Hidden OSPF/IS-IS routes appear in inet.3 when LDP tunneling is enabled on RSVP LSPs

When LDP is tunneled over RSVP LSPs, hidden OSPF and/or IS-IS routes appear in inet.3. This is normal behavior for LDP tunneling due to stealth IGP shortcuts (shortcuts that have not been explicitly configured).

Assume the following network diagram:

P1 has the following configuration:

With this setup, P1 installs an LDP route in inet.3 for the PE2 loopback:

However, P1 also installs a hidden IS-IS route inet.3 for the same address:

In order to tunnel LDP FECs inside the RSVP LSP, P1 enables IGP shortcuts in the background. This causes IS-IS or OSPF (whichever is being used) to install inet.3 routes for prefixes via RSVP LSPs through which they are reachable. However, because IGP shortcuts have not been explicitly configured, these shortcut routes are hidden. This makes them available to LDP for resolution, but prevents BGP from inadvertently using them as well. Additional information on IGP shortcuts is available at “Using Labeled-Switched Paths to Augment SPF to Compute IGP Shortcuts.”

This is normal, expected behavior. However, if IGP shortcuts are explicitly enabled, these routes will not longer be hidden.

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Prasanna

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