Hi Greg,
> Would
> IPFRR still be considered FRR if restoration of connectivity relies on
> flooding
I don't understand the question. Failed IPFRR is still FRR. But failed one.
We all know that each technology has its limitations. Not full
coverage is known limitation of IPFRR. And it is also understood that
when IPFRR fails routers have to resort to traditional convergence. But
this doesn't change the fact that flooding is not part of IPFRR.
Flooding is part of traditional convergence.
Flooding and traditional convergence are required even if IPFRR
succeeds because router can't run forever on repair routes. So flooding
is always part of the bigger picture, it just is not part of IPFRR
technology itself.
Anton
On 04/12/2011 06:45 AM, Greg Mirsky wrote:
Hi Curtis,
thank you for the explanation on IPFRR. I know that things are sometimes
are not what we call them but I believe that if MPLS FRR doesn't work
for some reason and LSP rerouted to restore service it's not FRR. Would
IPFRR still be considered FRR if restoration of connectivity relies on
flooding and route convergence, not on use of pre-calculated RIB?
Regards,
Greg
On Mon, Apr 11, 2011 at 8:16 PM, Curtis Villamizar <[email protected]
<mailto:[email protected]>> wrote:
In message <[email protected]
<mailto:[email protected]>>
Greg Mirsky writes:
>
> Dear Anton,
>
> I believe that MPLS-TE FRR does not address tasks 2 through 5 in the
> same sense as it is required in IPFRR. Certainly, protecting LSP has
> to be calculated by SPF, signaled and RIB/FIB/HW properly
updated. But
> these actions all done prior to when an MPLS-TE deemed protected.
Upon
> Fault detection the only action required is on the PLR.
>
> Regards,
> Greg
Greg,
IPFRR does not need tasks 2 through 5 either. OTOH, IPFRR coverage is
often less than full coverage.
In both MPLS FRR and IPFRR, if protection works it is handled entirely
by the PLR. In IPFRR, some PLRs have no fast protection and have to
rely on flooding. In IPFRR and MPLS FRR sometimes unexpected multiple
failures occur since a previously unknown shared resource is
discovered the hard way or an extroidinary event occurs (ie: two
fibers on same fault line, etc). In this case even protection from
the MPLS FRR PLR doesn't work.
In MPLS if a reroute is required, the CSPF load being N^2 log N (order
N CSPF computation have to be run), the LSA flooding has no
significant impact at all. In IPFRR where only one SPF has to be run,
flooding is still not the primary contributor to convergence time. It
may be a combination of 4 and 5 below.
Curtis
> On Mon, Apr 11, 2011 at 1:54 PM, Anton Smirnov
<[email protected] <mailto:[email protected]>> wrote:
>
> > Hi all,
> > even though I put OSPF-FN draft in the subject it is the
framework
> > approach FN-FRWK which draws more questions. At the very first
line it
> > reads:
> >
> > This document describes an architectural work that competes
with the
> >> IP Fast Re-Route (IPFRR) solution
> >>
> >
> > Lets compare speed of traffic restoration between the two. So,
traditional
> > OSPF convergence time consists of the sum of:
> >
> > 1. Failure detection
> > 2. LSA origination
> > 3. Per-hop flooding
> > 4. SPF (delay and calculation itself)
> > 5. RIB/FIB/hardware update
> >
> > 3, 4 and 5 all can be significant depending on network size,
number of
> > routes etc.
> >
> > FRR (both MPLS TE FRR and IPFRR) address 2-5. With good
implementation FRR
> > should be by order of magnitude as fast as 1.
> >
> > FN addresses only 3. It doesn't address 4 and 5. As I wrote
above in many
> > networks they are at least as significant as 3.
> >
> > So, by the speed of convergence FN doesn't look to come
anywhere close to
> > FRR.
> >
> >
> > Now, lets look at FN from another perspective. Router
announcing failure
> > doesn't benefit from FN. Its immediate neighbors do not benefit
from FN
> > either - 1 hop traditional flooding should be as fast as 1 hop
FN flooding.
> > It is only distant routers who benefit from the FN - and the
farther is
> > router from the failure the bigger is gain.
> > On the other hand, if there exists path alternative to the
failed one
> > then _typically_ it is not too far (in terms of hops) from the
failing one.
> > I.e. from point of view of router which is 15 hops away from
the point of
> > failure it is less likely that routes will change. BTW, ordered
FIB approach
> > builds on concept that 'old' routes on remote routers do not
cause traffic
> > blackholing or loops.
> >
> > The big problem with FN approach is that routers remote from
the point of
> > failure benefit most - but at the same time their convergence
is the least
> > important for end-to-end traffic restoration.
> > The worst case network for FN is fully meshed area. Since
each router is
> > 1 hop away from every other one FN will give no benefits.
> > The best case network for FN is an area consisting of one big
ring. In
> > this case alternative path is on diametrically opposite end of
the network
> > and convergence of remote routers is crucial.
> >
> > So yeah, FN will help remote routers to converge faster. But
how much
> > this will improve end-to-end traffic restoration in real
networks? I suspect
> > not much. Some degree of meshiness in network topology is the norm.
> >
> > FN is an interesting proposal but it is very far from being
convincing.
> > Pitching FN against FRR is a mistake.
> >
> > --
> > Anton
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