Troubleshooting Inter-VLAN ACL Configurations on Switches

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Inter-VLAN ACL configuration issues on XS3800 core switches (such as policies not taking effect or losing complete access) are typically caused by misconfigured traffic classifiers or the stateless nature of switch ACLs. When the frame format is mismatched or subnet scopes do not align, the switch fails to match and block the target traffic as intended.

To troubleshoot and ensure your inter-VLAN ACL rules function correctly, we recommend the following approaches:

1. Verify Classifier Packet Formats:
   • Check if your classifiers are configured with the 802.3tag packet format, which may fail to match normal IPv4 and ICMP traffic that uses Ethernet II.
   • Remove the packet-format and source-IP conditions to match the traffic directly by VLAN and destination network.
2. Align Subnet Mask Configurations:
   • Ensure the subnet mask on your DHCP server matches the subnet mask defined on the VLAN interface and the ACL classifier.
   • Adjust the ACL rules if clients receive IP addresses outside the configured classifier subnet (e.g., a /22 DHCP scope used alongside a /24 ACL mask).
3. Evaluate ACL Stateless Behavior:
   • Understand that switch ACLs evaluate packets independently without maintaining session state, meaning return traffic from a permitted connection may be blocked.
   • Consider deploying policies on a stateful firewall if you require bidirectional communication where only one side initiates the connection.

Note: Switch ACLs are stateless, meaning a policy configured to block traffic in one direction can unintentionally drop response packets for established sessions in the other.

If you observe that policies still do not take effect after correcting the packet formats and subnet masks, or if you require advanced stateful inspection, a dedicated firewall should be used.

If more assistance is needed, please feel free to contact us!