Operations on Labels

The router supports the following label operations:

• Push—Add a new label to the top of the packet. For IPv4 packets, the new label is the first label. The time-to-live (TTL) and s bits are derived from the IP packet header. The MPLS class of service (CoS) is derived from the queue number. If the push operation is performed on an existing MPLS packet, you will have a packet with two or more labels. This is called label stacking. The top label must have its s bit set to 0, and might derive CoS and TTL from lower levels. The new top label in a label stack always initializes its TTL to 255, regardless of the TTL value of lower labels.
• Pop—Remove the label from the beginning of the packet. Once the label is removed, the TTL is copied from the label into the IP packet header, and the underlying IP packet is forwarded as a native IP packet. In the case of multiple labels in a packet (label stacking), removal of the top label yields another MPLS packet. The new top label might derive CoS and TTL from a previous top label. The popped TTL value from the previous top label is not written back to the new top label.
• Swap—Replace the label at the top of the label stack with a new label. The S and CoS bits are copied from the previous label, and the TTL value is copied and decremented (unless the no-decrement-ttl or no-propagate-ttl statement is configured). A transit router supports a label stack of any depth.
• Multiple Push—Add multiple labels (up to three) on top of existing packets. This operation is equivalent to pushing multiple times.
• Swap and Push—Replace the existing top of the label stack with a new label, and then push another new label on top.