[c-nsp] MPLS LDP Sync w/ ISIS over point to point Link

[Spyros Kakaroukas]

I've been in a similar situation before and my understanding is as follows.

If you use loopbacks for your LDP peering and have a default route in your global table you will end up in a catch 22. Assume R1 and R2 . R1 is up and connected to the rest of your domain and has a default route installed. R2 is connected to R1 and is just coming up. They both send LDP hellos. R2 sees the LDP hellos sourced from R1's loopback. It does not have a route to that so it tries to bring up the IGP. R1 sees R2's hellos, sourced from R2's loopback. It does have a matching route for that ( the default ) so it will not bring up the IGP ( until the holddown expires, which is never by default ).

Holddown should fix this, as suggested. You may want to reevaluate whether you want ldp-igp sync in such a design though. Depending on your design goals, there might be other knobs or procedures more suited to them.

On 3 Feb 2015 01:09, dip <diptanshu.singh at gmail.com> wrote:
Without going too deep right now as I am outside I think "mpls ldp igp sync
holddown sec" should fix the problem .

On Monday, February 2, 2015, Troy Boutso <sensible115 at gmail.com> wrote:

> Hey
>
> I've been rolling out new routers to various sites throughout our
> organisation. And in doing so, I've been applying the "mpls ldp sync"
> command under the "router isis" subsection.
> This has been fine up until now. Because all other sites are running OSPF
> and ISIS together (as we are in the process of migrating away from an OSPF
> network to an ISIS based MPLS core network, etc).
>
> With this new site, I only planed on only bringing up the isis adjacency as
> it is a new site and no OSPF is required (because I don't need to migrate
> anything off). However the ISIS adjacency won't come up because it doesn't
> have an LDP session up yet. And the LDP session wont come up without the
> IGP coming up.
>
> This is some real chicken and egg stuff right here.
>
> It has become quiet clear that all my other routers in production which
> have LDP sessions are essentially relying on that OSPF adjacency to help
> form the initial LDP session.
> One day I plan to shut those down. Which could cause me big issues further
> down the road.
> I do have ldp session protection enabled ... but if a router was to reboot
> and have no ospf to help form the initial LDP, then it seems my isis
> adjecencie may never form. That is the worst case scenario
>
>
> Getting back to my point ... If I remove the mpls ldp sync on both routers
> the ISIS adjacency forms immediately. So this is definitely the culprit.
> How on earth is this feature supposed to work in a production environment?
> Am I missing something here?
>
> Am I supposed to manually form ldp sessions (targeted) or something?
> If anyone has experience with this, I'm all ears.
>
> Kind Regards
> Troy
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