Cisco Systems 1000 Series Manual De Usuario

For IP sessions, the keepalives are not enabled by default. Enabling keepalives for IP sessions provides 
the same capability as PPP keepalives except that ICMP or ARP is used to test the presence of 
subscribers. For more information about Using ARP for Keepalive Messages and Using ICMP for 
Keepalive Messages, see the feature documentation at: 

To prevent head-of-the-line blocking of the IP input process and save system resources, configure the 
vpdn ip udp ignore checksum command:

Router(config)# vpdn ip udp ignore checksum

When you configure this command, the router directly queues the L2TP Hello packets and Hello 
acknowledgements to the L2TP control process. We recommend that you configure this command in all 
the scaled LAC and LNS L2TP tunnel configurations.

If you do not configure the vpdn ip udp ignore checksum command, the L2TP software sends the 
packets to UDP to validate the checksum. When too many packets are queued to the IP input process, 
the router starts Selective Packet Discard (SPD) mechanism that causes IP packets to be dropped. 

Head-of-the-line blocking of the IP input process might occur in other nonL2TP configurations. A flush 
occurring on an input interface indicates that the SPD mechanism is discarding packets.

When you use the lcp:interface-config RADIUS attribute to reconfigure the virtual access subscriber 
interface, call setup rate could be reduced on the Cisco ASR 1000 Series Aggregation Services Routers 
because the lcp:interface-config command syntax includes an IOS interface configuration command. 
This command is any valid IOS command that can be applied to an interface. When the 
lcp:interface-config attribute is downloaded from the RADIUS server to the Cisco ASR 1000 Series 
Aggregation Services Routers, the command parser is activated to configure the interface according to 
AV-pair, determining if the option is valid and then applying the configuration to the virtual access 
interface (VAI).

The subscriber session scaling on the Cisco ASR 1000 Series Aggregation Services Routers is not 
impacted by using the lcp:interface-config RADIUS attribute any more than if the equivalent IOS 
interface command was applied directly onto the virtual-template configuration and was cloned onto the 
VAI using that method. Using either the lcp:interface-config RADIUS attribute or the virtual-template 
to apply configuration onto the VAI it is the type of configuration being applied which may in a few cases 
affect the maximum subscriber session scale of the Cisco ASR 1000 Series Aggregation Services 
Routers.