On Nov 9, 2007 12:01 PM, mike shand <[email protected]
The added classification of alternates looks fine, except I am
completely confused about your statement about NP. See inline.
Alia Atlas wrote:
On 10/12/07, Rüdiger A Martin <[email protected]
Alia, dear Alex,
we have taken a look at your document and would like to add a few
comments to the discussion already going on:
- The concept of node protecting LFAs is first mentioned on page 6. The
definition and explanation follows later.
True - The first place a node protecting LFA is mentioned, I've added a
reference to the section with the definition.
I don't see a clean way of defining it at/before that location.
There are several kinds of loop-free alternates. But the suggested
order of preference among those different LFAs does not become clear at
all in the draft. So there should be some paragraph where all possible
candidate types are listed and described with their pros and cons. That
description should include the protected failure types but also the
behavior in case of unprotected failures: e.g., I miss a clear statement
that downstream LFAs do not create any loops no matter what kind of
failures may happen. Based on this, a clear suggestion for an order of
preference can be given.
Yes, we didn't have consensus about the order of preferences - and it
on what other types of protection are available in the network. I
agree that this
would be useful. How does the following added section (after 3.6)
"3.7. LFA types and Trade-offs
LFAs can provide different amounts of protection and the decision
about which type to prefer is dependent upon network topology and
other techniques in use in the network. This section describes the
different protection levels and the trade-offs associated with each.
1. Primary Next-hop: When there are equal-cost primary next-hops,
using one as an alternate is guaranteed to not cause micro-loops
involving S. Traffic flows across the paths that the network will
converge to, but congestion may be experienced on the primary
paths since traffic is sent across fewer. All primary next-hops
are downstream paths.
2. Downstream Paths: A downstream path, unlike an LFA, is guaranteed
not to cause a micro-loop involving S regardless of the actual
failure detected. However, the expected coverage of such
alternates in a network is expected to be poor. All downstream
paths are LFAs.
3. LFA: An LFA can have good coverage of a network, depending on
topology. However, it is possible to get micro-loops involving S
if a more severe failure occurs than is protected against.
The different types of protection are abbreviated as LP (link-
protecting), NP (node-protecting) and SP (SRLG-protecting).
a. LP, NP, and SP: If such an alternate exists, it gives protection
against all failures.
b. LP and NP: Many networks may handle SRLG failures via another
method or may focus on node and link failures as being more
c. LP: A network may handle node failures via a high availability
technique and be concerned primarily about protecting the more
common link failure case.
d. NP: These only exist on interfaces that aren't point-to-point.
If link protection is handled in a different layer, then an NP
alternate may be acceptable.
I'm completely confused by this last statement. Surely it is possible
to have a node protecting LFA on a pt-pt interface?
Yes - but I have already categorized that case as an alternate that is LP and NP. This is discussing alternates that are
only NP. I'll make it clearer.
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