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Troubleshooting the Link Services Interface

 

To solve configuration problems on a link services interface:

Problem

Description: You are configuring a multilink bundle, but you also have traffic without MLPPP encapsulation passing through constituent links of the multilink bundle. Do you apply all CoS components to the constituent links, or is applying them to the multilink bundle enough?

Solution

You can apply a scheduler map to the multilink bundle and its constituent links. Although you can apply several CoS components with the scheduler map, configure only the ones that are required. We recommend that you keep the configuration on the constituent links simple to avoid unnecessary delay in transmission.

Table 1 shows the CoS components to be applied on a multilink bundle and its constituent links.

Cos Component

Multilink Bundle

Constituent Links

Explanation

Classifier

Yes

No

CoS classification takes place on the incoming side of the interface, not on the transmitting side, so no classifiers are needed on constituent links.

Forwarding class

Yes

No

Forwarding class is associated with a queue, and the queue is applied to the interface by a scheduler map. The queue assignment is predetermined on the constituent links. All packets from Q2 of the multilink bundle are assigned to Q2 of the constituent link, and packets from all the other queues are queued to Q0 of the constituent link.

Scheduler map

Yes

Yes

Apply scheduler maps on the multilink bundle and the constituent link as follows:

  • Transmit rate—Make sure that the relative order of the transmit rate configured on Q0 and Q2 is the same on the constituent links as on the multilink bundle.

  • Scheduler priority—Make sure that the relative order of the scheduler priority configured on Q0 and Q2 is the same on the constituent links as on the multilink bundle.

  • Buffer size—Because all non-LFI packets from the multilink bundle transit on Q0 of the constituent links, make sure that the buffer size on Q0 of the constituent links is large enough.

  • RED drop profile—Configure a RED drop profile on the multilink bundle only. Configuring the RED drop profile on the constituent links applies a back pressure mechanism that changes the buffer size and introduces variation. Because this behavior might cause fragment drops on the constituent links, make sure to leave the RED drop profile at the default settings on the constituent links.

Shaping rate for a per-unit scheduler or an interface-level scheduler

No

Yes

Because per-unit scheduling is applied only at the end point, apply this shaping rate to the constituent links only. Any configuration applied earlier is overwritten by the constituent link configuration.

Transmit-rate exact or queue-level shaping

Yes

No

The interface-level shaping applied on the constituent links overrides any shaping on the queue. Thus apply transmit-rate exact shaping on the multilink bundle only.

Rewrite rules

Yes

No

Rewrite bits are copied from the packet into the fragments automatically during fragmentation. Thus what you configure on the multilink bundle is carried on the fragments to the constituent links.

Virtual channel group

Yes

No

Virtual channel groups are identified through firewall filter rules that are applied on packets only before the multilink bundle. Thus you do not need to apply the virtual channel group configuration to the constituent links.

Problem

Description: To test jitter and latency, you send three streams of IP packets. All packets have the same IP precedence settings. After configuring LFI and CRTP, the latency increased even over a noncongested link. How can you reduce jitter and latency?

Solution

To reduce jitter and latency, do the following:

  1. Make sure that you have configured a shaping rate on each constituent link.
  2. Make sure that you have not configured a shaping rate on the link services interface.
  3. Make sure that the configured shaping rate value is equal to the physical interface bandwidth.
  4. If shaping rates are configured correctly, and jitter still persists, contact the Juniper Networks Technical Assistance Center (JTAC).

Determine If LFI and Load Balancing Are Working Correctly

Problem

Description: In this case, you have a single network that supports multiple services. The network transmits data and delay-sensitive voice traffic. After configuring MLPPP and LFI, make sure that voice packets are transmitted across the network with very little delay and jitter. How can you find out if voice packets are being treated as LFI packets and load balancing is performed correctly?

Solution

When LFI is enabled, data (non-LFI) packets are encapsulated with an MLPPP header and fragmented to packets of a specified size. The delay-sensitive, voice (LFI) packets are PPP-encapsulated and interleaved between data packet fragments. Queuing and load balancing are performed differently for LFI and non-LFI packets.

To verify that LFI is performed correctly, determine that packets are fragmented and encapsulated as configured. After you know whether a packet is treated as an LFI packet or a non-LFI packet, you can confirm whether the load balancing is performed correctly.

Solution Scenario—Suppose two Juniper Networks devices, R0 and R1, are connected by a multilink bundle lsq-0/0/0.0 that aggregates two serial links, se-1/0/0 and se-1/0/1. On R0 and R1, MLPPP and LFI are enabled on the link services interface and the fragmentation threshold is set to 128 bytes.

In this example, we used a packet generator to generate voice and data streams. You can use the packet capture feature to capture and analyze the packets on the incoming interface.

The following two data streams were sent on the multilink bundle:

  • 100 data packets of 200 bytes (larger than the fragmentation threshold)

  • 500 data packets of 60 bytes (smaller than the fragmentation threshold)

The following two voice streams were sent on the multilink bundle:

  • 100 voice packets of 200 bytes from source port 100

  • 300 voice packets of 200 bytes from source port 200

To confirm that LFI and load balancing are performed correctly:

Note

Only the significant portions of command output are displayed and described in this example.

  1. Verify packet fragmentation. From operational mode, enter the show interfaces lsq-0/0/0 command to check that large packets are fragmented correctly.
    user@R0#> show interfaces lsq-0/0/0

    Meaning—The output shows a summary of packets transiting the device on the multilink bundle. Verify the following information on the multilink bundle:

    • The total number of transiting packets = 1000

    • The total number of transiting fragments=1100

    • The number of data packets that were fragmented =100

    The total number of packets sent (600 + 400) on the multilink bundle match the number of transiting packets (1000), indicating that no packets were dropped.

    The number of transiting fragments exceeds the number of transiting packets by 100, indicating that 100 large data packets were correctly fragmented.

    Corrective Action—If the packets are not fragmented correctly, check your fragmentation threshold configuration. Packets smaller than the specified fragmentation threshold are not fragmented.

  2. Verify packet encapsulation. To find out whether a packet is treated as an LFI or non-LFI packet, determine its encapsulation type. LFI packets are PPP encapsulated, and non-LFI packets are encapsulated with both PPP and MLPPP. PPP and MLPPP encapsulations have different overheads resulting in different-sized packets. You can compare packet sizes to determine the encapsulation type.

    A small unfragmented data packet contains a PPP header and a single MLPPP header. In a large fragmented data packet, the first fragment contains a PPP header and an MLPPP header, but the consecutive fragments contain only an MLPPP header.

    PPP and MLPPP encapsulations add the following number of bytes to a packet:

    • PPP encapsulation adds 7 bytes:

      4 bytes of header+2 bytes of frame check sequence (FCS)+1 byte that is idle or contains a flag

    • MLPPP encapsulation adds between 6 and 8 bytes:

      4 bytes of PPP header+2 to 4 bytes of multilink header

    Figure 1 shows the overhead added to PPP and MLPPP headers.

    Figure 1: PPP and MLPPP Headers
    PPP and MLPPP Headers

    For CRTP packets, the encapsulation overhead and packet size are even smaller than for an LFI packet. For more information, see Example: Configuring the Compressed Real-Time Transport Protocol.

    Table 2 shows the encapsulation overhead for a data packet and a voice packet of 70 bytes each. After encapsulation, the size of the data packet is larger than the size of the voice packet.

    Table 2: PPP and MLPPP Encapsulation Overhead

    Packet Type

    Encapsulation

    Initial Packet Size

    Encapsulation Overhead

    Packet Size after Encapsulation

    Voice packet (LFI)

    PPP

    70 bytes

    4 + 2 + 1 = 7 bytes

    77 bytes

    Data fragment (non-LFI) with short sequence

    MLPPP

    70 bytes

    4 + 2 + 1 + 4 + 2 = 13 bytes

    83 bytes

    Data fragment (non-LFI) with long sequence

    MLPPP

    70 bytes

    4 + 2 + 1 + 4 + 4 = 15 bytes

    85 bytes

    From operational mode, enter the show interfaces queue command to display the size of transmitted packet on each queue. Divide the number of bytes transmitted by the number of packets to obtain the size of the packets and determine the encapsulation type.

  3. Verify load balancing. From operational mode, enter the show interfaces queue command on the multilink bundle and its constituent links to confirm whether load balancing is performed accordingly on the packets.
    user@R0> show interfaces queue lsq-0/0/0
    user@R0> show interfaces queue se-1/0/0
    user@R0> show interfaces queue se-1/0/1

    Meaning—The output from these commands shows the packets transmitted and queued on each queue of the link services interface and its constituent links. Table 3 shows a summary of these values. (Because the number of transmitted packets equaled the number of queued packets on all the links, this table shows only the queued packets.)

    Table 3: Number of Packets Transmitted on a Queue

    Packets Queued

    Bundle lsq-0/0/0.0

    Constituent Link se-1/0/0

    Constituent Link se-1/0/1

    Explanation

    Packets on Q0

    600

    350

    350

    The total number of packets transiting the constituent links (350+350 = 700) exceeded the number of packets queued (600) on the multilink bundle.

    Packets on Q2

    400

    100

    300

    The total number of packets transiting the constituent links equaled the number of packets on the bundle.

    Packets on Q3

    0

    19

    18

    The packets transiting Q3 of the constituent links are for keepalive messages exchanged between constituent links. Thus no packets were counted on Q3 of the bundle.

    On the multilink bundle, verify the following:

    • The number of packets queued matches the number transmitted. If the numbers match, no packets were dropped. If more packets were queued than were transmitted, packets were dropped because the buffer was too small. The buffer size on the constituent links controls congestion at the output stage. To correct this problem, increase the buffer size on the constituent links.

    • The number of packets transiting Q0 (600) matches the number of large and small data packets received (100+500) on the multilink bundle. If the numbers match, all data packets correctly transited Q0.

    • The number of packets transiting Q2 on the multilink bundle (400) matches the number of voice packets received on the multilink bundle. If the numbers match, all voice LFI packets correctly transited Q2.

    On the constituent links, verify the following:

    • The total number of packets transiting Q0 (350+350) matches the number of data packets and data fragments (500+200). If the numbers match, all the data packets after fragmentation correctly transited Q0 of the constituent links.

      Packets transited both constituent links, indicating that load balancing was correctly performed on non-LFI packets.

    • The total number of packets transiting Q2 (300+100) on constituent links matches the number of voice packets received (400) on the multilink bundle. If the numbers match, all voice LFI packets correctly transited Q2.

      LFI packets from source port 100 transited se-1/0/0, and LFI packets from source port 200 transited se-1/0/1. Thus all LFI (Q2) packets were hashed based on the source port and correctly transited both constituent links.

    Corrective Action—If the packets transited only one link, take the following steps to resolve the problem:

    1. Determine whether the physical link is up (operational) or down (unavailable). An unavailable link indicates a problem with the PIM, interface port, or physical connection (link-layer errors). If the link is operational, move to the next step.
    2. Verify that the classifiers are correctly defined for non-LFI packets. Make sure that non-LFI packets are not configured to be queued to Q2. All packets queued to Q2 are treated as LFI packets.
    3. Verify that at least one of the following values is different in the LFI packets: source address, destination address, IP protocol, source port, or destination port. If the same values are configured for all LFI packets, the packets are all hashed to the same flow and transit the same link.
  4. Use the results to verify load balancing.

Determine Why Packets Are Dropped on a PVC Between a Juniper Networks Device and a Third-Party Device

Problem

Description: You are configuring a permanent virtual circuit (PVC) between T1, E1, T3, or E3 interfaces on a Juniper Networks device and a third-party device, and packets are being dropped and ping fails.

Solution

If the third-party device does not have the same FRF.12 support as the Juniper Networks device or supports FRF.12 in a different way, the Juniper Networks device interface on the PVC might discard a fragmented packet containing FRF.12 headers and count it as a "Policed Discard."

As a workaround, configure multilink bundles on both peers, and configure fragmentation thresholds on the multilink bundles.