Subject: [rbridge] How many trees, and per what

From: Eric.Gray at marconi.com (Gray, Eric)
Date: Sun, 5 Nov 2006 15:38:50 -0500

Radia,

	I think we have a very similar understanding of what "tree"
means.  

	The step from a uni-directional "tree" rooted at the local
ingress to a bi-directional "tree" - potentially rooted elsewhere
- is, IMO, a very big step.

	Therefore, again IMO, this is something we need to decide -
soon - hence it is something we should talk about this week.

--
Eric

--> -----Original Message-----
--> From: Radia.Perlman at sun.com [mailto:Radia.Perlman at sun.com] 
--> Sent: Sunday, November 05, 2006 2:25 PM
--> To: Gray, Eric
--> Cc: Sanjay Sane (sanjays); Russ White; rbridge at postel.org
--> Subject: Re: RE: [rbridge] How many trees, and per what
--> 
--> What I think an ingress RBridge tree is, is the following:
--> 
--> Suppose you have an RBridge R. Everyone calculates an SPF 
--> tree with R as the root, with some deterministic
--> tie-breaker in the case of two equal cost paths from R to 
--> some node N, such as the ID of the parent. For instance,
--> if you can attach N to the tree with either parent P1 or 
--> P2, and get the same minimal cost from R to N, you
--> choose the parent with lowest ID to attach N to.
--> 
--> So that's the ingress RBridge tree rooted at R. If it's 
--> really an ingress tree it could be thought of as unidirectional,
--> since traffic would only originate at R.
--> 
--> If instead you go with the proposal that allows the ingress 
--> RBridge R to select a different tree for a multicast than
--> the one rooted at R, so it selects, say, the tree rooted at 
--> R2, then what we mean by a tree is a *bidirectional* tree
--> rooted at R2.
--> 
--> If the trees are bidirectional, the calculation of the tree 
--> is the same (do SPF with R2 as root). To figure out
--> which of your own links are in the R2 tree, you look at 
--> yourself in that tree, and use the port (neighbor) that connects
--> you to your parent in the tree, and all the ports 
--> (neighbors) that connect children in the tree to you. Those are
--> your links in the bidirectional tree.
--> 
--> For forwarding on a bidirectional tree, if you receive a 
--> packet marked as "use tree R2" from a neighbor that is
--> not one of the R2-tree links, then you discard the packet. 
--> Otherwise, you forward the packet onto all the other
--> links in the R2-tree other than the one you got it from.
--> 
--> I am really mystified as to what other definition there 
--> might be of trees. So please explain if you mean something else.
--> 
--> Thanks,
--> 
--> Radia
--> 
--> 
--> 
--> ----- Original Message -----
--> From: "Gray, Eric" <Eric.Gray at marconi.com>
--> Date: Sunday, November 5, 2006 10:04 am
--> Subject: RE: [rbridge] How many trees, and per what
--> 
--> > Clearly one of the things we need to talk about this week is what 
--> > exactlydo we mean by ingress rbridge tree. 
--> > 
-->