10. Appointed Forwarder Status Lost Counter (New)

   Strict conformance to the provisions of Section 4.8.3 of [RFC6325]
   on the value of the Appointed Forwarder Status Lost Counter can
   result in splitting of Interested VLANs and Spanning Tree Roots
   sub-TLVs [RFC7176] (or the corresponding Interested Labels sub-TLVs
   where a VLAN is mapped to an FGL) due to differences in this
   counter value for adjacent VLAN IDs (or 24-bit FGLs). This counter
   is a mechanism to optimize data plane learning by trimming the
   expiration timer for learned addresses on a per VLAN/FGL basis
   under some circumstances.

   The requirement to increment this counter by one whenever a TRILL
   switch loses appointed forwarder status on a port is hereby changed
   from the mandatory provisions of [RFC6325] to the enumerated
   provisions below.  To the extent that this might cause the
   Appointed Forwarder Status Lost Counter to be increased when
   [RFC6325] indicates it should not, this will cause data plane
   address learning timeouts at remote TRILL switches to be reduced.
   To the extent that this might cause the Appointed Forwarder Status
   Lost Counter to remain unchanged when [RFC6325] indicate it should
   be increased, this will defeat a reduction in such timeouts that
   would otherwise occur.
 
   (1) If any of the following apply, either data plane address
       learning is not in use or Appointed Forwarder status is
       irrelevant. In these cases the Appointed Forwarder Status Lost
       Counter MAY be left zero or set to any convenient value such as
       the value of the Appointed Forwarder Status Lost Counter for an
       adjacent VLAN ID or FGL..

       (1a) The TRILL switch port has been configured with the
       "disable end station traffic" bit on (also known as the trunk
       bit).

       (1b) The TRILL switch port has been configured IS-IS as an
       IS-IS point-to-point link.

       (1C) If the TRILL switch is relying on ESADI [RFC7357] or
       Directory Assist [RFC7379] and not using data plane learning.
  
   (2) In cases other than those numerated in point 1 above, the
       Appointed Forwarder Status Lost Counter SHOULD be incremented
       as described in [RFC6325]. Such incrementin has the advantage
       of optimizing data plane learning. Alternatively, the value of
       the Appointed Forwarder Status Lost Counters can deviate from
       that value, for example to make it match the value for an
       adjacent VLAN ID (or FGL), so as to permit greater aggregation
       of Interested VLANs and Spanning Tree Roots sub-TLVs.