This is a nice MPLS question I’ve received from one of the readers:

I have understood the Penultimate Hop Popping (PHP) process, but I don’t understand when a router would use UNTAGGED instead of POP TAG?

Instead of answering the question directly, let's walk through a series of simple Q&A pairs that will help you understand the whole process (remember: knowledge, not recipes!).

Where does the Untagged keyword appear? It only appears as the output label in the LFIB (Label Forwarding Information Base) that you can inspect with the show mpls forwarding-table.

What does the Untagged keyword mean? This keyword means that the router has no output label associated with the forwarding equivalence class (FEC ... usually an IP prefix). Since there is no output label, the router cannot perform a label swap (or pop) but has to remove the whole MPLS shim header.

Where would a router get the output label? It's received from the next-hop router.

When would a router have no output label? When there is no next-hop router or when the next-hop router did not advertise a label for the IP prefix.

When would there be no next-hop router? If the IP prefix is a directly connected subnet (including a loopback interface) or a summary route advertised by the router itself.

When would the next-hop router not advertise a label? The reasons a next-hop router would not advertise a label for an IP prefix include:

• It's not running MPLS.
• It's running MPLS but not CEF (MPLS labels are assigned to IP prefixes in CEF table).
• It's not reachable across an MPLS-enabled interface (both routers could be running MPLS, but the transit interface does not have the mpls ip configuration).
• The LDP session has not been established yet.
• There is a mismatch in LDP protocol (one router is running Cisco's proprietary TDP, the other one standard LDP).
• The next-hop router uses an access-list to filter the IP prefixes for which the MPLS labels are advertised.

Summary: you would see the Untagged label in the LFIB when the IP prefix is a directly connected interface, a summary route or the next-hop router has not advertised the label.