This article demonstrates with examples the difference in output between legacy and Next-Generation Broadband Edge Junos OS releases, when traffic is captured for L2TP-enabled subscribers on the access-facing MX-LAC interface.

In legacy Junos OS releases, information beyond the Authentication phase is considered as data traffic from the MX-LAC perspective. Therefore, this information is not shown when traffic is captured directly on the interface. However, it is observed that capturing traffic up to the Network Control Protocol (NCP) phase on MX-LAC is helpful for checking functionality, and troubleshooting problems related to subscriber connectivity, which is possible only with the Next-Generation Broadband Edge software.

Legacy

In legacy Junos OS releases, when traffic is captured on the access interface by using the monitor traffic interface <interface-name> command on MX-LAC, or when tcpdump is captured from the shell directly on the access-facing interface while connecting the L2TP-enabled subscribers, only information up to the Authentication phase is available. The rest of the control traffic is tunneled and sent to the L2TP network server (LNS) for session establishment. It is not shown in the output.

Example: Capturing traffic information on the access-facing interface in legacy Junos OS Subscriber Management releases: ​

MX-BNG> monitor traffic interface xe-2/0/0
19:29:53.216694  In PPPoE  [ses 1]LCP, Conf-Request (0x01), id 1, length 16
19:29:53.216699  In PPPoE  [ses 1]LCP, Conf-Request (0x01), id 1, length 16
19:29:53.217345 Out PPPoE  [ses 1]LCP, Conf-Request (0x01), id 197, length 20
19:29:53.217466 Out PPPoE  [ses 1]LCP, Conf-Ack (0x02), id 1, length 16
19:29:53.218204  In PPPoE  [ses 1]LCP, Conf-Nack (0x03), id 197, length 11
19:29:53.218210  In PPPoE  [ses 1]LCP, Conf-Nack (0x03), id 197, length 11
19:29:53.218550 Out PPPoE  [ses 1]LCP, Conf-Request (0x01), id 198, length 21
19:29:53.219154  In PPPoE  [ses 1]LCP, Conf-Ack (0x02), id 198, length 21
19:29:53.219158  In PPPoE  [ses 1]LCP, Conf-Ack (0x02), id 198, length 21
19:29:53.219539 Out PPPoE  [ses 1]CHAP, Challenge (0x01), id 100, Value efea1218987c5e30f3f2e75d0231f329, Name JUNOS
19:29:53.220239  In PPPoE  [ses 1]CHAP, Response (0x02), id 100, Value 027fe1bfc5cb0489d52b651feaacf199, Name [email protected]

Next-Generation Broadband Edge

In the new Next-Generation Broadband Edge releases, MX-LAC is able to intercept and show control traffic for the L2TP-enabled subscribers up to the Network Control Phase, when traffic is captured directly on the access interface. 

Example: Capturing traffic on the access-facing interface in Junos OS Next-Generation Broadband Edge releases:

MX-NGBNG> monitor traffic interface xe-2/0/0
18:09:30.867164  In PPPoE  [ses 1]LCP, Conf-Request (0x01), id 1, length 16
18:09:30.867167  In PPPoE  [ses 1]LCP, Conf-Request (0x01), id 1, length 16
18:09:30.867748 Out PPPoE  [ses 1]LCP, Conf-Request (0x01), id 121, length 20
18:09:30.867874 Out PPPoE  [ses 1]LCP, Conf-Ack (0x02), id 1, length 16
18:09:30.868376  In PPPoE  [ses 1]LCP, Conf-Ack (0x02), id 121, length 20
18:09:30.868382  In PPPoE  [ses 1]PAP, Auth-Req (0x01), id 85, Peer [email protected], Name password
18:09:30.868385  In PPPoE  [ses 1]LCP, Conf-Ack (0x02), id 121, length 20
18:09:30.868388  In PPPoE  [ses 1]PAP, Auth-Req (0x01), id 85, Peer [email protected], Name password         
18:09:31.029474 Out PPPoE  [ses 1]PAP, Auth-ACK (0x02), id 85, Msg Login successful

*****NCP Output******

18:09:31.029565 Out PPPoE  [ses 1]IPCP, Conf-Request (0x01), id 0, length 12
18:09:31.032252  In PPPoE  [ses 1]IPCP, Conf-Request (0x01), id 1, length 12
18:09:31.032259  In PPPoE  [ses 1]IP6CP, Conf-Request (0x01), id 1, length 16
18:09:31.032265  In PPPoE  [ses 1]IPCP, Conf-Ack (0x02), id 0, length 12
18:09:31.032268  In PPPoE  [ses 1]IPCP, Conf-Request (0x01), id 1, length 12
18:09:31.032271  In PPPoE  [ses 1]IP6CP, Conf-Request (0x01), id 1, length 16                  
18:09:31.032276  In PPPoE  [ses 1]IPCP, Conf-Ack (0x02), id 0, length 12
18:09:31.033176 Out PPPoE  [ses 1]IPCP, Conf-Nack (0x03), id 1, length 12
18:09:31.033260 Out PPPoE  [ses 1]LCP, Prot-Reject (0x08), id 0, length 22
18:09:31.037213  In PPPoE  [ses 1]IPCP, Conf-Request (0x01), id 2, length 12
18:09:31.037217  In PPPoE  [ses 1]IPCP, Conf-Request (0x01), id 2, length 12
18:09:31.037930 Out PPPoE  [ses 1]IPCP, Conf-Ack (0x02), id 2, length 12