On Wed, Nov 11, 2015 at 4:23 AM, Martin Winter <
[email protected]> wrote:
> Daniel,
>
> On 10 Nov 2015, at 6:27, Daniel Walton wrote:
>
> [updated Pradosh's email address on the cc]
>>
>> I think this was a case of Pradosh being cautious in his wording when he
>> wrote this patch. I cannot think of any scenario where this would cause a
>> routing loop and we have not had any customers have issues with routing
>> loops. The same used to be said about deterministic-med ("deploy it
>> everywhere at the same time") but that is another one where I have never
>> seen a scenario where deploying it in some places and not others actually
>> causes a loop. For me it is kind of like saying "deploy the same
>> route-maps everywhere else you are running a different decision algorithm
>> everywhere and you could get loops" which we know is not the case.
>>
>
> These cases you use for comparison can all be fixed in the configuration,
> by “configuring it correctly”. These are basically features which can be
> turned on or off.
>
> This change doesn’t seem to be possible to disable.
>
> So I worry that this change may only be fixed with
> a) replace Quagga with an older version without this feature
> b) replace all other boxes with Quagga as well
> c) remove the Quagga boxes.
> d) potentially some hack in a route-map/filter
>
> (Sorry got burned with such an issue between another vendor and Cisco in
> the my ISP past. Lucky at that time, we got the other vendor to change
> their code in < 24hrs. It still was an experience which had all mgmt
> question the use of non-cisco gear and I would love to avoid this with
> Quagga)
>
> Routing loops in BGP are typically caused when you have one node that is
>> forwarding down a path but is not advertising that path to his peers (ie
>> multipath scenario and the whole multipath-relax knob). The proposed
>> bestpath change does not cause that to happen....I think it is a safe
>> change.
>>
>
> Should I take this as it’s impossible for loops or just unlikely ?
> (Or are you not sure?)
>
> If there is a chance that this could cause loops, then I would love
> to see this being possible to enable / disable with a config knob.
IMO it is not possible for this to create a loop. Paul raised a good point
that it could cause some route churn though so it is probably worth adding
a knob to disable it if needed.
Daniel
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