Re: WGLC for draft-rtgwg-mrt-frr-architecture

On 21/12/2015 14:47, Chris Bowers wrote:
> Steward,
>
> I don't agree with the initial statement that the technique forms two trees rooted at a single node.  The designation of the GADAG root plays an important role in computing the red and blue MRT trees.  However,  red and blue MRT trees are computed using forward SPFs rooted at each source, which follow the directed links on the GADAG and do not propagate past the GADAG root.  The net result of these computations can be viewed as producing red and blue MRT trees rooted at each destination.  In any case, these trees are not rooted at the GADAG root.
Ah, OK.

However the GADAG root can move, does that not have an impact on the 
repair topology? It sounds from the above as if it might since you say 
you say that the root sets a propagation limit.

- Stewart
>
> Anil pointed out that the pseudo-code in draft-ietf-rtgwg-mrt-frr-algorithm didn't always make a clear distinction between the root of the forward SPF computation and the GADAG root, so we tried to clarify that in this set of changes.
>
> https://github.com/cbowers/draft-ietf-rtgwg-mrt-frr-algorithm/commit/ada619050ec9d773b7919a1c622f068d5a5a5e88
>
> Are there places in the architecture document where similar clarifications should be made?
>
> Chris
>
> -----Original Message-----
> From: rtgwg [mailto:rtgwg-bounces@ietf.org] On Behalf Of Stewart Bryant
> Sent: Friday, December 04, 2015 9:55 AM
> To: draft-rtgwg-mrt-frr-architecture@ietf.org; rtgwg@ietf.org; Alvaro Retana <aretana@cisco.com>
> Subject: Re: WGLC for draft-rtgwg-mrt-frr-architecture
>
> Another couple of comments on this draft.
>
> The technique you use of selecting a single node and forming two trees rooted at that node should really be noted up front in the summary.
>
> A consequence of this is that when you add a node or when the root node fails the trees and hence the FRR paths may change. To some extent this happens in LFA and RLFA, although the changes will tend to be confined to a local region, whereas with MTR I think that the  node may move to a completely different region. If that is the case then that has an impact on the FRR traffic management. By way of comparison, NV is the least impacted by this approach and the SR approach is impacted as much as LFA, but has the option of correcting this will a little effort.
>
> I think that there really needs to be some text on the matter in the architecture spec.
>
> - Stewart
>
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