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

From: Radia.Perlman at sun.com ([email-masked])
Date: Sun, 05 Nov 2006 11:24:42 -0800

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. 
>