Mailing List Archive

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!

--
BR!



James Chen
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Re: track-igp-metric in LDP [ In reply to ]
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@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@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!
>
> --
> BR!
>
>
>
> James Chen
> _______________________________________________
> juniper-nsp mailing list juniper-nsp@puck.nether.net
> https://puck.nether.net/mailman/listinfo/juniper-nsp
>


--
BR!



James Chen
_______________________________________________
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https://puck.nether.net/mailman/listinfo/juniper-nsp
Re: track-igp-metric in LDP [ In reply to ]
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@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@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@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!
> >
> > --
> > BR!
> >
> >
> >
> > James Chen
> > _______________________________________________
> > juniper-nsp mailing list juniper-nsp@puck.nether.net
> > https://puck.nether.net/mailman/listinfo/juniper-nsp
> >
>
>
> --
> BR!
>
>
>
> James Chen
> _______________________________________________
> juniper-nsp mailing list juniper-nsp@puck.nether.net
> https://puck.nether.net/mailman/listinfo/juniper-nsp
>
_______________________________________________
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Re: track-igp-metric in LDP [ In reply to ]
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@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@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@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@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!
>> >
>> > --
>> > BR!
>> >
>> >
>> >
>> > James Chen
>> > _______________________________________________
>> > juniper-nsp mailing list juniper-nsp@puck.nether.net
>> > https://puck.nether.net/mailman/listinfo/juniper-nsp
>> >
>>
>>
>> --
>> BR!
>>
>>
>>
>> James Chen
>> _______________________________________________
>> juniper-nsp mailing list juniper-nsp@puck.nether.net
>> https://puck.nether.net/mailman/listinfo/juniper-nsp
>>
>

--
BR!



James Chen
_______________________________________________
juniper-nsp mailing list juniper-nsp@puck.nether.net
https://puck.nether.net/mailman/listinfo/juniper-nsp
Re: track-igp-metric in LDP [ In reply to ]
On 2/Aug/20 16:21, Chen Jiang wrote:
> 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?

If you are forwarding in MPLS, yes.

That means an FEC built by LDP will be assigned the correct metric that
was calculated by the IGP. This way, you don't have your IGP telling you
A-B-C is the shortest path, but LDP says A-B-D-C is the shortest path.

Here's an example of some output when both inet6.3 and inet6.0 agree due
to "track-igp-metric":

tinka@er-01-jnb.za-re0# run show route 2c0f:feb0:0:2::8

inet6.0: 92801 destinations, 182856 routes (92796 active, 4 holddown, 2
hidden)
+ = Active Route, - = Last Active, * = Both

2c0f:feb0:0:2::8/128
                   *[IS-IS/18] 2d 07:53:40, metric 24700
                      to fe80::1205:caff:fe86:4ac3 via et-0/0/2.0
                    > to fe80::5287:89ff:fef3:25c3 via et-0/0/2.0
                      to fe80::1205:caff:fe86:4b10 via et-4/0/2.0
                      to fe80::5287:89ff:fef3:2610 via et-4/0/2.0

inet6.3: 1508 destinations, 1508 routes (1508 active, 0 holddown, 0 hidden)
+ = Active Route, - = Last Active, * = Both

2c0f:feb0:0:2::8/128
                   *[LDP/9] 2d 07:53:40, metric 24700
                      to fe80::1205:caff:fe86:4ac3 via et-0/0/2.0, Push
25215
                    > to fe80::5287:89ff:fef3:25c3 via et-0/0/2.0, Push
25214
                      to fe80::1205:caff:fe86:4b10 via et-4/0/2.0, Push
25215
                      to fe80::5287:89ff:fef3:2610 via et-4/0/2.0, Push
25214

{master}[edit]
tinka@er-01-jnb.za-re0#

Same applies to IPv4.

Mark.
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Re: track-igp-metric in LDP [ In reply to ]
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@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@geordish.org <mailto:me@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@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@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@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!

_______________________________________________
juniper-nsp mailing list juniper-nsp@puck.nether.net
https://puck.nether.net/mailman/listinfo/juniper-nsp
Re: track-igp-metric in LDP [ In reply to ]
On 2/Aug/20 18:06, Olivier Benghozi wrote:
> 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).

If you're a multi-vendor house, Cisco do this by default.

Mark.
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Re: track-igp-metric in LDP [ In reply to ]
I had to do this with my cgnat deployment. I had an unforeseen and
undesirable result of all my customer facing mpls pe's using one and only
one of my cgnat boundary mx960's. (this was via an mpls l3vpn) not good, I
need the pe's to flow towards the igp-closest mx960 towards the internet.
Thus load balancing via multiple mx960's and using separate and unique
public ipv4 pools. This track-igp-metric did the trick. It allowed my
cgnat boundary nodes to be seen as more or less attractive from the customer
facing mpls pe perspective.

As someone stated, i think it's rsvp, mp-ibgp for vpns and perhaps other
things that make use of inet.3 in it's best path calculations

-Aaron

-----Original Message-----
From: juniper-nsp <juniper-nsp-bounces@puck.nether.net> On Behalf Of Chen
Jiang
Sent: Sunday, August 2, 2020 5:11 AM
To: Juniper List <juniper-nsp@puck.nether.net>
Subject: [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!

--
BR!



James Chen
_______________________________________________
juniper-nsp mailing list juniper-nsp@puck.nether.net
https://puck.nether.net/mailman/listinfo/juniper-nsp

_______________________________________________
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