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V-101147

The Cisco switch must have all trunk links enabled statically.

Finding ID
CISC-L2-000200
Rule ID
SV-110251r1_rule
Severity
Cat II
CCE
(None)
Group Title
SRG-NET-000512-L2S-000005
CCI
CCI-000366
Target Key
(None)
Documentable
No
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.

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

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.