The Cisco switch must have all trunk links enabled statically.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-220640	CISC-L2-000200	SV-220640r539671_rule		Medium

Description
When trunk negotiation is enabled via Dynamic Trunk Protocol (DTP), considerable time can be spent negotiating trunk settings (802.1q or ISL) when a node or interface is restored. While this negotiation is happening, traffic is dropped because the link is up from a Layer 2 perspective. Packet loss can be eliminated by setting the interface statically to trunk mode, thereby avoiding dynamic trunk protocol negotiation and significantly reducing any outage when restoring a failed link or switch.

Description

When trunk negotiation is enabled via Dynamic Trunk Protocol (DTP), considerable time can be spent negotiating trunk settings (802.1q or ISL) when a node or interface is restored. While this negotiation is happening, traffic is dropped because the link is up from a Layer 2 perspective. Packet loss can be eliminated by setting the interface statically to trunk mode, thereby avoiding dynamic trunk protocol negotiation and significantly reducing any outage when restoring a failed link or switch.

STIG	Date
Cisco IOS Switch L2S Security Technical Implementation Guide	2022-09-12

Details

Check Text ( C-22355r507966_chk )
By default, DTP is enabled on all Cisco switches. Review the switch configuration to verify that trunk links will not form a trunk via negotiation as shown in the example below: SW2#show interfaces switchport Name: Gi0/0 Switchport: Enabled Administrative Mode: dynamic auto Operational Mode: static access Administrative Trunking Encapsulation: negotiate Operational Trunking Encapsulation: native Negotiation of Trunking: On If trunk negotiation is enabled on any interface, this is a finding.

Check Text ( C-22355r507966_chk )

By default, DTP is enabled on all Cisco switches. Review the switch configuration to verify that trunk links will not form a trunk via negotiation as shown in the example below:

SW2#show interfaces switchport
Name: Gi0/0
Switchport: Enabled
Administrative Mode: dynamic auto
Operational Mode: static access
Administrative Trunking Encapsulation: negotiate
Operational Trunking Encapsulation: native
Negotiation of Trunking: On

If trunk negotiation is enabled on any interface, this is a finding.

Fix Text (F-22344r507967_fix)
Configure the switch to enable trunk links statically as shown in the configuration below: SW2(config-if)#switchport trunk encapsulation dot1q SW2(config-if)#switchport mode trunk SW2(config-if)#switchport nonegotiate