The MPLS WG Archive

[jin.lizhong@zte.com.cn]

Hi, Ice
  pls. see the following lines
began with jin.lizhong


==========================================================
From:IJsbrand
Wijnands <ice@cisco.com>
To:jin.lizhong@zte.com.cn

Hi,


>
> 2.3.1.1.  Determining one's 'upstream LSR'
>
>    A node Z that is part of P2MP LSP <X, Y> determines
the LDP peer U
>    which lies on the best path from Z to the root node X.
If there are
>    more than one such LDP peers, only one of them is picked.
 U is Z's
>    "Upstream LSR" for <X, Y>.
>
> Which is the "best path" for Z. Two ways can determine the
best path.
> 1. Z can look up the normal "route table", and get the best
path  
> according the route.(The route table may be generated by OSPF, ISIS,
 
> RIP, or even OSPF-TE)
>
> 2. Z can get the best path according the multicast table (this table
 
> may be generated by various multicast routing protocols), and the
P2MP  
> LSP or MP2MP LSP will only work on the routers that support  
> corresponding multicast routing protocols.

Can you give an example of a multicast routing protocol that populates
 
such a table?

jin.lizhong:Different multicast routing protocols
may generate different trees
(the tree may be generated by DVMRP or PIM-DM protocol).
Z can determine the
upstream LSR by following the multicast routing protocol
messages. Then 
the P2MP or MP2MP LSP is identical to the L3 multicast
tree. Which is 
the best path for Z is hard to say, the path determined
by unicast routing
protocols or multicast routing protocols.


>
> The two ways are not compatiable, and the two P2MP LSPs or MP2MP LSPs
 
> are defferent. Maybe this draft should describe the two ways, and
 
> specify one way or both the two ways. The best path selection problem
 
> should be solved.

Are you suggesting we should set priority to one table over the other?

It looks to me that the table selection is a local policy and does not
 
necessarily need to be the same on each LSR. You may want to use a  
MuRIB between 2 LSRs just to make Multicast divert from Unicast for a  
specific link. How these two tables are used is up to the operator to  
define. It does not seems to be something the draft must spell out. So
 
I think this is out side the scope of the document.

jin.lizhong:Firstly, we should describe the
"best path" in more detail.
The two selection methods are not compatiable and
P2MP or MP2MP LSP will
not be setup sucessfully if different methods are
used in Root Node, Transit
Node, and Leaf Node. What's more, if Root Node does
not support multicast
routing protocols and Leaf Node only support multicast
routing lookup 
method. Then the Leaf Node will not find the Root
Node, for the Root Node
is out of the sight of Multicast domains. 
Secondly, we should specify the concept of "best
path". Then we can set a
priority to one table over the other, or we can only
select one selection
method.THX

Thx,

Ice.

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