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List:       juniper-nsp
Subject:    [j-nsp] ospf lfa: ecmp vs. srlg ?
From:       Alexandre Snarskii <snar () snar ! spb ! ru>
Date:       2019-11-20 10:00:51
Message-ID: 20191120100051.GA17550 () staff ! retn ! net
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Hi!

Sorry for a bit cryptic subject, let me explain it in details here:
looks like OSPF LFA is able to consider SRLG marking when calculating
backup paths in most cases but not when primary path is already ECMP
over two or more links: in this case backup is not calculated at all, 
despite the fact that both links share the same SRLG markings and can 
go down in the same time...

show ospf backup spf <node> shows:

XX.XXX.X25.14
  Self to Destination Metric: 22
  Parent Node: XX.XXX.X25.1
  Primary next-hop: ae8.11 via XX.XXX.X33.244
  Primary next-hop: ae2.9 via XX.XXX.X33.246
  Backup Neighbor: XX.XXX.X25.2 via: Direct
    Neighbor to Destination Metric: 15, Neighbor to Self Metric: 7
    Self to Neighbor Metric: 7, Backup preference: 0x0
    Not evaluated, Reason: Primary next-hop multipath

this route is indeed multipath:

snar@Router> show route XX.XXX.X25.14 table inet.0 detail | match via
                Next hop: XX.XXX.X33.244 via ae8.11 weight 0x1
                Next hop: XX.XXX.X33.246 via ae2.9 weight 0x1, selected

however, both primary interfaces using the same dwdm transmission underlay
and srlg marking on interfaces have four segments in common.

For non-multipath nodes backup route is correctly calculated and uses
different transmission system:

snar@Router> show route XX.XXX.X25.46 table inet.0 detail | match via 
                Next hop: XX.XXX.X33.244 via ae8.11 weight 0x1, selected
                Next hop: XX.XXX.X34.53 via ae4.8 weight 0xf000

Is there any way to force backup calculation for multipath nodes 
without changing metric to make remote node non-multipath ?

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