VLAN 1 must be pruned from all trunk and access ports that do not require it.

From Layer 2 Switch Security Technical Implementation Guide

Part of VLAN 1 traffic traverses across unnecessary trunk

SV-3972r2_rule VLAN 1 must be pruned from all trunk and access ports that do not require it.

Vulnerability discussion

VLAN 1 is a special VLAN that tags and handles most of the control plane traffic such as Spanning-Tree Protocol (STP), Cisco Discovery Protocol (CDP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP) all VLAN 1 tagged traffic. VLAN 1 is enabled on all trunks and ports by default. With larger campus networks, care needs to be taken about the diameter of the VLAN 1 STP domain; instability in one part of the network could affect VLAN 1, thereby influencing control-plane stability and therefore STP stability for all other VLANs.

Check content

Review the device configuration to determine if VLAN 1 is pruned from all trunk and access switch ports. If VLAN 1 is not pruned from trunk or access switch ports where it's not required, this is a finding.

Fix text

Best practice for VLAN-based networks is to prune unnecessary ports from gaining access to VLAN 1 and insure that it does not traverse trunks not requiring VLAN 1 traffic.

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