The Cisco switch must have the default VLAN pruned from all trunk ports that do not require it.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-220692	SRG-NET-000512-L2S-000009	CISC-L2-000230	SV-220692r991948_rule	2024-08-22	3

Description
The default VLAN (i.e., VLAN 1) is a special VLAN used for control plane traffic such as Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP). 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 STP domain for the default VLAN. Instability in one part of the network could affect the default VLAN, thereby influencing control-plane stability and therefore STP stability for all other VLANs.

Description

The default VLAN (i.e., VLAN 1) is a special VLAN used for control plane traffic such as Spanning-Tree Protocol (STP), Dynamic Trunking Protocol (DTP), VLAN Trunking Protocol (VTP), and Port Aggregation Protocol (PAgP). 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 STP domain for the default VLAN. Instability in one part of the network could affect the default VLAN, thereby influencing control-plane stability and therefore STP stability for all other VLANs.

ℹ️ Check
Review the switch configuration and verify that the default VLAN is pruned from trunk links that do not require it. SW1# show interface trunk -------------------------------------------------------------------------------- Port Native Status Port Vlan Channel -------------------------------------------------------------------------------- Eth1/1 1 trunking -- Eth1/2 1 trunking -- -------------------------------------------------------------------------------- Port Vlans Allowed on Trunk -------------------------------------------------------------------------------- Eth1/1 1-998,1000-4094 Eth1/2 1-998,1000-4094 If the default VLAN is not pruned from trunk links that should not be transporting frames for the VLAN, this is a finding.

ℹ️ Check

Review the switch configuration and verify that the default VLAN is pruned from trunk links that do not require it. SW1# show interface trunk -------------------------------------------------------------------------------- Port Native Status Port Vlan Channel -------------------------------------------------------------------------------- Eth1/1 1 trunking -- Eth1/2 1 trunking -- -------------------------------------------------------------------------------- Port Vlans Allowed on Trunk -------------------------------------------------------------------------------- Eth1/1 1-998,1000-4094 Eth1/2 1-998,1000-4094 If the default VLAN is not pruned from trunk links that should not be transporting frames for the VLAN, this is a finding.

✔️ Fix
Step 1: Prune VLAN 1 from any trunk links as necessary. SW1(config)# int e1/2 SW1(config-if)# switchport trunk allowed vlan except 1, 999 SW1(config-if)# end Step 2: Verify VLAN 1 is not allowed on the trunk link. SW1# show interface trunk -------------------------------------------------------------------------------- Port Native Status Port Vlan Channel -------------------------------------------------------------------------------- Eth1/1 1 trunking -- Eth1/2 1 trunking -- -------------------------------------------------------------------------------- Port Vlans Allowed on Trunk -------------------------------------------------------------------------------- Eth1/1 1-998,1000-4094 Eth1/2 2-998,1000-4094

✔️ Fix

Step 1: Prune VLAN 1 from any trunk links as necessary. SW1(config)# int e1/2 SW1(config-if)# switchport trunk allowed vlan except 1, 999 SW1(config-if)# end Step 2: Verify VLAN 1 is not allowed on the trunk link. SW1# show interface trunk -------------------------------------------------------------------------------- Port Native Status Port Vlan Channel -------------------------------------------------------------------------------- Eth1/1 1 trunking -- Eth1/2 1 trunking -- -------------------------------------------------------------------------------- Port Vlans Allowed on Trunk -------------------------------------------------------------------------------- Eth1/1 1-998,1000-4094 Eth1/2 2-998,1000-4094