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Hi
Dan,
1) The amount of information advertised
The amount of
information carried and the stability of that information is similar to that
used in TE networks today. In those networks, when a TE LSP is set up or
torn down, there is a change in the bandwidth availability on all links
traversed. This causes an update to the TE information advertised in the IGP
for those links.
True. However, typically the number of TE LSPs that are
established is a static number. Even in extreme cases, folks typically use
O(n^2) LSPs to form a complete mesh of their nodes.
In
the applications that I think that you're contemplating, there are many more
links, with the ability of end users to set up their own links throughout the
network. Please correct me if I'm misunderstanding.
In general, this is not seen as a critical issue. There are several
possibilities to reduce the impact of TE advertisements if they become a
problem. For example, as implemented in some of our current IP network, each
router can impose a threshold (time and/or bandwidth delta) before which it
will not re-issue an advertisement.
Effectively rate limiting the link state database flooding, which
is the goal.
In networks that require IGP
convergence (for IP data traffic) and TE advertisement (for MPLS-TE traffic)
there is some concern that the distribution of TE information might impact the
IGP convergence time. In these cases, implementations may prioritise normal
LSAs ahead of opaque LSAs, or may use separate protocol instances.
The
former is more difficult in IS-IS, as TE information is typically distributed as
part of the same LSP that carries the physical link information. Separate
protocol instances is of course possible, but it would seem like it would become
more of a manageability issue (do you need identical topologies?) and would not
obviate the need for CPU cycles.
2) Speed of convergence of wavelength
availability
In a TE network, the need for an up-to-date TED depends to
a good degree on the relative size of LSPs and residual bandwidth, and on the
rate of arrival of new LSPs. Increasingly, core TE LSPs may be quite large,
but in general, the rate of arrival of large LSPs is quite low, and the
holding times are quite high.
So, the bottom line in a TE network is
that when an LSP needs more bandwidth than is actually available, it is
crucial that the TED is up-to-date (or "real-time" like some folks may call
it).
I'm
not sure that I would characterize it as 'crucial'. It would seem like
there is always going to be *some* window where remote nodes have imperfect
information, just due to speed of light delays. Ergo, the head end must
always be prepared for its path to be rejected during setup and then recompute
the path based on new information.
But as Igor has pointed out,
this is impossible to achieve in a distributed system. Indeed, it is
impossible to achieve even in a centralised system since network failures may
occur. That means that there is always a possibility that a computed path will
be signaled and will fail to be established. We should be familiar with this
situation in scenarios such as contention during restoration after network
failure, and the signaling protocols are designed to fail and try again
(usually using "crankback" information about the failed link, but also
assuming that the IGP may have converged in the mean time).
Note
that crankback (recomputing the path DURING setup) is not always
necessary. Sometimes recomputation at the head end will
suffice.
We can look at a couple of
things:
- What is the arrival rate of lambda LSPs?
- What is the
required setup time of lambda LSPs?
With the exception of restoration
LSPs, I think we may say that a 5-10 second delay in LSP setup time
for 1 in 50 LSPs would be acceptable, but you may have a different
experience.
In my
experience nearly static LSPs are perfectly acceptable, and setup time is not an
issue. ;-)
My
concern however is simpler: limiting the rate of link state database
flooding. This is vital to ensure the stability of the
IGP.
Regards,
Tony
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