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[SRX] VDSL link does not come up with some ISPs

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Article ID: KB28670 KB Last Updated: 25 Jan 2014Version: 1.0
Summary:

Some Branch SRX devices have the capability to support a VDSL link. However, it is reported that the VDSL link will not train up with some ISPs, while an ADSL link on the same card will work without any problems with the same ISPs.

Symptoms:

Note: This article deals with a specific issue related to the implementation of VDSL on Branch SRX devices. For general VDSL related information and for configuration examples, please use this link.

At the time of writing of this article, the following Branch SRX platforms have either the capability to support a VDSL mini-PIM or have an integrated VDSL card:

  • SRX110 - Integrated ADSL/VDSL card
  • SRX210, SRX220 and SRX240 - Support the VDSL mini-PIM

It is observed that the VDSL link fails to come up with some ISPs while an ADSL link trains up without any issues. It is also noted that other vendor VDSL modems train up successfully with the same ISPs. On such devices, when the monitor traffic command is executed on the logical VDSL interface, i.e., the pt interface, it seems like the PPPoE negotiation commences but the LCP packets never make it to the ISP:


root# run monitor traffic interface pt-2/0/0
verbose output suppressed, use <detail> or <extensive> for full protocol decode
Address resolution is ON. Use <no-resolve> to avoid any reverse lookup delay.
Address resolution timeout is 4s.
Listening on pt-2/0/0, capture size 96 bytes

10:29:53.074007 Out PPPoE [ses 4230]LCP, Conf-Request (0x01), id 68, length 16
10:29:53.618794 In PPPoE [ses 4230]LCP, Conf-Request (0x01), id 36, length 20
10:29:53.619949 Out PPPoE [ses 4230]LCP, Conf-Ack (0x02), id 36, length 20
10:29:55.963432 Out PPPoE [ses 4230]LCP, Conf-Request (0x01), id 69, length 16
10:29:56.718290 In PPPoE [ses 4230]LCP, Conf-Request (0x01), id 36, length 20
10:29:56.719528 Out PPPoE [ses 4230]LCP, Conf-Ack (0x02), id 36, length 20
10:29:58.939040 Out PPPoE [ses 4230]LCP, Conf-Request (0x01), id 70, length 16
10:29:59.819201 In PPPoE [ses 4230]LCP, Conf-Request (0x01), id 36, length 20
10:29:59.820439 Out PPPoE [ses 4230]LCP, Conf-Ack (0x02), id 36, length 20
10:30:01.848902 Out PPPoE [ses 4230]LCP, Conf-Request (0x01), id 71, length 16
10:30:02.909497 In PPPoE [ses 4230]LCP, Conf-Request (0x01), id 36, length 20
10:30:02.910846 Out PPPoE [ses 4230]LCP, Conf-Ack (0x02), id 36, length 20
10:30:04.782562 Out PPPoE [ses 4230]LCP, Conf-Request (0x01), id 72, length 16

From this, it can be seen that the SRX sends a Conf-Request packet but the ISP does not seem to receive the packet and responds with a Conf-Request as well.  The SRX then sends a Conf-Ack to the Conf-Request by the ISP but there is no response from the ISP.

This behavior repeats until the PPPoE negotiation times out, restarts and loops through the same steps.

Cause:
According to Junos OS' implementation of VDSL on the SRX platforms, the IEEE802.1p field in the VLAN header of the LCP packets is set to a priority of 6 (internetwork-control).  However, not all ISPs accept high-priority LCP packets and drop them at the edge of their network. This is why the LCP packets seem to never make it to the ISP.

When running the monitor traffic command on the pt interface with the extensive keyword, the differences in the priority bits can be easily seen:


10:44:46.520712 In  << Incoming LCP packet from the ISP
         Juniper PCAP Flags [Ext, In], PCAP Extension(s) total length 16
          Device Media Type Extension TLV #3, length 1, value: Ethernet (1)
          Logical Interface Encapsulation Extension TLV #6, length 1, value: PPPoE over Ethernet (62)
          Device Interface Index Extension TLV #1, length 2, value: 38144
          Logical Interface Index Extension TLV #4, length 4, value: 74
          -----original packet-----
          00:30:88:15:f8:fd > 64:87:88:52:98:d4, ethertype 802.1Q (0x8100), length 60: vlan 600, p 0, ethertype PPPoE S, PPPoE [ses 6600]LCP (0xc021), length 20: LCP, Conf-Request (0x01), id 156, length 20 
<< Priority 0 (best-effort)

10:44:46.345574
Out   <<< Outgoing LCP packet from the SRX
         Juniper PCAP Flags [Ext], PCAP Extension(s) total length 16
          Device Media Type Extension TLV #3, length 1, value: Ethernet (1)
          Logical Interface Encapsulation Extension TLV #6, length 1, value: PPPoE over Ethernet (62)
          Device Interface Index Extension TLV #1, length 2, value: 38144
          Logical Interface Index Extension TLV #4, length 4, value: 74
          -----original packet-----
          64:87:88:52:98:d4 > 00:30:88:15:f8:fd, ethertype 802.1Q (0x8100), length 40: vlan 600, p 6, ethertype PPPoE S, PPPoE [ses 6600]LCP (0xc021), length 16: LCP, Conf-Request (0x01), id 11, length 16    
<< Priority 6 (internetwork-control)

Solution:

When faced with a similar issue, run the following command from operational mode:

monitor traffic interface pt-<fpc/pic/port> extensive

If the outgoing LCP packets show the priority-bit set to 6, then configure the following under edit mode and commit:

set class-of-service host-outbound-traffic ieee-802.1 default be

The above command should enable the PPPoE negotiations to complete successfully and the VDSL link to come up.


Note: This command is only available in versions 12.1R5, 12.1X44-D26, 12.1X45-D20 and 12.1X46-D10 and later.

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