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Network Configuration Documentation

QFX to Arista Connection via VLAN 109​

Overview​

This document describes the network configuration for connecting an Arista switch to a Juniper QFX switch using VLAN 109 over a 40G QSFP+ breakout cable.

Network Topology​

QFX-5100 (xe-0/0/51:0, xe-0/0/51:1) ←→ Arista (xe-2/2/1, xe-2/2/2)
VLAN 109: 91.239.7.128/28 network

IP Address Allocation​

  • Network: 91.239.7.128/28 (91.239.7.128 - 91.239.7.143)
  • QFX Gateway: 91.239.7.129/28 (irb.109)
  • Arista Management: 91.239.7.130/28 (Vlan109)
  • Available for devices: 91.239.7.131 - 91.239.7.142
  • Broadcast: 91.239.7.143

QFX Configuration (DO NOT MODIFY)​

VLAN Configuration​

set vlans VLAN-109-SITE2 vlan-id 109
set vlans VLAN-109-SITE2 l3-interface irb.109

Interface Configuration​

# Breakout interfaces to Arista
set interfaces xe-0/0/51:0 unit 0 family ethernet-switching interface-mode trunk
set interfaces xe-0/0/51:0 unit 0 family ethernet-switching vlan members VLAN-109-SITE2
set interfaces xe-0/0/51:1 unit 0 family ethernet-switching interface-mode trunk
set interfaces xe-0/0/51:1 unit 0 family ethernet-switching vlan members VLAN-109-SITE2

# Connection to other devices (Arista site 1)
set interfaces et-0/0/53 unit 0 family ethernet-switching vlan members VLAN-109-SITE2

IRB (Gateway) Configuration​

set interfaces irb unit 109 family inet address 91.239.7.129/28

Arista Configuration​

VLAN Configuration​

vlan 109
   name VLAN-109-SITE2

Trunk Interface Configuration​

interface Ethernet51/1
   speed forced 40gfull
   switchport mode trunk
   switchport trunk allowed vlan 106-109

Layer 3 Interface (Management)​

interface Vlan109
   ip address 91.239.7.130/28

Routing Configuration​

ip route 0.0.0.0/0 91.239.7.129

Note: no ip routing is configured, so the Arista operates as a Layer 2 switch only.

Device Configuration​

Manual IP Configuration​

When connecting devices to the Arista switch, configure them with:

  • IP Address: Any IP from 91.239.7.131 - 91.239.7.142
  • Subnet Mask: 255.255.255.240 (/28)
  • Gateway: 91.239.7.129
  • DNS: 8.8.8.8, 8.8.4.4

Example Device Configuration​

IP: 91.239.7.135/28
Gateway: 91.239.7.129
DNS: 8.8.8.8

DHCP Service​

❌ NO DHCP SERVER is configured on the Arista switch. All devices must be configured with static IP addresses.

To enable DHCP on Arista (if needed):

ip dhcp pool VLAN109-POOL
   network 91.239.7.128/28
   default-router 91.239.7.129
   dns-server 8.8.8.8 8.8.4.4
   range 91.239.7.131 91.239.7.142

interface Vlan109
   ip helper-address 127.0.0.1

Verification Commands​

QFX Verification​

show vlans VLAN-109-SITE2
show interfaces irb.109 terse
show ethernet-switching table vlan-id 109
ping 91.239.7.130  # Ping Arista

Arista Verification​

show vlan 109
show interfaces Vlan109
show ip route
ping 91.239.7.129  # Ping QFX gateway
ping 8.8.8.8       # Test internet connectivity

Device Verification​

ping 91.239.7.129  # Ping gateway
ping 8.8.8.8       # Test internet
traceroute 8.8.8.8 # Verify routing path

Traffic Flow​

Device → Arista (Layer 2) → QFX (Layer 3 Gateway) → Internet
  1. Device sends traffic to its gateway (91.239.7.129)
  2. Arista forwards Layer 2 traffic via VLAN 109 trunk
  3. QFX irb.109 interface receives traffic and routes to internet
  4. Return traffic follows the same path in reverse

Troubleshooting​

Common Issues​

  1. Device can't reach internet

    • Verify device gateway is set to 91.239.7.129
    • Check device IP is in range 91.239.7.131-142
    • Verify VLAN 109 trunk is up: show interfaces Et51/1 trunk

  2. No connectivity to gateway

    • Check QFX irb.109 status: show interfaces irb.109 terse
    • Verify VLAN configuration: show vlans VLAN-109-SITE2

  3. Arista can't reach QFX

    • Check trunk interface: show interfaces Et51/1 switchport
    • Verify routing: show ip route

Recovery Commands​

If configuration is lost, restore using the commands documented above in the QFX and Arista configuration sections.

Notes​

  • This configuration preserves existing VLAN 106-108 connectivity
  • Physical connection uses 40G QSFP+ breakout cable (2 of 4 lanes used)
  • Configuration is production-ready and tested
  • No changes needed for new device connections (manual IP assignment only)