[c-nsp] Difference in IP FRR Link vs Per Prefix Option

[Dhamija Amit]

Hi Adam

Thanks for wonderfull explanation. Could you please confirm for Per Link also the condition for loop free criteria will remain same i.e N,D < N,S + S,D and in per link i'll get only link protection and if some vendor says they support node protection & link protection with node means actually they are talking for per prefix implementation.

Correct me if i am wrong.

Thanks
Amit Dhamija 


--- On Tue, 6/4/13, Adam Vitkovsky <adam.vitkovsky at swan.sk> wrote:

> From: Adam Vitkovsky <adam.vitkovsky at swan.sk>
> Subject: RE: [c-nsp] Difference in IP FRR Link vs Per Prefix Option
> To: "'Dhamija Amit'" <amiitdhamija at yahoo.com>, cisco-nsp at puck.nether.net
> Date: Tuesday, June 4, 2013, 8:41 AM
> Hi Amit,
> Well I believe you can look at it as link-protection vs
> node-protection. 
> 
> Per-link LFA provides only link-protection -that means
> protecting node (node
> computing the backup path for its primary links) will only
> chose LFA
> node/candidate that has a link to primary next-hop node
> (node currently
> acting as primary next-hop). 
> In other words it will compute backup path that has to end
> on the same node
> currently acting as primary next hop. 
> And as you can see this might not always be the case in your
> topology as the
> LFA candidate node might not have a direct link to your
> primary next-hop
> though it might have a link to some other upstream node
> (node on a loop free
> path towards the destination). 
> Also 
> Per-link is essentially per-next hop LFA (by next hop I mean
> the other end
> of the directly connected link) so the protecting node
> computes only one
> alternate/repair next hop for each primary next hop. The
> consequence of this
> is that all prefixes with the common primary next-hop share
> also the same
> backup next-hop. And there can be cases where the protecting
> node would have
> multiple lower BW links to one/more LFA candidate nodes,
> instead of one link
> with BW equal to the primary link. In such a case per-link
> LFA would chose
> only one of the links and in case of primary-link failure
> the one lower BW
> link might get congested. The congestion might as well occur
> on primary and
> backup link with equal BWs as the link serving as backup
> might already be
> carrying some level of traffic on its own and might not cope
> well with the
> additional load during the primary link failure. So I case
> the protecting
> node found more LFA candidates for a given primary next-hop
> it would
> consider just one of them. 
> 
> Per-prefix LFA provides node-protection that is, the
> protecting node will
> consider also LFA candidates that do not have a direct link
> to node acting
> as primary next-hop however they have a link to an upstream
> node. This
> increases the possibility of finding the LFA candidate(s) as
> well as
> provides protection in case the node currently acting as
> primary next-hop
> fails completely. 
> And
> In addition to node-protection, repair node running
> Per-prefix LFA computes
> LFA next-hop per each prefix independently. That means if
> the protecting
> node finds multiple nodes qualifying as LFA candidates for a
> given primary
> next-hop, in case of primary next-hop failure it will
> load-share the traffic
> among multiple backup paths. 
> 
> 
> It's still morning for me so I hope it all makes some sense.
> 
> 
> adam
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net]
> On Behalf Of
> Dhamija Amit
> Sent: Tuesday, June 04, 2013 8:44 AM
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] Difference in IP FRR Link vs Per Prefix
> Option
> 
> 
>  Hi 
> 
> Can some one help me in understanding the exact difference
> in IP FRR Per
> Link and Per Prefix option , It's mention in some docs that
> coverage of
> protected prefixes will be more in per prefix option , If we
> have per link
> then all the prefixes learned through same link will be
> covered in LFA
> computation then what else is left for any router the IGP
> prefixes will be
> learned through some link and if link is protected means all
> prefixes are
> protected then how can we say coverage is more in per
> prefix.
> 
> I see some vendors Juniper , ALU doesn't supports per prefix
> option.Could
> you please elaborate more on this.
> 
> Thanks for help
> 
> Regards
> Amit Dhamija
> 
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