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Cisco 10000 Series Router Software Configuration Guide
OL-2226-23
Chapter 5 Configuring the Layer 2 Tunnel Protocol Access Concentrator and Network Server
Layer 2 Access Concentrator
Session Load Balancing
The session load balancing feature enables the LAC to direct sessions across multiple LNS devices. The
LAC retrieves L2TP tunnel (VPDN) information from local configuration or a RADIUS server. Both
configuration methods support load balancing, but using RADIUS is more scalable than the local
method. When you enable the session load balancing feature using RADIUS, the server sends L2TP
tunnel information using multiple Tunnel-Server-Endpoint attributes in one tagged attribute group.
Multiple instances of the initiate-to ip command in VPDN group configuration mode configures the
session load balancing feature locally. For information on the command syntax, see the
“Sessions per
Tunnel Limiting” section on page 5-5.
When you enable the session load balancing feature, the LAC uses a priority or round-robin load
balancing algorithm to forward PPP sessions destined to the same domain among multiple tunnels.
Note Load balancing occurs with respect to the load a particular LAC generates. The LAC is not aware of the
true load on a set of LNS devices. The true load on the LNS devices is an aggregation of all LAC devices
using the LNS devices.
Session Load Failover
The session load failover feature works with the session load balancing feature to enable the LAC to
direct sessions across multiple LNS devices. If the primary set of LNS devices fails, the session load
failover feature enables the LAC to direct sessions to a set of failover LNS devices. The LAC uses the
failover LNS devices only if all of the primary set of devices are unavailable. Failover occurs if the LAC:
• Sends an excessive number of Start-Control-Connection-Requests (SCCRQs) (no response from
peer)
• Receives a Stop Control Channel (StopCNN) message from its peer during tunnel establishment
• Receives a Call-Disconnect-Notify (CDN) message during session establishment
• Receives vendor-specific attributes (VSAs) or standard RADIUS AV pairs indicating failover
The RADIUS Tunnel-Preference attribute is used to form load failover groups. When the values of the
Tunnel-Preference attributes for different tagged attribute groups are the same, the
Tunnel-Server-Endpoint for each of those attribute groups has the same failover priority.
Feature History for LAC
Cisco IOS Release Description Required PRE
12.2(15)BX This feature was introduced on the Cisco 10000 series
router.
PRE2
12.3(7)XI1 This feature was integrated into Cisco IOS Release
12.3(7)XI1.
PRE2
12.2(28)SB This feature was integrated into Cisco IOS Release
12.2(28)SB.
PRE2