[j-nsp] track-igp-metric in LDP

[Olivier Benghozi]

That's right: if you want your LDP labeled traffic to follow your IGP costs instead of using unexpected paths (and you probably want it in fact, as if you want to do something else you usually use RSVP/MPLS-TE or Segment Routing), you just need track-igp-metric (and therefore it's always useful/needed/best practice).
As Michael wrote, by default (without this knob) you would have all LSP/LDP paths at the same cost (which is probably stupid).

> Le 2 août 2020 à 16:21, Chen Jiang <ilovebgp4 at gmail.com> a écrit :
> 
> Hi! Dave
> 
> Thanks for clarifying. So you mean BGP path selection tie breaker for IGP
> metric is happening in inet.3(LDP) but not inet.0(IGP like OSPF/IOSIS), so
> we need "track-igp-metric" to introduce metric into LDP for
> track-igp-metric. is it correct?
> 
> BR!
> 
> James Chen
> 
> On Sun, Aug 2, 2020 at 8:17 PM Dave Bell <me at geordish.org <mailto:me at geordish.org>> wrote:
> 
>> If you are running next hop self on your BGP routes at the edge, the best
>> path will be via a loopback in an  LSP.
>> 
>> If you have two identical routes, one of the tie breakers is IGP
>> preference. If that knob isn’t set the IGP cost will be 1 for everything,
>> and you will progress down to less helpful tue breakers like router id.
>> 
>> Regards,
>> Dave
>> 
>> On Sun, 2 Aug 2020 at 11:58, Chen Jiang <ilovebgp4 at gmail.com> wrote:
>> 
>>> Hi! Michael
>>> 
>>> Thanks for your clarification.
>>> 
>>> Sure, it will let LDP use IGP metric, but is there any benefit?
>>> 
>>> Cause per my understanding LDP only works for label distributing, not path
>>> selection, and LDP always follows the IGP path, so what is the difference
>>> or benefit to add additional configuration knob to let LDP use IGP metric?
>>> 
>>> BR!
>>> 
>>> Chen Jiang
>>> 
>>> On Sun, Aug 2, 2020 at 6:32 PM Michael Hallgren <mh at xalto.net> wrote:
>>> 
>>>> Hi James,
>>>> 
>>>> From memory, Junos assigns metric 1 by default to "LDP routes", not IGP
>>>> metric, unless you push this button.
>>>> 
>>>> Cheers,
>>>> mh
>>>> ------------------------------
>>>> *De :* Chen Jiang <ilovebgp4 at gmail.com>
>>>> *Envoyé :* dimanche 2 août 2020 12:10
>>>> *À :* Juniper List
>>>> *Objet :* [j-nsp] track-igp-metric in LDP
>>>> 
>>>> Hi! Experts
>>>> 
>>>> Sorry for disturbing, I am curious about track-igp-metric knob under
>>> LDP,
>>>> is there any scenarios it will be useful? I think ldp is just a label
>>>> distribution protocol, the forwarding path always follows the IGP
>>> shortest
>>>> path, is there any benefit for using track-igp-metric?
>>>> 
>>>> Thanks for your help!