In a Layer 2 environment, subscribers or customers connected to SAPs A or B can create a denial of service attack by sending packets sourcing the gateway MAC address. This moves the learned gateway MAC from the uplink SDP/SAP to the subscriber’s or customer’s SAP causing all communication to the gateway to be disrupted. If local content is attached to the same VPLS (D), a similar attack can be launched against it. Communication between subscribers or customers is also disallowed but split horizon is not sufficient in the topology shown in Figure: MAC learning protection.
The 7450 ESS, 7750 SR, and 7950 XRS routers enable MAC learning protection capability for SAPs and SDPs. With this mechanism, forwarding and learning rules apply to the non-protected SAPs. Assume hosts H1, H2, and H3 (Figure: MAC learning protection) are non-protected while IES interfaces G and H are protected. When a frame arrives at a protected SAP/SDP, the MAC is learned as usual. When a frame arrives from a non-protected SAP or SDP, the frame must be dropped if the source MAC address is protected and the MAC address is not relearned. The system allows only packets with a protected MAC destination address.
The system can be configured statically. The addresses of all protected MACs are configured. Only the IP address can be included and use a dynamic mechanism to resolve the MAC address (cpe-ping). All protected MACs in all VPLS instances in the network must be configured.
To eliminate the ability of a subscriber or customer to cause a DoS attack, the node restricts the learning of protected MAC addresses based on a statically defined list. Also, the destination MAC address is checked against the protected MAC list to verify that a packet entering a restricted SAP has a protected MAC as a destination.