Chris
I now have this as
Since normal link state routing takes into account the IS-IS overload bit,
[RFC6987] and costing out of links as described in [RFC5286], the forward
SPFs performed by the PLR rooted at the neighbors of the PLR also
need to take this into account. A repair tunnel path from a neighbor
of the PLR to a repair tunnel endpoint will generally avoid the nodes and
links excluded by the IGP overload/costing out rules. However, there
are two situations where this behavior may result in a repair path
traversing a link or router that should be excluded:
1. When the first hop on the repair tunnel path (from the PLR to
a direct neighbor) does not follow the IGP shortest path. In this
case, the PLR MUST NOT use a repair tunnel path whose first
hop is along a link whose cost or reverse cost is LSInfinity (for OSPF)
or the maximum cost (for IS-IS) or, has the overload bit set (for IS-IS)
2. The IS-IS overload bit and the mechanism of [RFC6987]
only prevent transit traffic from traversing a node. They do
not prevent traffic destined to a node. The per-neighbor
forwards SPFs using the standard IGP overload rules will not
prevent a PLR from choosing a repair tunnel endpoint that is
advertising a desire to not carry transit traffic. Therefore, the
PLR MUST NOT use a repair tunnel endpoint with the IS-IS
overload bit set, or where all outgoing interfaces have the
cost set to LSInfinity for OSPF.
Please can you verify that I have not done anything bad in the
editing.
I am trying to figure out if I need more OSPF and ISIS refs. I could
argue that anyone that needs to implement this will know them
and anyone that needs the ref should not be implementing it :)
However I am not sure the IESG will agree :)
Stewart
On 30/05/2014 13:28, Stewart Bryant wrote:
Chris
I would like to take this together with final review coments
from the WGCs
Stewart
On 28/05/2014 19:54, Chris Bowers wrote:
See inline [CB] for comment on the new section 4.4.
Chris
*From:*Stewart Bryant [mailto:[email protected]]
*Sent:* Friday, May 23, 2014 10:11 AM
*To:* Chris Bowers; [email protected]
*Subject:* Re: comments on draft-ietf-rtgwg-remote-lfa-05
On 23/05/2014 13:43, Chris Bowers wrote:
Authors,
Below are two comments on draft-ietf-rtgwg-remote-lfa-05.
1) A section should be added to the document clarifying what the
expected behavior is for RLFA when routers advertise themselves
as not desiring to carry transit traffic or links have been
costed out. RFC5286 has a useful section on this topic
entitled "Interactions with IS-IS Overload, RFC 3137, and Costed
Out Links". The current document would benefit from additional
clarity in this topic.
New section 4.4.
The consideration concerning interactions with IS-IS Overload,
[RFC3137],
and costed out links as described in [RFC5286] apply. In selecting a PQ
node a PLR MUST exclude any candidate that is reachable (including
via ECMP)
from the PLR via a path subject to one of the above exclusions.
The method of determining the exclusion is a local matter.
[CB] I think the following text explains the expected behavior more
clearly.
"Since normal IGP routing takes into account the IS-IS overload bit ,
RFC 3137, and costing out of links, the forward SPFs performed by the
PLR rooted at neighbors of the PLR also need to take this into
account. Therefore a repair tunnel path from a neighbor of the PLR
to a repair tunnel endpoint will generally avoid the nodes and links
excluded by the IGP overload/costing out rules. However, there are
two situations where this behavior may result in a repair path
traversing a link or router that should be excluded.
1) The first hop on the repair tunnel path (from the PLR to a direct
neighbor) does not necessarily follow the IGP shortest path.
Therefore, the PLR MUST NOT use a repair tunnel path whose first hop
is along a link whose cost or reverse cost is LSInfinity (for OSPF)
or the maximum cost (for IS-IS) or, has the overload bit set (for
IS-IS).
2) The IS-IS overload bit and the mechanism of RFC 3137 only prevent
transit traffic from traversing a node. It does not prevent traffic
destined to a node. So the per-neighbor forwards SPFs using standard
IGP overload rules will not prevent a PLR from choosing a repair
tunnel endpoint that is advertising a desire to not carry transit
traffic. Therefore, the PLR MUST NOT use a repair tunnel endpoint
with the IS-IS overload bit set (or all outgoing interfaces with cost
set to LSInfinity for OSPF). "
2) The current document doesn't appear to say anything about the
expected behavior of RLFA when the IGP has multiple areas or
levels. It would be useful to either address this topic, or
explicitly narrow the scope of the document to describing the
behavior when the IGP has a single area or level.
I have added to the bottom of the intro:
This document considers the case when the repair path is confined to
either a
single area or to the level two routing domain. In all other cases,
the chosen
PQ node should be regarded as a tunnel adjacency of the repairing node,
and the considerations described in Section 6 of [RFC5286] taken
into account.
- Stewart
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