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Re: Fwd: I-D ACTION:draft-atlas-ip-local-protect-uturn-01.txt

Subject: Re: Fwd: I-D ACTION:draft-atlas-ip-local-protect-uturn-01.txt
From: Alia Atlas
Date: Thu, 28 Oct 2004 19:05:40 -0400
There are other examples for SRLGs that may be of substantial interest.
For example, multiple L3 interfaces may fail simultaneously, because they use the same physical port or line-card.
It would be interesting to discuss what set of SRLGs need to be considered
for protection.
Similar to looking for node-protection, SRLG-protection might be
significantly less common in current networks.
Just as a node failure can cause forwarding loops when only a
link-protecting alternate is available, an SRLG failure can cause
forwarding loops when the available alternate doesn't offer SRLG protection.
Naturally, computing SRLG protection is more expensive; at the very least
for loop-free alternates, one has to drag the list of SRLGs traversed along
the paths computed for a router's neighbors.
I am concerned that we don't go down a path which won't handle correlated
failures (even if they aren't handled in the initial considerations).

At 01:21 PM 10/28/2004, Curtis Villamizar wrote:
For example:

  Node protection can be thought of as a simultaneious failure of all
  edges that terminate at a given node.

  Links (edges) that travel over a common length of fiber (DWM) may be
  considered to have a high probablitity of correlated failure.

  Links that rely on other common equipment or resources including
  power may experience correlated failure.  Whether to consider the
  probablilty of failure of any given reasource as significant enough
  is up to the operator.

  In some cases fiber along the same physical path can fail.  Classic
  examples, are fiber on the same bridge that collapsed, on both sides
  of the same railroad track, fiber in gas pipes dug up be a
  contractor digging up the wrong unused gas pipes, along the same
  (San Deigo) earthquake fault.  The operator needs to decide if any
  case is sufficiently probable.

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