Layer 2 PDU Rate Limiter
Available only on certain switches, such as the Supervisor Engineer 720 for the Catalyst 6500, a third option to stop the DoS from causing damage exists. It takes the form of a hardware-based Layer 2 PDU rate limiter. It limits the number of Layer 2 PDUs (BPDUs, DTP, Port Aggregation Protocol [PAgP], CDP, VTP frames) destined for the supervisor engine's processor. The feature works only on Catalyst 6500/7600 that are not operating in truncated mode. The switch uses truncated mode for traffic between fabric-enabled modules when both fabric-enabled and nonfabric-enabled modules are installed. In this mode, the router sends a truncated version of the traffic (the first 64 bytes of the frame) over the switching fabric. (For more information about the various modes of operation of the Catalyst 6500 switch, see the third entry in the section, "References.") The Layer 2 PDU rate limiter is configured as follows:
Router(config)# mls rate-limit layer2 pdu 200 20 ^ 200 L2 PDUs per second, burst of
20 packets
Fine-tuning the rate limiter can be time consuming and error prone, because it is global to the switch and applicable to traffic received across all VLANs for various Layer 2 protocols. However, it can be safely enabled with a fairly high threshold. As a rough guideline, 2000 PDUs per second is a high watermark figure for an enterprise class switch. (The rate limiter prevents only a DoS attack. It does not stop the other attacks described in this chapter [root hostile takeover, and so on].)
Continue reading here: IEEE 8021Q Overview
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