Label Operations
Figure 1 depicts an LDP LSP being tunneled through an RSVP LSP. (For definitions of label operations, see Label Description.) The shaded inner oval represents the RSVP domain, whereas the outer oval depicts the LDP domain. RSVP establishes an LSP through routers B, C, D, and E, with the sequence of labels L3, L4. LDP establishes an LSP through Routers A, B, E, F, and G, with the sequence of labels L1, L2, L5. LDP views the RSVP LSP between Routers B and E as a single hop.
When the packet arrives at Router A, it enters the LSP established by LDP, and a label (L1) is pushed onto the packet. When the packet arrives at Router B, the label (L1) is swapped with another label (L2). Because the packet is entering the traffic-engineered LSP established by RSVP, a second label (L3) is pushed onto the packet.
This outer label (L3) is swapped with a new label (L4) at the intermediate router (C) within the RSVP LSP tunnel, and when the penultimate router (D) is reached, the top label is popped. Router E swaps the label (L2) with a new label (L5), and the penultimate router for the LDP-established LSP (F) pops the last label.
Figure 1: Swap and Push When LDP LSPs Are Tunneled Through RSVP LSPs
Figure 2 depicts a double push label operation (L1L2). A double push label operation is used when the ingress router (A) for both the LDP LSP and the RSVP LSP tunneled through it is the same device. Note that Router D is the penultimate hop for the LDP-established LSP, so L2 is popped from the packet by Router D.
Figure 2: Double Push When LDP LSPs Are Tunneled Through RSVP LSPs
