The management VLAN is not pruned from any VLAN trunk links belonging to the managed network’s infrastructure.

From Infrastructure L3 Switch Secure Technical Implementation Guide - Cisco

Part of The management VLAN is not pruned from trunk links

SV-19340r1_rule The management VLAN is not pruned from any VLAN trunk links belonging to the managed network’s infrastructure.

Vulnerability discussion

The OOBM access switch will connect to the management interface of the managed network elements. The management interface can be a true OOBM interface or a standard interface functioning as the management interface. In either case, the management interface of the managed network element will be directly connected to the OOBM network.An OOBM interface does not forward transit traffic; thereby, providing complete separation of production and management traffic. Since all management traffic is immediately forwarded into the management network, it is not exposed to possible tampering. The separation also ensures that congestion or failures in the managed network do not affect the management of the device. If the device does not have an OOBM port, the interface functioning as the management interface must be configured so that management traffic does not leak into the managed network and that production traffic does not leak into the management network. ISL and 802.1q trunking enables multiple VLANs to traverse the same physical links between layer 2 switches or between a layer 2 switch and a router. If the management VLAN is not pruned from any VLAN trunk links belonging to the managed network’s infrastructure, management traffic has the potential to leak into the production network.

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The management VLAN must be pruned from any VLAN trunk links belonging to the managed network’s infrastructure. By default all the VLANs that exist on a switch are active on a trunk link. Since the switch is being managed via OOBM connection, management traffic should not traverse any trunk links. The following Catalyst IOS configuration is an example of a trunk link with the management VLAN (i.e. 10) pruned from a trunk. interface fastEthernet0/1 switchport trunk encapsulation dot1q switchport mode dynamic desirable switchport trunk native vlan 3 switchport trunk allowed vlan 2-9 This can also be verified with the show interface trunk command as shown below: Switch-A# show interface trunk Port Mode Encapsulation Status Native vlan Fa0/1 desirable 802.1q trunking 3 Port Vlans allowed on trunk Fa0/1 2-9 Port Vlans in spanning tree forwarding state and not pruned Fa0/1 2-5 Note: VTP pruning allows the switch to not forward user traffic for VLANs that are not active on a remote switch. This feature dynamically prunes unneeded traffic across trunk links. VTP pruning needs to be enabled on the server for the VTP domains—after which all VTP clients in the VTP domain will automatically enable VTP pruning. To enable VTP pruning on a Cisco IOS switch, you use the vtp pruning VLAN configuration or global configuration command. Since, the management VLAN will be active on all managed switchs, VTP will never prune this VLAN. Hence, it will have to be manually removed as shown above.

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Prune the management VLAN from any VLAN trunk links belonging to the managed network’s infrastructure.

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