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LDP generates local bindings for all the routes in the routing table. I configured an ISIS topology such that:

R1 ----- R2 ----- R3 ----- R4 ----- R5
L1      L1/2      L2      L1/2      L1

Initially, R2 was advertising (Not advertising actually but sending an LSP with ATT bit set) to R1 telling that it has connectivity to reach level 2. R1 thus installed a default route to reach level 2. In order to run LDP, I leaked routes from level 2 to level 1 so now R1's routing table has all the specific routes. (But the default route is still present. Why is it required now?). As expected, LDP created labels for all the specific routes in routing table but it did not create a label for the L1 default route with next hop as R2. So my question is:

  1. In ISIS, if routes are leaked, why is a default route still required?
  2. If it is there in routing table, why is a label not present in LFIB?
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Some implementations set the ATT bit automatically and provide you a command or knob under a configuration stanza to disable it. i.e on Juniper: ignore-attached-bit

Ignore the attached bit on IS-IS Level 1 routers. Configuring this statement enables the routing device to ignore the attached bit on incoming Level 1 link-state PDUs. If the attached bit is ignored, no default route, which points to the routing device which has set the attached bit, is installed.

Others, require you to actually manually configure the ATT bit. i.e Cisco: attached-bit send

To configure an Intermediate System-to-Intermediate System (IS-IS) instance with an attached bit in the Level 1 link-state packet (LSP), use the attached-bit send command in address family configuration mode.

So whichever it is, you certainly have power over what you want to achieve, in this case you have decided to perform leaking to have specific routes advertised to your internal area. In this example you have given, leaving the attached bit and thinking about a worst case scenario where there are no failsafe mechanisms, you could end up with a blackhole if you loose connectivity to the specific routes because you still have the default one.

On the other hand, I'm not sure which parameters have been taken into consideration for the leaking, so if it were to be something dynamic where the routes are only leaked if certain conditions you specified are met, then if one of those evaluates to false you will stop seeing those routes and this could potentially result in connectivity loss if there is no other way to work around, hence default route.

In regards to LDP not creating a FEC for the default route introduced by the attached bit in ISIS.

Personally I haven't tested this yet, as soon my hands get on some equipment will do, but for what Google has showed me:

From Microtik:

/mpls ldp set distribute-for-default-route=yes does. It seems to add an entry in the MPLS forwarding table for the default route

Reason being:

  • MPLS switching does not have "matching longest prefix" feature of IP routing.

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