TCP/IP Tutorial and Technical Overview - IBM Redbooks

For example, a network might contain an LSP . In this
example, R2 and R3 are not directly connected, but are peers endpoints of an
LSP tunnel. The actual sequence of LSRs traversed through the network is . Figure A-3 shows this configuration.
Level 1
Level 2
R1 R2
Figure A-3 LSP tunnels
Lb La IP hdr Payload
Lc La IP hdr Payload
LSP Tunnel
R21 R22
R3 R4
La IP hdr Payload
A packet traversing this network travels along a level-1 LSP: .
Then, when traveling from R2 to R3, uses a level-2 LSP: .
From the perspective of the level-1 LSP, R2’s peer devices are R1 and R3. From
the level-2 perspective, R2’s peer device is R21.
Using this diagram, the following actions occur when a packet is sent through the
LSP tunnel:
1. R2 receives a labeled packet from R1. The packet contains a single label.
The depth of the label stack is one.
2. R2 pops this label and pushes a label understood by R3. This label is called
La.
3. R2 must also include a label understood by R21. R2 pushes the label on top
of the existing level-1 label. This label is called Lb. The label stack contains
two entries.
4. R2 forwards the packet to R21.
5. R21 pops the level-2 label (Lb) appended by R2 and pushes a level-2 label
understood by R22. This label is called Lc. R21 does not process the level-1
label. The label stack contains two entries.
6. R21 forwards the packet to R22.
Appendix A. Multiprotocol Label Switching 935