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Re: LFAs and ECMP

Subject: Re: LFAs and ECMP
From: Alia Atlas
Date: Mon, 14 Feb 2005 11:38:55 -0500
At 06:33 AM 2/14/2005, mike shand wrote:
At 21:14 13/02/2005 -0500, Alia Atlas wrote:
The LFA draft currently doesn't really discuss ECMP and the protection that may or may not be available via ECMP. For a particular destination D, S may have multiple equal-cost primary next-hops.
In the following figure, S has three equal-cost paths via E1, E2 and E3.

       20         3
  [N]-----[ S ]--------[E3]
   |      5 |            |
20 |        |            |
   |    ---------        | 2
   |  5 |       | 5      |
   |   [E1]    [E2]------|
   |    |       |
   | 10 |    10 |
   |---[A]     [B]
        |       |
      2 |--[D]--| 2

In this example, the primary next-hop to E1 can get link and node protection from the primary next-hop to E3; it can only get node protection from E2. E2 can't get node protection from E1 or E3; E2 can get node protection and link protection from the loop-free neighbor N.
Simply having multiple primary next-hops does not guarantee the desired
type of protection. The basic question is whether it is preferable to
use an equal-cost path that doesn't provide all the desired types of
protection or an alternate path that does give the desired protection?
Should the draft discuss this issue or describe the use of other primary
next-hops as alternates in any more detail?

Yes, I think so. We had some text in our tunnels draft which addressed some of this (section 4.6) , but the LAN case as you describe makes it more complicated. I think if we are going to say you can use ECMP at all (which I think we should), we need to say exactly under what conditions it is OK.
So describe the above example as a case of how ECMP doesn't guarantee
protection - and then up to the implementation/operator to decide what the
preference between protection type and cost is?

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