Cisco 10000 - Manual (Page 79)

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Cisco 10000 Series Router Software Configuration Guide
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Chapter 2 Scalability and Performance
Configuring the Cisco 10000 Series Router for High Scalability
Placing PPPoA Sessions in Listening Mode
For better scalability and faster convergence of PPPoA, PPPoEoA, or LAC sessions, set sessions to
passive mode, using the atm pppatm passive command in ATM subinterface configuration mode. This
command places PPP or L2TP sessions on an ATM subinterface into listening mode. For large-scale
PPP terminated aggregation (PPPoA and PPPoEoA) and L2TP (LAC), the atm pppatm passive
command is required.
Instead of sending out Link Control Protocol (LCP) packets to establish the sessions actively, the
sessions listen to the incoming LCP packets and become active only after they receive their first
LCP packet. When PPPoX is in passive mode, the LAC brings up the sessions only when the subscribers
become active and does not waste processing power polling all the sessions.
The following example configures passive mode for the PPPoA sessions on an ATM multipoint
subinterface:
Router(config)# interface atm 1/0.1 multipoint
Router(config-subif)# atm pppatm passive
Router(config-subif)# range range-pppoa-1 pvc 100 199
Router(config-subif-atm-range)# encapsulation aal5mux ppp virtual-template 1
Scaling L2TP Tunnel Configurations
To prevent head-of-the-line blocking of the IP input process and save system resources, configure the
following command in global configuration mode:
Router(config)# vpdn ip udp ignore checksum
When you configure this command, the router directly queues L2TP Hello packets and Hello
acknowledgements to the L2TP control process. We recommend that you configure this command in all
scaled LAC and LNS L2TP tunnel configurations.
If you do not configure the vpdn ip udp ignore checksum command, the L2TP software sends the
packet to UDP to validate the checksum. When too many packets are queued to the IP input process, the
router starts selective packet discard (SPD), which causes IP packets to be dropped.
Note Head-of-the-line blocking of the IP input process might occur in other non-L2TP configurations. A flush
occurring on an input interface indicates that SPD is discarding packets.