Description

This KB explains the Junos expected behavior during an A10-NSP interface flap.

Solution

Diagram: 

       +                                                           |
       |                                                           |
       |                      Subscriber                A10-NSP    |
 +---+ |    +---------+       Access     +-------------+Interface  |           +--------------+
 |   +-|----+         |------------------+ MX BNG      +-----------|-----------+ 3rd Party    |
 |CPE| |    |DSLAM    |       Interface  |             |           |           | BNG          |
 +---+ |    +---------+                  +-------------+           |           +--------------+
       |                                                           |
       |                                                           |
       |          Wholesale Service Provider Network               |

Behavior:

Failure of an A10-NSP interface (core-facing) in Layer 2 Bit Stream Access (L2BSA ) routing instance initiates a logout of all the wholesale subscriber sessions attached to that particular interface. This event is treated as logical Access Node Control Protocol (ANCP) Port Down event in Junos.  The affected L2BSA subscriber sessions are subject to logout; which includes issuance of RADIUS Accounting-Stop with final statistics for associated service and client sessions, and removing dynamic VLAN IFLs (logical interfaces).  

After subscriber access-lines are logged out​, Junos keeps them in the pending state until the core-facing interface comes up or any new core-facing interface is added to the routing instance. The CLI command ' show auto-configuration out-of-band pending ' reports the subscribers in this state. There is no timeout value for keeping subscriber access-lines in this state.

When the core-facing interface comes up again, sessions in the pending state are required to go through the initial login sequence; including authentication, dynamic VLAN IFL creation, service activation(s), and RADIUS Accounting-Start messages. ​Junos treats this event as a logical ANCP Port UP event for a subscriber access-line.
 

Recommendation:

In order to reduce the probability of an L2BSA subscriber logout due to core IFD (physical interface device) failures, it is recommended to use an aggregate Ethernet interface as a A10-NSP with multiple member interfaces on different FPCs (Flexible PIC Concentrators).