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Configure 802.1q VLAN trunking between Netscreen and Cisco switches

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Article ID: KB5506 KB Last Updated: 24 Jun 2019Version: 4.0
Summary:
Configure 802.1q VLAN trunking between Juniper Netscreen firewalls and Cisco switches
Symptoms:
Conditions:
  • The trunking protocol on the switch is 802.1q 
  • All VLANs are allowed to pass traffic through the trunk.

Symptom:
  • The root interface on the NetScreen is not reachable from the router which is connected to the native (default) VLAN 1.

Background:
  • The native VLAN in Cisco terminology means it will not tag the packet for this VLAN and packets arriving on that interface are assumed to be in VLAN 1. The root interface on the NetScreen is not tagged.
  • For example, if you change the native VLAN on the Cisco switch to VLAN 6, this means packets originated from VLAN 6 will not be tagged, only nodes connected to VLAN 6 can reach the root interface of the NetScreen.
Solution:
The native VLAN association on the trunk port was mistakenly changed on the switch to VLAN 6. This means only devices on VLAN 6 could reach the NetScreen device. The trunk port on the switch should have a native VLAN set to 1. This is specific to Cisco switches. If not using Cisco, contact the switch vendor to determine what it uses for its native VLAN.  For more information see Cisco's Forum message: https://learningnetwork.cisco.com/message/5070
 
Modification History:
2019-06-15: content reviewed for accuracy.
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