assign switch ports not in use to an inactive VLANs

[yaredo70]

Hi All,

The note is from Cisco switch STIG requirement that I need to implement in all my switches in the network. 

Rule Title: The Cisco switch must have all disabled switch ports assigned to an unused VLAN.

Discussion: It is possible that a disabled port that is assigned to a user or management VLAN becomes enabled by accident or by an attacker and as a result gains access to that VLAN as a member.

Check Text: Step 1: Review the switch configurations and examine all access switch ports. Each access switch port not in use should have membership to an inactive VLAN.

interface GigabitEthernet0/0
switchport access vlan 999
shutdown
!
interface GigabitEthernet0/1
switchport access vlan 999
shutdown
…
…
…
interface GigabitEthernet0/9
switchport access vlan 999
shutdown

Step 2: Verify that traffic from the inactive VLAN is not allowed on any trunk links as shown in the example below:

interface GigabitEthernet1/1
switchport trunk allowed vlan 1-998,1000-4094
switchport trunk encapsulation dot1q
switchport mode trunk

Note: Switch ports configured for 802.1x are exempt from this requirement.

If there are any access switch ports not in use and not in an inactive VLAN, this is a finding.

Fix Text: Assign all switch ports not in use to an inactive VLAN.

Step 1: Assign the disabled interfaces to an inactive VLAN.

SW3(config)#int range g0/0 – 9
SW3(config-if-range)# switchport access vlan 999

Step 2: Configure trunk links to not allow traffic from the inactive VLAN.

SW3(config)#int g1/1
SW3(config-if)#switchport trunk allowed vlan except 999

My first question is, how can I check that the switch has already been configured with trunk links to not allow traffic from the inactive VLAN?

second question - what would happen if I configured the port with the below command?

Step 1: Assign the disabled interfaces to an inactive VLAN.

SW3(config)#int range g0/0 – 9
SW3(config-if-range)# switchport access vlan 999

Step 2: Configure trunk links to not allow traffic from the inactive VLAN.

SW3(config)#int g1/1
SW3(config-if)#switchport trunk allowed vlan except 999

[Flavio Miranda]

@yaredo70 

"My first question is, how can I check that the switch has already been configured with trunk links to not allow traffic from the inactive VLAN?"

show interface trunk   - This command will list all the interface in trunk and the vlans on it.

second question - what would happen if I configured the port with the below command?

Step 1: Assign the disabled interfaces to an inactive VLAN.

SW3(config)#int range g0/0 – 9
SW3(config-if-range)# switchport access vlan 999

With this command you would add the interface from Gi0/0 to Gi0/9 to vlan 999

Step 2: Configure trunk links to not allow traffic from the inactive VLAN.

SW3(config)#int g1/1
SW3(config-if)#switchport trunk allowed vlan except 999

 You can also use the command  "switchport trunk remove vlan 999"