Routing

I have MX480 with two Routing engine (re0,re1). By default, re0 is a master, re1 is a backup. I've update re1 with version 18.1R2.5, then made re1  as a master ( request chassis routing-engine master switch). But re0, I couldn't update, I created case, Juniper send me new routing engine. So, my question is: When I pull it back to the MX480 in slot0 what would happened ? re1 still be the master or re0 will become the master.

By default, Junos OS uses re0 as the master Routing Engine and re1 as the backup Routing Engine. Unless otherwise specified in the configuration.

https://www.juniper.net/documentation/en_US/release-independent/junos/topics/task/operational/host-redundancy-master-backup-routing-engine-nog.html

could you share the following output from router:

> show configuration chassis

Routing Engine status:
Slot 1:
Current state Master
Election priority Backup
Temperature 30 degrees C / 86 degrees F
CPU temperature 38 degrees C / 100 degrees F
DRAM 3584 MB (4096 MB installed)
Memory utilization 10 percent
5 sec CPU utilization:
User 1 percent
Background 0 percent
Kernel 4 percent
Interrupt 0 percent
Idle 95 percent
1 min CPU utilization:
User 1 percent
Background 0 percent
Kernel 4 percent
Interrupt 0 percent
Idle 94 percent
5 min CPU utilization:
User 1 percent
Background 0 percent
Kernel 4 percent
Interrupt 0 percent
Idle 94 percent
15 min CPU utilization:
User 1 percent
Background 0 percent
Kernel 4 percent
Interrupt 0 percent
Idle 95 percent
Model RE-S-2000
Serial ID 9012022222
Start time 2018-07-01 02:19:26 ALMT
Uptime 86 days, 12 hours, 57 minutes, 35 seconds
Last reboot reason Router rebooted after a normal shutdown.
Load averages: 1 minute 5 minute 15 minute
0.62 0.74 0.70

Hi,

From the below o/p, there no "(default)" next to "current state" which means you have configured RE0 to be master and RE1 under the "set chassis redundancy routing-engine <0/1> <master/backup>"  stanza.

Routing Engine status:
Slot 1:
Current state Master
Election priority Backup

So you in your case, after the backup Routing Engine takes mastership, it continues to function as master even after the originally configured master Routing Engine has successfully resumed operation (that is when you insert new RE in slot-0). You must manually restore it to its previous backup status. (However, if at any time one of the Routing Engines is not present, the other Routing Engine becomes master automatically, regardless of how redundancy is configured.)

https://www.juniper.net/documentation/en_US/junos/topics/topic-map/configuring-routing-engine-redundancy-tm.html