Subject: Question regarding multi-homed prefix LFA computation

From: Kui Zhang <[email-masked]>
Date: Thu, 14 Feb 2008 11:14:35 +0800

Hi all,

I have a doubt about multi-homed prefix LFA computation described in
draft-ietf-rtgwg-ipfrr-spec-base-10:

"If the alternate next-hop for the

   prefix p is simply inherited from the router advertising it on the

   shortest path to p, then the prefix p's alternate next-hop would be

   the link to C. This would provide link protection, but not the node

   protection that is possible via A.

                      5   +---+  4   +---+  5  +---+

                    ------| S |------| A |-----| B |

                    |     +---+      +---+     +---+

                    |       |                    |

                    |     5 |                  5 |

                    |       |                    |

                  +---+ 5 +---+   5       7    +---+

                  | C |---| E |------ p -------| F |

                  +---+   +---+                +---+

                       Figure 6: Multi-homed prefix"

It seems router A is not a valid node-protecting LFA, which will cause a
forwarding loop.

And the following words seem incorrect since we have to take the cost from
the advertising router to the advertised prefixes into consideration.

   "If there exist multiple multi-homed prefixes that share the same

   connectivity and the difference in metrics to those routers, then a

   single node can be used to represent the set."

For example, in figure 6, if the cost for prefix p to F decreases to 4, then
router A will be a valid node-protecting LFA.

Do I miss something here? Thanks for helping me in advance.

Regards,

Zhangkui

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---

From: mike shand <[email-masked]>
Date: Thu, 14 Feb 2008 12:17:35 +0000

Kui Zhang wrote:

Hi all,

I have a
doubt about multi-homed prefix LFA computation described in
draft-ietf-rtgwg-ipfrr-spec-base-10:

“If
the alternate next-hop for the

prefix
p is simply inherited from the router advertising it on the

5   +---+  4   +---+  5  +---+

|     +---+
+---+     +---+

|
|
|

|     5
|
5 |

|
|
|

+---+ 5 +---+   5       7
+---+

+---+
+---+
+---+

Figure 6: Multi-homed prefix”

It seems
router A is not a valid node-protecting LFA, which will cause a
forwarding loop.

And the
following words seem incorrect since we have to take the cost from the
advertising router to the advertised prefixes into consideration.

“If there exist multiple multi-homed prefixes that share the same

single node can be used to represent the set.”

For example,
in figure 6, if the cost for prefix p to F decreases to 4, then router
A will
be a valid node-protecting LFA.

Do I miss
something here? Thanks for helping me in advance.

There does seem to be something wrong here as you point out. Actually
even decreasing F-p to 4 doesn't completely fix it because then A would
have an ECMP path to p (both cost 14). The cost would need to reduce to
3 to fix this.

However, it would be better to fix it by (say) increasing the cost of
link SA to 8. This would ensure that the later text about prefix X was
also correct.

But maybe Alia had something else in mind?

Alia?

Mike

Regards,

Zhangkui

---

From: "Alia Atlas" <[email-masked]>
Date: Thu, 14 Feb 2008 09:08:03 -0500

It does look like the costs are not correct to have A provide a
node-protecting alternate.  As Mike suggests, I'll change the cost of SA to
be 8 instead of 4.

As to the multiple-prefix with the same difference, that is correct.
Certainly the costs between the advertising routers and the prefixes need to
be considered, but they can be grouped intelligently for computation.  Let
me see if I can explain better - perhaps we can but some better text in
there.

Say there is another prefix p2 that is also attached to E, with a cost of 3,
and to F with a cost of 5.  Then one could just look at the difference
between the costs for the attachments for the computation.

Compare   cost(S to E) + 0     versus   cost(S to F) + 2

Now, one can add 3 to both parts without changing which is larger - and that
will give the final cost to p2.
Similarly, one can add 5 to both parts without changing which is larger -
and that will give the final cost to p.

Alia

On Thu, Feb 14, 2008 at 7:17 AM, mike shand <[email-masked]> wrote:

> For example, in figure 6, if the cost for prefix p to F decreases to 4,
> then router A will be a valid node-protecting LFA.
>
>
>
> There does seem to be something wrong here as you point out. Actually even
> decreasing F-p to 4 doesn't completely fix it because then A would have an
> ECMP path to p (both cost 14). The cost would need to reduce to 3 to fix
> this.
>
> However, it would be better to fix it by (say) increasing the cost of link
> SA to 8. This would ensure that the later text about prefix X was also
> correct.
>
> ------------------------------
>
> rtgwg mailing [email-masked]://www.ietf.org/mailman/listinfo/rtgwg
>
>
>

---

From: Kui Zhang <[email-masked]>
Date: Fri, 15 Feb 2008 11:49:21 +0800

Hi Alia and Mike,

For a multi-homed prefix, I suggest we can also point out, if the logical
topological transformation is not desirable, consider it as a single-homed
prefix. In this case, we should also point out the destination router D used
to compute the alternate next hop for this prefix should be the one
preferred in the normal route calculation process.

Besides, the dependency between link-protecting LFA and node-protecting LFA
condition seems not so clear, can we point out if a candidate neighbor meets
the node-protecting inequality, it will always meet the link-protecting
inequality? I read the following words in section 3.6.  Selection Procedure,
which seems implying such a dependency exactly.

“Loop-Free Node-Protecting Alternate -  This next-hop satisfies

      Inequality 1 and Inequality 3.  The path avoids S, S's primary

      neighbor E, and the link from S to E.”

zhangkui

  _____  

From: Alia Atlas [mailto:[email-masked]] 
Sent: Thursday, February 14, 2008 10:08 PM
To: mike shand
Cc: Kui Zhang; [email-masked]
Subject: Re: Question regarding multi-homed prefix LFA computation

Alia

Kui Zhang wrote: 

Hi all,

                      5   +---+  4   +---+  5  +---+

                    |     +---+      +---+     +---+

                    |       |                    |

                    |     5 |                  5 |

                    |       |                    |

                  +---+ 5 +---+   5       7    +---+

                  +---+   +---+                +---+

Alia?

    Mike

Regards,

Zhangkui

  _____

---

From: "Alia Atlas" <[email-masked]>
Date: Thu, 14 Feb 2008 23:39:13 -0500

Hi Zhang Kui,

> preferred in the normal route calculation process.
I don't believe that you can consider a multi-homed prefix as a single-homed
prefix.  The original SPF that is in use before the failure will not have
done that.  There's no way of deciding how that prefix was attached.

> Besides, the dependency between link-protecting LFA and node-protecting
> LFA condition seems not so clear, can we point out if a candidate neighbor
> meets the node-protecting inequality, it will always meet the
> link-protecting inequality? I read the following words in section 3.6.
> Selection Procedure, which seems implying such a dependency exactly.
>
>
>
> "Loop-Free Node-Protecting Alternate -  This next-hop satisfies
>
>       neighbor E, and the link from S to E."
>
> It is not the case that a node-protecting LFA is guaranteed to also be a
link-protecting LFA.
I believe that there is a clear counter-example given in the section
discussing broadcast interfaces (such as Ethernet).

Alia

> *From:* Alia Atlas [mailto:[email-masked]]
> *Sent:* Thursday, February 14, 2008 10:08 PM
> *To:* mike shand
> *Cc:* Kui Zhang; [email-masked]
> *Subject:* Re: Question regarding multi-homed prefix LFA computation
>
>
>
> Say there is another prefix p2 that is also attached to E, with a cost of
> 3, and to F with a cost of 5.  Then one could just look at the difference
> Now, one can add 3 to both parts without changing which is larger - and
> that will give the final cost to p2.