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Date:   Thu, 16 May 2019 18:07:13 +0800
From:   Hangbin Liu <>
To:     Robert Shearman <>
        "Eric W. Biederman" <>,
        Ying Xu <>
Subject: [MPLS] Why can we encapsulate outgoing label with reserved values?

Hi Robert, Eric,

In commit 5a9ab0176198 ("mpls: Prevent use of implicit NULL label as outgoing
label") I saw we disabled setting label 3. Is there a reason that why we did
not disable other reserved values(0-15)?

I saw function mpls_label_ok() checks the index and reject all reserved values.
With this two different handles, we can encap a reserved label, but could not
exchange or decapsulate the reserved labels. e.g.

# ip route add encap mpls 3 via inet
Error: Implicit NULL Label (3) can not be used in encapsulation.
# ip route add encap mpls 0 via inet
# ip -f mpls route add 0 via inet
Error: Invalid label - must be MPLS_LABEL_FIRST_UNRESERVED or higher.


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