# [rbridge] How many trees, and per what

Sun Nov 5 12:51:06 PST 2006

```Hopefully by "very similar" understanding of what we are talking about, you mean "identical". If there is some disconnect
over what we understand the technical proposal to be, we should clear that up before trying to argue the merits
of the technical proposal.

----- Original Message -----
From: "Gray, Eric" <Eric.Gray at marconi.com>
Date: Sunday, November 5, 2006 12:38 pm
Subject: Re: [rbridge] How many trees, and per what

>
> 	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 -
>
> --
> Eric
>
> --> -----Original Message-----
> --> 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
> -->
> --> 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,
> -->
> -->
> -->
> -->
> --> ----- 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
> -->
> what
> --> > exactlydo we mean by ingress rbridge tree.
> --> >
> -->
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```