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L2TP
Forwarding Controller
Copyright © 2010, Juniper Networks, Inc.
The total time required for the standby module to become active is dependent on the
size of the configuration parameters. On a normal basis, it takes about 2-3 minutes for
the new primary module to become active, in which case, clients running small intervals
of keepalives expire. This system of expiry of keepalives poses a limitation on the stateful
switchover model. This limitation is similar to the restriction seen during the upgrade
phase of the unified ISSU process in which traffic forwarding is interrupted for a brief
period. To work around this restriction, echo requests for the sessions that terminate on
the failed line module are redirected to a different hardware. For failures on tunnel server
modules (ES2 4G LMs with Service IOA), the access module handles such problems.
L2TP configuration and operation data are maintained in the line module and this
information is mirrored to the standby module. After the switchover of the primary tunnel
server module to the secondary module occurs, the L2TP application on the line module
restores the configuration and operation data to the newly active primary module. This
mechanism is similar to the warm start procedure during unified ISSU. The L2TP
application on the SRP module handles the line module events related to the primary
and secondary modules.
When a stateful line module switchover occurs, the forwarding controller (FC) tables
that refer to the failed line module are updated with stream IDs that map to the line
module (ES2 4G LM with Service IOA) that has taken over the role of the primary module.
FC tables use a combination of slot ID, stream ID, and key hash table. The modifications
to the FC tables enables packets to be sent to the newly functioning primary module
after the switchover is complete.
During the stateful line module switchover, PPP subscriber sessions on an LNS device in
an L2TP tunnel might be terminated due to the lack of PPP keepalive responses from
the LNS device. To prevent the termination of subscriber sessions, the access module in
the LNS device handles the PPP echo requests from all active subscriber sessions (on
behalf of the failed line module) and responds with valid PPP echo reply messages. After
a successful switchover, the access module in the LNS stops responding to the PPP echo
request messages.
When the access module in the LNS receives an event from the application, such as PPP,
to denote a failure with the primary line module, the access module starts processing
the PPP echo requests that are destined for the LNS. The access module in the LNS
concludes the handling of PPP echo requests after it receives a notification that the
switchover is complete.
The following configuration events also take place during a stateful switchover on tunnel
server modules that are installed on E120 and E320 routers that operate as LNS devices
in an L2TP tunnel:
All possible next hop attributes, which signify the IP address of the node that is closer
to the advertised prefix (such as MPLS and ATM sessions), at the LNS are supported.
PPP keepalive messages are not considered for the session statistics calculated during
stateful switchover.
Chapter 4: Managing Stateful Line Module Switchover
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