[j-nsp] Segment Routing Real World Deployment (was: VPC mc-lag)

[adamv0025 at netconsultings.com]

> From: Mark Tinka [mailto:mark.tinka at seacom.mu] 
> Sent: Monday, July 09, 2018 11:54 AM
>
>> On 9/Jul/18 11:58, adamv0025 at netconsultings.com wrote:
>> That is faulty fibre causes BGP session problems but not targeted-LDP session problems? 
>
> Faulty fibre will affect any control plane sessions.

Well that really depends on the type of fault, let me explain:

Fault A) flapping rate is within the dampening reuse limit of the core interface so the link stays down.
In a network where there's redundancy between two edge PE nodes.
A failed fibre somewhere in the network should not affect just any control plane protocol.
Quite the opposite.
Does it affect IS-IS or LDP "sessions" (more appropriately adjacencies) yes it does.
But does it affect BGP or targeted LDP (basically TCP)sessions between the two edge PE nodes -or BGP session from PEs to RRs? -no it should not (if there's redundancy in the network).
And by the same token it should not cause downtime on the PW between the two edge PEs (if there's redundancy in the network)
-yes the PW will now be routed via an alternate path circumventing the faulty fibre but it will remain UP (unless bound to a TE path forcing it to stay and fail) and if the switchover is based on FRR customer should hardly notice anything.    

Fault B) flapping rate is outside the dampening reuse limit of the core interface so the link goes up and down in dampening reuse time intervals.
This is a nasty one, how nasty depends on your interface state dampening scheme.
But with FRR it should be just inconvenience without much effect to customer traffic carried by the PW that keeps on constantly switching paths, (maybe resulting in some out of order packets if primary and backup paths are not symmetric).  

Fault C) link never fails, 
It exhibits very high packet drop rate to screw your SLAs but not high enough to take bfd session down (affecting 3 hellos in a row). 
Maybe the remedy for this one could be LFM sessions instead of BFD if your network suffers from low quality fibres -as LFM should take into account Frame and Symbol error count for all data passing through the interface not just LFM PDUs, which you can then use as a threshold to bring the link down. 
    

adam

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