Label Operations

Each LSR along an LSP is responsible for examining the MPLS label, determining the LSP next hop, and performing the required label operations. LSRs can perform five label operations:

• Push—Adds a new label to the top of the packet. For IPv4 packets arriving at the inbound router, the new label is the first label in the label stack. For MPLS packets with an existing label, this operation adds a label to the stack and sets the stacking bit to 0, indicating that more MPLS labels follow the first.

  When it receives the packet, the inbound router performs an IP route lookup on the packet. Because the route lookup yields an LSP next hop, the inbound router performs a label push on the packet, and then forwards the packet to the LSP next hop.

• Swap—Replaces the label at the top of the label stack with a new label.

  When a transit router receives the packet, it performs an MPLS forwarding table lookup. The lookup yields the LSP next hop and the path index of the link between the transit router and the next router in the LSP.

• Pop—Removes the label from the top of the label stack. For IPv4 packets arriving at the penultimate router, the entire MPLS label is removed from the label stack. For MPLS packets with an existing label, this operation removes the top label from the label stack and modifies the stacking bit as necessary—sets it to 1, for example, if only a single label remains in the stack.

  If multiple LSPs terminate at the same outbound router, the router performs MPLS label operations for all outbound traffic on the LSPs. To share the operations among multiple routers, most LSPs use penultimate hop popping (PHP).

• Multiple push—Adds multiple labels to the top of the label stack. This action is equivalent to performing multiple push operations.

  The multiple push operation is used with label stacking, which is beyond the scope of this guide.

• Swap and push—Replaces the top label with a new label and then pushes a new label to the top of the stack.

  The swap and push operation is used with label stacking, which is beyond the scope of this guide.