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:[email protected]] On Behalf Of Dhamija Amit Sent: Tuesday, June 04, 2013 8:44 AM To: [email protected] 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 _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/ _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
