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:

PE1(10.0.0.81)---P1(10.0.0.82)-----P2(10.0.0.83)----PE2(10.0.0.84)

 |---------------------------LDP---------------------|

                   |----RSVP LSPs---|

P1 has the following configuration:

protocols {
  rsvp {
    interface xe-10/1/7.92;
  }
  mpls {
    explicit-null;
    label-switched-path to-P2 {
      to 10.0.0.83;
      ldp-tunneling; <=== LDP tunneling is configured on the LSP to P2
    }
    interface xe-10/1/7.91;
    interface xe-10/1/7.92;
  }
  isis { <=== 'traffic-engineering inet shortcuts' is not enabled
    level 1 disable;
    level 2 wide-metrics-only;
    interface xe-10/1/7.91 {
      level 2 metric 20;
    }
    interface xe-10/1/7.92 {
      level 2 metric 20;
    }

    interface lo0.82 {
      passive;
    }
  }
  ldp {
    track-igp-metric;
    interface xe-10/1/7.91;
    interface lo0.82;
    session 10.0.0.83;
  }
}

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

[email protected]> show route 10.0.0.84 table inet.3


inet.3: 4 destinations, 7 routes (3 active, 0 holddown, 3 hidden)

+ = Active Route, - = Last Active, * = Both


10.0.0.84/32 *[LDP/9] 00:10:01, metric 40 <== PE2's loopback address

> to 192.168.92.1 via xe-10/1/7.92, label-switched-path to-P2

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

[email protected]> show route 10.0.0.84 table inet.3 hidden


inet.3: 4 destinations, 7 routes (3 active, 0 holddown, 3 hidden)

+ = Active Route, - = Last Active, * = Both


10.0.0.84/32 [IS-IS] 00:12:28, metric 40

> to 192.168.92.1 via xe-10/1/7.92, label-switched-path to-P2

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.

[edit] 

[email protected]# set protocols isis traffic-engineering family inet shortcuts 


[edit] 

[email protected]# commit synchronize and-quit 

re0: 

configuration check succeeds 

re1: 

commit complete

re0: 

commit complete 

Exiting configuration mode 


[email protected]> show route 10.0.0.84 table inet.3 


inet.3: 4 destinations, 7 routes (4 active, 0 holddown, 0 hidden) 

+ = Active Route, - = Last Active, * = Both 


10.0.0.84/32 *[LDP/9] 00:04:39, metric 40   
  > to 192.168.92.1 via xe-10/1/7.92, label-switched-path to-P2   
  [IS-IS/18] 00:04:40, metric 40 <=== previously hidden route is now active   
  > to 192.168.92.1 via xe-10/1/7.92, label-switched-path to-P2

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