TCP/IP Tutorial and Technical Overview - IBM Redbooks

stack. Using this information, the node encodes a new label stack and forwards
the resulting packet.
Figure A-2 depicts label swapping in an MPLS environment.
In
Label
Addr
Prefix
Out
Port
Out
Label
- 10.0 5 50
1
R1
In
Label
In
Port
Addr
Prefix
Figure A-2 Label swapping in an MPLS environment
Note: In a label swapping environment, the next hop router is always
determined from MPLS information. This might cause the packet to traverse a
different path than the one obtained using conventional routing algorithms.
Penultimate hop popping
This is the ability to pop an MPLS label at the penultimate node rather than at the
egress node. From an architectural perspective, this type of processing is
permitted. The purpose of a label is to forward a packet through the network to
the egress node. After the penultimate node has decided to send the packet to
the egress node, the label no longer has any function. It does not need to be
included in the packet.
The penultimate node pops the stack and forwards the packet based on the next
hop address obtained from the NHLFE. When the egress node receives the
packet, one of two activities occur:
► The packet contains a label. This occurs when the penultimate node
processed a packet with at least two labels. In this scenario, the label now at
the top of the stack is the label the egress node needs to process to make a
forwarding decision.
Out
Port
Out
Label
50 1 10.0 3 20
51 2 10.0 3 21
In
Label
In
Port
Addr
Prefix
Out
Port
Out
Label
20 5 10.0 1 -
21 5 10.0 1 -
22 3 10.0 1 -
5 1 3 5 1
2
R2
R3 R5
10.0.1.5 Payload 50 10.0.1.5 Payload 20 10.0.1.5 Payload 10.0.1.5 Payload
R4
Appendix A. Multiprotocol Label Switching 933
3