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Limit PPPoE PADI per specific time period

In ISP environment, some of the clients have their internet service disabled or somehow their modem is configured with wrong username and password. That will produce massive Accept-Rejects, because client modem continually try to request PPPOE connection.
That could cause high cpu consumption for Radius Servers and produce much log which burden troubleshooting effort.

Session throttle feature of Cisco IOS allows us to limit number of PPPoE PADI over period of time and help to prevent this situation.
How to use:
In the Ethernet environment:

       bba-group pppoe group1
            virtual-template 1
            sessions per-mac throttle 10 60 300

The above configuration cause BRAS not to answer PPPOE PADI if 10 request has passed within 60 seconds. The silent time will last for 300 seconds.

 

L2 Protocol Tunneling can cause loop

Turnning on l2protocol-tunnel cdp and vtp on the ports will cause loop in the network. Why?