MPLS Default Route LFIB Issue

Hi All,

Firstly, I'm new to MPLS so this could quite easily be my lack of understanding.

Can anyone think of a reason why the default route in the global table would be showing as explicit-null when the next two hops are P routers and the egress LSR is 3 hops away?

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CORE1#sh mpls forwarding-table 8.8.8.8

Local Outgoing Prefix Bytes Label Outgoing Next Hop

Label Label or Tunnel Id Switched interface

None explicit-n 0.0.0.0/04096327 Te1/0/1 192.168.10.14

explicit-n 0.0.0.0/00 Te1/0/2 192.168.10.22

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The default route is in OSPF everywhere and LDP is up between all devices where MPLS is enabled. There are plenty of other routes in the global table with labels assigned correctly and routing to those is fine. For default route pings, it works from CORE1 itself, but from SW1 (without MPLS enabled) it doesn't route beyond CORE1.

We are in a transition period, which is why the first device isn't a PE for now and just has routes in the global table.

SW1(NO MPLS) - CORE1(P) - WAN1(P) - WAN2(P) - CORE2(PE) - LEGACYSW(NO MPLS) - FW(DEFAULT ORIG)

We have explicit-null set everywhere to standardise the QoS configurations. But even without this it would presumably be popping it. It's as if WAN1 thinks 0.0.0.0 is directly connected so is sending the null.

This was attempted in a tight change window and we had to roll back, so getting additional logs etc isn't possible unfortunately. We might just have to go for another window to get 'show tech' output etc, but it would be nice to go into it with some troubleshooting ideas.

I've tried to simulate it in GNS3 and it seems to work, which isn't helpful for reproducing it.

Any thoughts would be greatly appreciated.