In reviewing various presentations for Remote LFA fast re-route it is typically noted that the following configuration needs to be applied to support passive targeted LDP sessions for Remote LFA to function properly.
mpls ldp discovery targeted-hello accept
I am however not really turning up anything concrete on why this is required. Without the above configuration I am noting what appears to be active targeted sessions based on the show mpls ldp discovery output. Are these not available for Remote LFA for the without the above configuration?
ASR9001-Aggr1#show mpls ldp discovery
Tue Sep 27 21:48:41.091 UTC
Local LDP Identifier: 10.144.255.7:0
Discovery Sources:
Interfaces:
TenGigE0/0/2/0 : xmit/recv
VRF: 'default' (0x60000000)
LDP Id: 10.144.255.6:0, Transport address: 10.144.255.6
Hold time: 15 sec (local:15 sec, peer:15 sec)
TenGigE0/0/2/1 : xmit/recv
VRF: 'default' (0x60000000)
LDP Id: 10.144.255.8:0, Transport address: 10.144.255.8
Hold time: 15 sec (local:15 sec, peer:15 sec)
Targeted Hellos:
10.144.255.7 -> 10.80.255.79 (active), xmit/recv
LDP Id: 10.80.255.79:0
Hold time: 90 sec (local:90 sec, peer:90 sec)
10.144.255.7 -> 10.80.255.80 (active), xmit/recv
LDP Id: 10.80.255.80:0
Hold time: 90 sec (local:90 sec, peer:90 sec)
10.144.255.7 -> 10.144.255.1 (active), xmit/recv
LDP Id: 10.144.255.1:0
Hold time: 90 sec (local:90 sec, peer:90 sec)
I don't fully understand the difference between the active and the passive sessions and the implications and am not turning up a useful explanation. Is there a reference out there that explains this is a useful way, especially in the context of remote LFA?
