Juniper EX Virtual Chassis Configuration: Step-by-Step CLI Guide

A Virtual Chassis turns up to ten Juniper EX switches into one logical device that shares a control plane, runs one configuration and gives you active/standby RE redundancy cheaply. This guide walks through the CLI that makes it happen: preprovisioning members so the configuration exists before the hardware does, designating and cabling Virtual Chassis ports, setting mastership priority and member IDs, handling mixed and non-provisioned modes, and verifying the fabric is actually formed. The examples use EX4300/EX4650 conventions but the concepts carry across the EX range.

Two ways to build one

Step 1: Preprovision the members

user@ex-0> configure
[edit virtual-chassis]
user@ex-0# set preprovisioned
user@ex-0# set member 0 serial-number ABC123 role routing-engine
user@ex-0# set member 1 serial-number DEF456 role routing-engine
user@ex-0# set member 2 serial-number GHI789 role line-card
user@ex-0# set member 2 no-split-detection
user@ex-0# set member 3 serial-number JKL012 role line-card
user@ex-0# set member 3 no-split-detection
user@ex-0# set mastership-priority 200
user@ex-0# commit and-quit

Roles matter. A routing-engine member can become master; a line-card member cannot, which is how you prevent a closet switch from taking over the chassis during a split. Setting no-split-detection on line-card members avoids the split-detection shutdown that triggers when a large chassis loses half its VCP links - intended for two-member edge topologies, dangerous if used blindly.

Step 2: Create and cable the VCPs

! Dedicated VCP ports on EX4300 (the rear 40G ports)
user@ex-0# set virtual-chassis vcp-ports vcp-0
user@ex-0# set virtual-chassis vcp-ports vcp-1

! Uplink ports converted to VCPs where needed
user@ex-0# set virtual-chassis vcp-ports vcp-2
user@ex-0# set virtual-chassis vcp-ports vcp-3
user@ex-0# set interfaces xe-0/2/0 virtual-chassis-vc-port 2
user@ex-0# set interfaces xe-0/2/1 virtual-chassis-vc-port 3

! Automatic VCP conversion for newly added members
user@ex-0# set virtual-chassis auto-sw-update

user@ex-0# commit and-quit
user@ex-0> request virtual-chassis vc-port set interface vcp-0 member 3

Converting an uplink port to a VCP means it is no longer usable as a data port - check your uplink budget before you commit. For multi-member chassis, follow a ring topology: VCP0 of member N to VCP1 of member N+1 and close the loop. Rings survive a single link failure without splitting.

Step 3: Bring the members up and verify

user@ex-0> show virtual-chassis
user@ex-0> show virtual-chassis status
user@ex-0> show virtual-chassis vc-port
user@ex-0> show virtual-chassis protocol adjacency
user@ex-0> show virtual-chassis member 2

Virtual Chassis ID: 1a2b3c4d
Member ID  Status    Serial No    Model    Prio  Role      Mixed
0 (FPC 0)  Prsnt     ABC123       ex4300   200   Master*   No
1 (FPC 1)  Prsnt     DEF456       ex4300   128   Backup    No
2 (FPC 2)  Prsnt     GHI789       ex4300   128   Linecard  No

Every member must show Prsnt and a non-zero Role, and the VCP links must be Up in both directions. A member that appears in show virtual-chassis status but shows no VCP links is cabled to the wrong port or the port was never converted.

Mastership, split and recovery

user@ex-0> request virtual-chassis routing-engine ...            ! promotion
user@ex-0> show virtual-chassis status | match Master
user@ex-0> show chassis alarms
user@ex-0> show log messages | match -i 'vc|split'

! recover a member that lost its configuration
user@ex-1> request system zeroize
user@ex-1> request virtual-chassis renumber member 1 new-member-id 4

When a chassis splits, both fragments may run independently and both keep the same IP addresses - the classic split-brain. Fix it structurally: run a full VCP ring, avoid no-split-detection on members that can realistically split, and add a second management path so a broken fragment is still reachable. Maintain the same discipline on other vendors' stack technologies; our guides on ArubaOS-CX VSF, Dell OS10 VLT, Arista EOS MLAG and Huawei iStack vs CSS cover the same failure modes, and initial EX setup is the prerequisite step.

原文链接：https://www.juniper.net/documentation/us/en/software/junos/virtual-chassis/topics/concept/virtual-chassis-overview.html