MPLS Transit Router Label Operation

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An MPLS (Multiprotocol Label Switching) transit router is a type of router that forwards traffic through an MPLS network. In this context, MPLS is a protocol that uses labels to direct traffic through the network, improving its speed and efficiency.

Among the label operations available in MPLS, the most common are: push, pop, swap, and swap and push.

• Push: This operation adds a new label to the top of the MPLS stack. When a router pushes a label, it assigns a new forwarding equivalence class (FEC) to the packet, based on its destination IP address. The label then becomes the top label in the stack and is used by subsequent routers to forward the packet.

• Pop: This operation removes the top label from the MPLS stack. When a router pops a label, it forwards the packet based on the next label in the stack, or based on the underlying IP header if there are no more labels. The popping router also updates the Time to Live (TTL) field in the IP header before forwarding the packet.

• Swap: This operation replaces the top label in the MPLS stack with a new label. When a router swaps a label, it assigns a new FEC to the packet, based on the new label. The router then forwards the packet based on the new label.

• Swap and push: This operation combines the swap and push operations. It replaces the top label in the MPLS stack with a new label and pushes another label onto the top of the stack. This allows the router to assign a new FEC to the packet based on the new label, while also forwarding it based on the new top label.

Of these label operations, the most likely operation performed by an MPLS transit router is "swap". This is because transit routers are responsible for forwarding packets between different parts of the network, and often need to change the label to reflect the new destination of the packet. When a transit router swaps a label, it replaces the existing label with a new label that reflects the next hop in the path to the packet's final destination.