The MPLS-OPS Archive

[Spice Sylvia <falsesylvia@yahoo.co.uk>]

Eric,

Consider the case as mentioned by Wilson:

If the best IGP path on the loose hop R2 to reach R4
was R2, would it be right or wrong for the LSP to be
setup via R2-->R1?

Would not using that path via R1 also not amount to
"suboptimal routing" in this case?? Or is that
appropriate?

I admit you could say that this is a bad way to define
an ERO, and hence he asked, why does one need to
bother about hops/intermediate nodes in the ERO.

Incase you only defined realistic constraints, and not
"EROs", this would not happen in the 1st place.
...correct?
The only time an ERO makes sense is when the complete
path is defined via EROs (strict and no loose)

I also do not quite understand why the Bandwidth
reservation is considered to be only in the "control
plane". Is there a technical limitation on the same?

-SF

 --- Eric Osborne <eosborne@cisco.com> wrote: > ...
> 
> >> TE signalling in general always assumes that
> every
> >> node has a unique
> >> identifier, most commonly referred to as the
> Router
> >> ID.
> >>
> >
> > a. Why should it? Do we actually care at the time
> of
> > LSP setup as to what "route we take"?
> 
> Yes, we do.  This is the whole purpose behind an ERO
> - to be able to send  
> an LSP a specific way across the network.  In order
> to calculate this on  
> the router itself (which is one way, but not the
> only way, to determine a  
> path), we need unique identifiers we can run a
> constrained SPF [CSPF] with.
> 
> > Wouldnt a better example be a "sort of EIGRP
> database"
> > based Path Computation where we only get relevant
> data
> > and compute an LSP based on the same?
> > A simple example would be hops and bandwidth.
> 
> The less visibility you have past your neighbors,
> the more crankback,  
> explorer packets, and other such odd stuff you need
> to find a path from  
> source to destination.  And since you're dealing
> with a multi-variable  
> problem ("I need to find the lowest cost path with
> bandwidth X"), your  
> EIGRP model wouldn't work real well.  EIGRP does
> what it does because what  
> we know is _minimum_ bandwidth across the path that
> has the lowest cost.   
> To do TE with this, you'd have to build something
> that says "I have a path  
> of cost 100 with BW 10; another of cost 103 with BW
> 14; another with cost  
> 110 with BW 75", etc.  The more paths you have, the
> more information  
> explosion you have.
> 
> 
> > Does it matter what path I take as long as those
> > constraints are met? Do I need to know
> intermediate
> > node identifiers or even give them such
> identifiers?
> >
> 
> Apart from operational stuff like manageability,
> node identifiers are  
> useful for:
> 
> - doing offline path calculation and then uploading
> said calculated paths  
> to the routers in question
> - things like FRR, which need to know which NHops
> and NNHops (and in  
> theory, NNNNN..NHops) an LSP goes through, so they
> can do the right thing  
> with protection.
> 
> >
> >
> >
> >> > #2. If such a scenraio does occur, ie. an LSP
> is
> >> > indeed setup where there is a loop, and there
> are
> >> no
> >> > constraints specified, is it safe to assume
> that
> >> there
> >> > is a loop in the original topology itself?
> >>
> >> No, I don't think that's always true.  If the ERO
> is
> >> part stric and part
> >> loose, and the strict part sends a reservation in
> >> effect off of the
> >> shortest path tree to a destination, then an
> >> expanding node (the one that
> >> deals with the loose ERO) may well decide to send
> >> the reservation back
> >> down the shortest path tree and cross the
> segments
> >> it's already gone
> >> over.  This seems like the sort of thing that
> could
> >> be easily safeguarded
> >> against in an implementation, and I've never run
> >> into this issue in the
> >> real world, so I'm kinda speculating here.
> >>
> >
> > Yes that is definately possible...but it does it
> > affect anything? I am speculating this on a white
> > board now so pardon me if I make some really dumb
> > mistakes.
> 
> Rember, there are no stupid questions, only stupid
> people....:)
> 
> >
> > Say we suppose the ERO is the ordered set of
> "router
> > ids".
> >
> > I have a part strict and part loose ERO which says
> > "use node 1 and then 2" you can use any other node
> in
> > the middle but "4" has to be there plus give me
> say "x
> > bandwidth through and through till my end point"
> >
> > you could end up going back to 1 and getting to 4.
> >
> > However, what is wrong with it?
> 
> If you cross the same node X twice in your path,
> what's the point?  Why  
> would I want to drive from Dover to Bristol through
> London by going  
> Dover->London->Birmingham->London->Bristol?  It's
> unnecessarily  
> suboptimal; looping through a node that's in your
> path later on is, as far  
> as I can see, of no benefit and of almost certain
> drawback.
> 
> 
> >
> > as long as the "end point" is known, and your
> > "constraints are met", what would happen in this
> case
> > is:
> >
> > label from 1 to 2, another label from 2 to 1 and
> then
> > to another label from 1 to 4.
> >
> > can you think of wilder examples as the ones I can
> > think of do not seem to cause any problems?
> 
> Depends what you mean by "problem".  It's true that
> packets would  
> eventually arrive at their destination, so in the
> strictest sense, nothing  
> is broken if you do what you're suggesting. 
> However, it's wildly  
> suboptimal - I end up reserving BW at point 2 which
> some other LSP could  
> have used, and for no benefit whatsoever.
> 
> >
> > It seems to me that no matter what I do, if it is
> > totally loose, I would always end up getting along
> the
> > IGP, if it is strict, I may get a looped path but
> it
> > really would not matter.
> 
> If you are totally loose, it's like you don't have
> an ERO at all, in which  
> case (at least in our implementation), the headend
> sends the packet to a  
> next-hop along the shortest cost path to the
> destination, and so on down  
> the line.  It should be obvious that lack of any
> explicit routing doesn't  
> work when there's a bandwidth requirement, but
> without bandwidth, this  
> essentially means the headend presignals one of the
> lowest-cost paths from  
> the set of ECMPaths to the destination.  I don't
> generally see the point  
> in TE with no ERO, with the possible exception of
> NNHop FRR.
> 
> 
> 
> eric
> 
> >
> > This was one of my questions.
> >
> >
> >> You could even hit this, I suppose, with a
> >> succession of loose EROs.  But
> >> certainly with *no* ERO, if there's a loop in
> >> signalling it's because
> >> there's a loop in the routing.
> 


	
	
		
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