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V-220640

CAT II (Medium)

The Cisco switch must have all trunk links enabled statically.

Rule ID

SV-220640r539671_rule

STIG

Cisco IOS Switch L2S Security Technical Implementation Guide

Version

V3R1

CCIs

CCI-000366

Discussion

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.

Check Content

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

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