Junos OS

Hi. I'm learning configuration groups and have a question. suppose these commands:

set groups TEST_GROUP interface "<em[45]>" unit 0 family inet mtu 5000

set interface em4 apply-groups TEST_GROUP

with regard to these commands, what is the purpose of "<em[45]>" or mentioning any interface namber in line 1 (first command) when we will apply this group on any interface that we want. for example, my first command mentions em4 and em5. so if I go to interface em3 and apply that group there, what will happen? does that interface take effect from the group? if yes, so what is the purpose of mentioning interface numbers while defining the groups itself?! tnx. 

No, if you use em3, the group will not be applied to that interface.

This configuration should be a safety measure to make sure that you don't apply the group to other interfaces that you don't want to by mistake.

Check this quick test:

{master:0}[edit]
root@EX4200# show interfaces ge-0/0/1
apply-groups TEST;

{master:0}[edit]
root@EX4200# show interfaces ge-0/0/2
apply-groups TEST;

{master:0}[edit]
root@EX4200# show interfaces ge-0/0/3
apply-groups TEST;

{master:0}[edit]
root@EX4200# show groups TEST
interfaces {
"<ge-0/0/[12]>" {
mtu 6789;
}
}

{master:0}[edit]
root@EX4200# show interfaces ge-0/0/1 | display inheritance
##
## '6789' was inherited from group 'TEST'
##
mtu 6789;

{master:0}[edit]
root@EX4200# show interfaces ge-0/0/2 | display inheritance
##
## '6789' was inherited from group 'TEST'
##
mtu 6789;

{master:0}[edit]
root@EX4200# show interfaces ge-0/0/3 | display inheritance

{master:0}[edit]
root@EX4200#

=====

If this worked for you please flag my post as an "Accepted Solution" so others can benefit. A kudo would be cool if you think I earned it.

yes you're right. but router accepts the command and even it shows the "apply-groups" under interface configuration mode. but it has no effects because of what you said, security measue. when the interfaces inherit the mtu from configuration group, it is displayeed with "show inter em3 | display inherit" command. tnx again 😉