The Cisco switch must have all trunk links enabled statically.
Severity | Group ID | Group Title | Version | Rule ID | Date | STIG Version |
|---|---|---|---|---|---|---|
| medium | V-220640 | SRG-NET-000512-L2S-000005 | CISC-L2-000200 | SV-220640r539671_rule | 2024-06-06 | 3 |
| 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. |
| ℹ️ Check |
|---|
| 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 |
|---|
| 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 |