Figure 49 shows an LSP that uses label stacking. The ingress node, LSR 1, receives an unlabeled data packet and prepends label d to the packet. LSR 2 receives the packet, removes label d and uses it as an index in its forwarding table to find the next label, prepending label e to the packet. LSR 3 removes label e and prepends label s (negotiated with LSR 5) to the packet. LSR 3 pushes label x on top of label s. LSR 4 pops the top (outermost) label, x, and pushes label r on top of label s. LSR 5 pops label r, determines that it must pop label s, and pushes label z on the empty stack. Finally, the egress node, LSR 6, removes label z and determines where to forward the packet outside the MPLS domain.
Figure 49: Label Stacking
The configuration shown in Figure 49 is an example of an LSP within an LSP (a tunnel within a tunnel). The first LSP consists of LSR 1, LSR 2, LSR 3, LSR 5, and LSR 6. The second LSP consists of LSR 3, LSR 4, and LSR 5. The two LSPs have different ingress and egress points. LSR 1 and LSR 6 are LERs. Less obviously, LSR 3 and LSR 5 are also LERs, but for the internal LSP.
 |
Note: Label stacking is typically employed for LSR peers that are not directly connected. Figure 49 is a simplified example to illustrate the concept of label stacking. |
 | Copyright © 2008, Juniper Networks, Inc. All rights reserved. Trademark Notice. | |