2MP LSP X, and PE3 and PE5 are the egress routers. PE2 is the ingress router for P2MP LSP Y, and PE4 and PE6 are the egress routers. As can be seen, the two P2MP LSPs have P1, P2, P3 and P4 in common. P2MP LSP Z, denoted by the wide gray lines, can be created in the core with P1 as the ingress and P3 and P4 as the egresses. P2MP LSPs X and Y can be ...