A LFA configuration tip for explicitly excluding one interface or more.

It introduces a LFA configuration tip for explicitly excluding one interface or more in the production scenario, because not all the interfaces are suitable to work as a backup in LFA.

The production might need to consider  the bandwidth, priority and physical loading fiber issue . etc.

So whenever it deploys LFA, the customer needs to exclude some interfaces which isn't appropriate for protecting the main interface.

By default, all OSPF/ISIS interfaces that belong to the default instance or to a specific routing instance are eligible as a backup interface for interfaces configured with link-protection or node-link protection.

You can specify that any OSPF/ISIS interface be excluded from functioning as a backup interface to protected interfaces. 

The knob example as below:

set protocols ospf area x.x.x.x interface interface-name no-eligible-backup;
set protocols isis interface interface-name no-eligible-backup;

These configuration doesn't relay on "igp  back-spf-options"  or "routing-option backup-selection" setting.

It would work as the fist priority compared with other LFA computation.

You can use the following list to check the backup state for the designed interface.

show isis backup spf results
show isis backup coverage
show isis interface detail

show ospf backup spf results
show ospf backup neighbor

Reference links:

https://www.juniper.net/documentation//us/en/software/junos/is-is/topics/concept/isis-node-link-protection-understanding.html

https://www.juniper.net/documentation//us/en/software/junos/ospf/topics/topic-map/configuring-loop-free-alternate-routes-for-ospf.html