[c-nsp] MPLS interface continuity and OSPF configuration ME-3600X

[Adam Vitkovsky]

Hi Eric,

Routers in MPLS do not base their forwarding decision on whether MPLS is enabled on a particular link or not, they still base their routing decision on IGP metric.
So as in your case better IGP metric forced packets to be forwarded over the newly enabled link.
Unfortunately since the new link between R2 and R3 was not enabled for MPLS, R2 striped all the labels from the packets and sent them as pure IP packets to R3 and R3 was forced to do IP lookup to forward the packets further which resulted in dropping the packets as R3 did not have a route for prefixes specified as destinations in incoming packets.

To avoid this happening in the future you can use MPLS LDP-IGP Synchronization as it sets max metric on OSPF links not enabled for MPLS.
router ospf 100
 mpls ldp sync

So in your case the newly configured link (even if configured with better metric) would gain max metric if MPLS would have not been configured on it.
That's OSPF steering traffic away of links on which LDP is not working properly or is not enabled at all.
  

adam
> -----Original Message-----
> From: cisco-nsp [mailto:cisco-nsp-bounces at puck.nether.net] On Behalf Of
> Eric Louie
> Sent: 01 May 2015 20:10
> To: CiscoNSP
> Subject: [c-nsp] MPLS interface continuity and OSPF configuration ME-3600X
> 
> I had a strange anomaly happen yesterday
> 
> We have 2 ME-3600's as part of an MPLS network.  There are MPLS interfaces
> on both sides of them and between them.  so, like this
> 
> R1 ---- R2 ---- R3 ---- R4
> 
> and all the links between are MPLS IP
> 
> My NOC enabled a new circuit between R2 and R3, and "forgot" to use mpls
> ip
> on both interfaces.  When they decreased the OSPF cost on the two
> interfaces (as the preferred route), traffic from R1 to R4 stopped at R2.
>  (I didn't get the corresponding result R4 to R1, but our monitoring server
> is behind R4, and could not reach any devices connected to R1)  The old
> MPLS circuit was still connected and enabled, just costed out via OSPF.
> 
> So, it "appears" that when the new link was costed in, because it was not
> an MPLS link, the traffic didn't know how to get through the new R2-R3
> link, since the MPLS link was now costed out by MPLS.
> 
> making the new link an MPLS link solved the problem.
> 
> My questions are:
> Is this "by design"?  In other words, if we have MPLS links on both sides
> of a pair of routers, does that MPLS configuration need to be contiguous
> throughout?
> 
> Is this a condition of configuring MPLS, that all intermediate paths need
> to be either tunnelled or configured?  (something that I might have missed
> in my MPLS learning)
> 
> Did we run into a bug?  IOS 15.2(4)S
> 
> Is this a TAC question?
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