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

[Adam Vitkovsky]

Looks like bug or a legacy way of working, 
The command: "mpls ldp sync" should not have any impact on IGP sessions formation. 
What it is supposed to do is to set substantially higher metric on a link over which LDP is not operational making the path uninteresting from a routing point of view. However the high metric won't make routes behind this link unreachable. 
That is why it is important never to use the holddown timer as you would never want the MPLS traffic to be routed over a link that is not ready for label switching yet. 

adam
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> Troy Boutso
> Sent: 02 February 2015 21:27
> To: cisco-nsp at puck.nether.net
> Subject: [c-nsp] MPLS LDP Sync w/ ISIS over point to point Link
> 
> 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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