Describe Push, Swap, Pop Functions In MPLS

In the context of Multiprotocol Label Switching (MPLS), the terms "Push," "Swap," and "Pop" refer to operations related to label stacking. MPLS uses labels to make forwarding decisions, and these operations help manage the label stack as packets travel through the network. Here's a brief overview of each operation:

1. Push:

   • Function: Adds a label to the top of the existing label stack.
   • When It's Used: Typically used when a packet enters an MPLS domain. An MPLS router (Label Edge Router or LER) at the edge of the network adds a new label to the packet's label stack before forwarding it into the MPLS network.
   • Example: Suppose a packet arrives with no label stack or with an existing stack. The LER pushes a new label onto the stack, which signifies the path the packet should take through the MPLS network.

2. Swap:

   • Function: Replaces the label at the top of the label stack with a new label.
   • When It's Used: Used within the MPLS network by Label Switching Routers (LSRs) to update the label as the packet moves through different segments of the network. The new label guides the packet to its next hop.
   • Example: As a packet moves through the MPLS network, the current LSR swaps the top label with a new one that directs the packet to the next hop router. This operation helps in routing the packet efficiently through the network.

3. Pop:

   • Function: Removes the top label from the label stack.
   • When It's Used: Typically used when a packet exits the MPLS network and is handed off to an external network or to the final destination. The Label Edge Router (LER) at the egress of the MPLS network pops the MPLS label, revealing the original packet.
   • Example: When the packet reaches the egress router, the MPLS label is popped off, and the packet, now without an MPLS label, is forwarded based on its original IP header or other header information.