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How to Create a Virtual Switch Chassis with Junos

Creation of a Virtual Chassis unit with interconnected switches establishes a single unit that you can manage as though it were a single chassis. One switch member in the Virtual Chassis is the primary or master switch, and a second member is a backup that provides redundancy if the master member fails for some reason.

The forwarding (and routing) tables remain synchronized with those of the master member. If a failover to the backup occurs, this member switch can immediately step in to continue handling traffic on the LAN.

If you use the default configuration, you connect the two switches with the dedicated 64-Gbps VCPs on the rear panels and power on the switches. The Virtual Chassis is operational. Connect each of the two VCPs on the chassis member to a VCP on the other member. You don’t have to configure these ports.

After you power on the two switches in the Virtual Chassis unit, the JUNOS picks one of them to be the master (the other one becomes the backup) and assigns member IDs to each one. You can see this information on the switch’s LCD display.

Another way to check the master-backup assignment is to view the status of the Virtual Chassis unit:

user@junos-switch> show virtual-chassis status
Virtual Chassis ID: 0019.e250.47a0
           Mastership   Neighbor List
Member ID Status Serial No Model priority Role  ID Interface
0 (FPC 0) Prsnt AK0207360276 ex4200-48p  128 Master* 1 vcp-0
                1 vcp-1
1 (FPC 1) Prsnt AK0207360281 ex4200-24t  128 Backup 0 vcp-0
                0 vcp-1
Member ID for next new member: 4 (FPC 4)

The Mastership priority column shows that both members have an equal chance of being elected as the master. In this case, member 0 is the master. Also check that the dedicated VCPs are up and running:

user@junos-switch> show virtual-chassis vc-port all-members
Interface  Type    Status
PIC / Port
vcp-0   Dedicated  Up
vcp-1   Dedicated  Up
Interface  Type    Status
PIC / Port
vcp-0   Dedicated  Up
vcp-1   Dedicated  Up

Junos OS treats each switch in a Virtual Chassis unit as though it were a FPC in the slot of an M-series router chassis. So, the software calls the first member switch fpc0, and it calls the second switch fpc1.

When you commit any configuration changes to the Virtual Chassis unit, use the commit synchronize command, rather than the plain commit command. This command ensures that configuration changes are saved on both the master and backup switches.

Now you can power on the new third switch. You don’t need to perform any configuration on this switch, and the expanded Virtual Chassis unit is ready to use.

You should use the same mastership priority for master and backup. To understand why, consider what happens when one switch has higher priority and, thus, is always the master. If that master switch has a problem that is causing it to reboot itself, it will continue to become the master, only to reboot a minute later. In other words, you lose the benefits of having a backup.

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