MPLS and RSVP
7450 ESS MPLS Guide Page 25
A stack can carry several labels, organized in a last in/first out order. The top of the label stack
appears first in the packet and the bottom of the stack appears last (Figure 2).
Figure 2: Label Packet Placement
The label value at the top of the stack is looked up when a labeled packet is received. A successful
lookup reveals:
• The next hop where the packet is to be forwarded.
• The operation to be performed on the label stack before forwarding.
In addition, the lookup may reveal outgoing data link encapsulation and other information needed
to properly forward the packet.
An empty label stack can be thought of as an unlabeled packet. An empty label stack has zero (0)
depth. The label at the bottom of the stack is referred to as the Level 1 label. The label above it (if
it exists) is the Level 2 label, and so on. The label at the top of the stack is referred to as the Level
m label.
Labeled packet processing is independent of the level of hierarchy. Processing is always based on
the top label in the stack which includes information about the operations to perform on the
packet's label stack.
OSSG014
Layer 2 Header Top Label … Bottom Label Data Packet
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