Subject: RE: Some Queries

From: "Martin, Christian" <[email-masked]>
Date: Tue, 9 Jan 2001 14:34:06 -0500

> 
> > Hi Vijay,
> >
> > > Then we start to add hierarchy here. One design being the 
> lets mesh the
> > > aggregation routers instead of the edge routers. Problem 
> with that design
> > > is that now the MPLS domain does not go all the way to 
> the edge, causing
> > > the edge to slosh ip traffic flows based on topology 
> changes inside your
> > > network, causing your bandwidth settings on LSPS to fail 
> miserably, as
> > > traffic may now start traversing LSPs which were 
> previously (thanks to the
> > > wonders of SPF) never loaded, thus the automatic TE 
> scripts set their
> > > reserved bandwidth to 0.
> >
> > So you advertise the LSPs into your SPF, and give them a 
> fixed metric.
> > Now your SPFs behave: end of slosh.  (Confess!)
> 
> Could you elaborate how this scheme works, how this solves the problem
> described by Vijay above?

Since the edge boxen now see the core LSPs as part of the SPT, they can make
more accurate decisions as to which edge-core path to use when forwarding
packets towards the core.  What Vijay was referring to was the problem of
having TE'ed LSPs having misrepresented reservable bandwidth because
edge-to-edge flows are unconstrained, and thus, do not follow the TE design.
When scripts go in and observe bandwidth utilization on BB trunks, and see
that they are full relative to the reservable bandwidth, they reduce that
number to 0.  The fixed metric approach that Kireeti mentioned appears to
not offer any added benefit, unless the metrics are fixed relative to the
parameters on the links, which I think was his intention.  (Unless Kireeti
was suggesting that a fixed metric (same everywhere) would prevent frequent
SPF churn, which it would)  In other words, by representing the mesh of LSPs
as a dimensionless cloud, the injection of LSP information to the IGP
provides little improvement over not injecting the LSPs into the IGP, in
terms of path optimization.  However, by injecting meaningful metrics, ie,
those based on the link speed, reservable bandwidth, etc, and not adjusting
them automatically, you optimize the end-to-end path.  Again, this doesn't
solve the SPF churn issue as well as using fixed metrics, however, it may
help mitigate link sauturation.  Either way, it is a sound approach,
however, Vijay would know better as he has seen this applied in some _very_
large MPLS networks.

It should also be noted that this approach is not uncommon given that not
all edge devices support MPLS, yet may need (or should) to take advantage of
an optimized SPT from edge-to-edge.  

regards,
chris

> thanks,
> cyl

---

From: Kireeti Kompella <[email-masked]>
Date: Tue, 9 Jan 2001 12:32:38 -0800 (PST)

Hi Chris, 

> The fixed metric approach that Kireeti mentioned appears to
> parameters on the links, which I think was his intention.

"Fixed metric" means that the LSP's metric doesn't change, even if
it re-routes.  One of Vijay's problems is that a topology change could
cause serious traffic shifts (to use his colorful language, slosh),
negating the carefully constructed TE patterns and associated bandwidths.

Kireeti.