Will RSVP let LSPs establish across higher ISIS metric?

[loris.marcellini]

Hey folks,

I have a curiosity and haven't had the chance to try this live given it's hard to saturate a link in my lab lol

If I have the following topology with the numbers being the ISIS metric for the given links:

DC0

   r1-------50-----------r2--DC1

   |

   |

   100
   |

   |

   r3

   |

   DC1

RSVP-TE is preferring the green link and establishes all LSPs across it until it hits the 80% reservable threshold. At this point, should r1 (headend) need to establish a new LSP to reach DC1, will it use the blue link despite the metric being unfavourable or just congest the green link until there's traffic that needs to be forwarded?

I never read anything about this in RFC or books and I am not sure whether RSVP entity that are at max capacity would be retired/ignored from the ISIS nodes graph or not. Could you help me understand and provide some resources on this?

Thanks, L.

[loris.marcellini]

@Harold Ritter do you have any insights?

[Harold Ritter · Accepted solution]

Hi @loris.marcellini ,

Sorry for the delay.

> will it use the blue link despite the metric being unfavourable or just congest the green link until there's traffic that needs to be > forwarded?

RSVP-TE does not take link congestion into account, so the blue link is not used unless the green link goes down.

Regards, 

[loris.marcellini]

Thanks so much. That's what I thought but then someone told me it actually does and I then is when I have started looking for proves of the opposite in RFCs etc but could not find anything.

Thanks for the reply as usual!

Cheers, L.

[loris.marcellini]

Hey Harold, looking through some more Juniper documentation, I found that:

-CSPF prunes the traffic engineering database of all the links that are not full duplex and do not have sufficient reservable bandwidth.

So from this, my understanding is that the link will not even get selected to be part of the ERO and RSVP won't even try to signal it over the higher metric link. 

Does this make the answer to my question different in any way? RSVP-TE does not care about available bandwidth but CSPF does, does that imply that yes, the blue link will actually be used at some point when reservable bandwidth is maxed out on the green one?

Source:

https://www.juniper.net/documentation/us/en/software/junos/mpls/topics/topic-map/lsp-computation.html