Routing
Routing

IS-IS link protection question

‎01-29-2019 08:54 AM

Hello,

I'm testing some IS-IS link-protection and am having some strange results.

I have a pretty simple topology setup as below:

PE1--------------P1
   |              | 
   |              |
   |              |
   |              | 
 PE2-------------P2

On PE1 i have IS-IS link-protection configured on both links.

The primary path from PE1 to PE2 is the direct link between the two.

however i thought a backup path would be calculated via P1 when link-proteection is enabled, but this does not seem to be the case as shown here:

 

root@PE1> show isis backup spf results PE2.00 
 IS-IS level 1 SPF results:
  0 nodes

 IS-IS level 2 SPF results:
PE2.00
  Primary next-hop: ge-0/0/3.0, IPV4, PE2, SNPA:  50:0:0:11:0:3
    Root: PE2, Root Metric: 10, Metric: 0, Root Preference: 0x0
      Not eligible, IPV4, Reason: Primary next-hop link fate sharing 
    Root: P1, Root Metric: 10, Metric: 20, Root Preference: 0x0
      track-item: PE2.00-00
      track-item: PE1.00-00
      Not eligible, IPV4, Reason: Path loops 
  1 nodes
root@PE1> show isis backup spf results no-coverage PE2.00 
 IS-IS level 1 SPF results:
  0 nodes

 IS-IS level 2 SPF results:
PE2.00
  Primary next-hop: ge-0/0/3.0, IPV4, PE2, SNPA:  50:0:0:11:0:3
    Root: PE2, Root Metric: 10, Metric: 0, Root Preference: 0x0
      Not eligible, IPV4, Reason: Primary next-hop link fate sharing 
    Root: P1, Root Metric: 10, Metric: 20, Root Preference: 0x0
      track-item: PE2.00-00
      track-item: PE1.00-00
      Not eligible, IPV4, Reason: Path loops 
  1 nodes

perhaps my understanding of link-protection is not correct.  Can somone explain why i'm gettinmg "Not eligible, IPV4, Reason: Path loops"

 

 

1 REPLY 1
Routing

Re: IS-IS link protection question

[ Edited ]
‎01-29-2019 09:19 AM

Hello,

Assuming link metric are equal, P1 has 2 ECMP paths towards PE2. Including one back through PE1 itself.

ISIS LFA algorithm on PE1 figures this out and refuses to use P1 as backup nexthop.

Anything but triangle topo is difficult for LFA - You have to either add extra links, adjust link metrics, build tunnels or abandon LFA together and use RLFA or TI-LFA.

HTH

Thx

Alex

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