LSRs perform the operation of label swapping, imposition, or disposition depending on their location in the MPLS domain. In certain cases, the incoming label maps to special outgoing labels that define the operation to be performed at the upstream LSR or router. These labels are propagated by the downstream LSR during label distribution to the upstream LSR. The following outlines the types of outgoing labels that can be associated with a packet:
- Untagged – The incoming MPLS packet is converted to an IP packet and forwarded to the destination (MPLS to IP Domain transition). This is used in the implementation of MPLS VPN (discussed in Chapter 3).
- Implicit-null or POP label – This label is assigned when the top label of the incoming MPLS packet is removed and the resulting MPLS or IP packet is forwarded to the next-hop downstream router. The value for this label is 3 (20 bit label field). This label is used in MPLS networks that implement penultimate hop popping discussed in the next section.
- Explicit-null Label – This label is assigned to preserve the EXP value of the top label of an incoming packet. The top label is swapped with a label value of 0 (20 bit label field) and forwarded as an MPLS packet to the next-hop downstream router. This label is used in the implementation of QoS with MPLS.
- Aggregate – In this label, the incoming MPLS packet is converted to an IP packet (by removing all labels if label stack is found on incoming packet), and an FIB (CEF) lookup is performed to identify the outgoing interface to destination (used in MPLS VPN implementations, which is discussed in Chapter 3).
Figure 1-12 illustrates the usage of these label types.
Figure 1-12. Special Label Types