L2Tp Reassembly On Lac - Cisco ASR 9000 Series Configuration Manual

Establishing Subscriber Sessions
• Preventing creation of additional VPDN sessions. See,
This is a sample user-profile for L2TP LAC:
abc_xyz@domain.com Password="abc"
Service-Type = Outbound-User,
Tunnel-Type = L2TP,
Tunnel-Medium-Type = IP,
Cisco-avpair = "vpdn:ip-addresses=3.3.3.3",
Cisco-avpair = "vpdn:source-ip=1.1.1.1"
Note For L2TP LAC session to be up, the user-profile coming from the RADIUS server to the BNG must have
Service-Type = Outbound-User configured for the user.
Restriction
Provisioning PPP LAC session is subjected to a restriction that only ASR 9000 Enhanced Ethernet Line Cards
are supported as core facing line cards.

L2TP Reassembly on LAC

The L2TP Reassembly feature on L2TP Access Concentrator (LAC) ensures reassembly of fragmented L2TP
data packets in the intervening network, between the LAC and L2TP Network Server (LNS). Data packets
are fragmented when they exceed the Maximum Transmission Unit (MTU) of the IPv4 core. Enabling this
feature prevents the fragmented packets from getting dropped and ensures the subsequent forwarding of these
data packets.
When L2TP Reassembly feature is disabled on LAC, fragmented data packets are dropped. The feature does
not affect the reassembly of non-L2TP packets. To ensure that packets for non-L2TP applications are properly
reassembled regardless of whether load balancing occurs for each packet, it is recommended that:
• A separate loopback address be configured only for L2TP traffic. No other applications on the router
should use this IP address.
• Multiple loopback addresses be used for L2TP, but no other applications across all VRFs should use
these addresses.
In cases of reassembly errors or fragmentation timeout, the maximum period a traffic flow is kept, before it
is forwarded to the Route Switch Processor (RSP) is 250ms.
Restrictions
Enabling L2TP reassembly feature is subjected to these restrictions:
• Only ASR 9000 Enhanced Ethernet Line Cards support L2TP reassembly feature
• Only IPv4 fragmented packets are reassembled
• Only packets with two fragments are reassembled
• The fragments must not overlap
• The fragmented IP headers must not contain options
OL-28375-03
Cisco ASR 9000 Series Aggregation Services Router Broadband Network Gateway Configuration Guide, Release
Provisioning PPP LAC Session
Configuring Softshut for VPDN, on page 116.
4.3.x
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