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When would an MPLS LSR have Untagged output label?

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!).

It's highly recommended you read the first few chapters of the MPLS and VPN Architectures book before the rest of this post.

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.

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  1. Another great article! You have a great explaining approach !!!

  2. Very good article to clarify my understanding!!

    By the way, what would the keyword "aggregate" mean, this is another keyword I dont quite understand while I am checking the mpls forwarding-table.

  3. Dear Ivan, when a loopback network is advertised as let say, on the neighboring router its seen as pop tag, on local router i cant see it in sh mpls forwarding. If the loopback is not running mpls why router is advertising implicit null label for it ? why dont leave it untagged ?

  4. Correction...

    extract from above:
    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.

    interfaces directly connected are not shown in LFIB since the router has already requested a POP action for them. and therez no need to look into LFIB..

  5. Not true on LC-ATM interfaces (luckily we don't see them too often :) ) or on VRF interfaces.

  6. Ivan,

    summary routes are not untagged labels as well, summarized routes are tagged with imp-null.

    kindly elaborate ur statement regarding VRF interfaces...