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[ScreenOS] 'PPP on interface serial1/0 finds possible loopback' reported in event log

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Article ID: KB21130 KB Last Updated: 11 Sep 2019Version: 2.0
Summary:

This article describes the relevancy of the following message:

PPP on interface serial1/0 finds possible loopback

Symptoms:

Environment:

Serial interface with PPP encapsulation(T3 link) configuration:

set ppp profile "test"
set ppp profile "test" static-ip
set ppp profile "test" auth type pap
set ppp profile "test" auth local-name "netscreen"
set ppp profile "test" auth secret "netscreen"
set interface "serial1/0" ppp profile test
set interface "serial1/0" encap ppp
set interface serial1/0 ip 10.1.1.1/30
set interface serial1/0 hold-time up 10
set interface serial1/0 t3-options fcs 32
set interface serial1/0 t3-options compatibility-mode digital-link subrate 44.2Mb
set interface serial1/0 t3-options loopback mode local

The following message is displayed in get event:

2011-05-24 19:47:07 system notif 00572 PPP on interface serial1/0 finds possible loopback
 
Solution:
Customer enabled loopback testing method as local to check the circuit. With this, the packets transmitted by the firewall are looped to the firewall itself. The loopback test was successful and that is the reason that the above alert messages are reported in the event log.

Loopback tests allow you to isolate pieces of the circuit, and test them separately. Begin loopback tests on the customer premises with channel service unit/data service unit (CSU/DSU) loopback tests. Then proceed on to loopback tests that involve the telco or provider. Loopback tests prove that the router equipment, CSU/DSU, and connecting cables are not faulty, conduct further tests with the telco or provider of the circuit. You can configure loopback capability between the local T3 interface and the remote CSU. You can configure the loopback to be local or remote.This applies to all WAN/serial circuits T3/E3 (DS3), T1, E1.

With local loopback, the interface can transmit packets to the CSU, but receives its own transmission back again and ignores data from the CSU. This is configured with the following command:
set/unset interface serial1/0 t3-options loopback mode local
With remote loopback, packets sent from the CSU are received by the T3 interface, forwarded if there is a valid route, and immediately retransmitted to the CSU.  This is configured with the following command:
set/unset interface serial1/0 t3-options loopback mode remote
Modification History:

2019-09-10: Article reviewed for accuracy. No changes made. Article is correct and complete

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