[Bier] Comments on <draft-zzhang-bier-tether-02>

[Xiejingrong <[e-mail]>]

Hi,
I have some comments on this draft below:

     This can easily be prevented if BFR1 does an SPF calculation with the
     helper BFRx as the root.  For any BFERn reached via X from BFR1, if
     BFRx's SPF path to BFERn includes BFR1 then BFR1 must not use the
     helper.  Instead, BFR1 must directly tunnel packets for BFERn to X's
     BFR (grand-)child on BFR1's SPF path to BFERn, per section 6.9 of
     [RFC8279].

     In the following example, there is a connection between BFR1 and
     BFRx.  If the link metrics are all 1 on the three sides of
     BFR1-X-BFRx triangle, loop won't happen
[XJR]
This is an example of static condition. There may be some dynamic conditions too.
For example, the link BFRx-x goes down, and the original tunnel from BFR1 to BFRx will cause loop.
the tunnel will destroy and packet may lose (which wouldn't in this topology if not using the method).

           but if the BFRx-X metric is 3
     while other two sides of the triangle has metric 1 then BFRx will
     send BIER packets tunneled to it from BFR1 back to BFR1, causing a loop.
[XJR]
As above, when BFRx-X metric changes from 1 to 3, there may be packet loss too.
I mean the additional caculation is required not only at the first time, but at time when configuration/link-state changes.
If the BFR1 is a Non-BIER router1, then BFER1 has to tunnel to BFRx, and this procedure can not  share the LFA caculation.
Is the load of additional caculation a concern ?

Thanks
Jingrong