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Identifying the Source of RED Dropped Packets on PTX Series Routers

This topic describes how to identify the source of random early detection (RED) dropped packets.

Junos OS and PTX Series hardware CoS features use virtual output queues (VOQs) on the ingress to buffer and queue traffic for each egress output queue.

VOQ is a queuing strategy that eliminates congestion drops on the egress and alleviates head-of-line blocking. Head-of-line blocking is a condition in which a queue of packets is blocked from making progress because the packet at the head of the queue is waiting for resources to become available, while other packets behind this packet could be serviced. For example, if the ingress has a single queue for an egress Packet Forwarding Engine, then packets destined for a slow, congested interface can block packets destined for a fast, uncongested interface attached to the same egress Packet Forwarding Engine.

With VOQ, virtual queues are maintained on the ingress Packet Forwarding Engines, instead of on the egress Packet Forwarding Engine. However, the scheduling of the ingress virtual output queues is controlled by the egress Packet Forwarding Engine. For every egress output queue (shallow buffer), the VOQ architecture provides virtual queues on each and every ingress Packet Forwarding Engine. These queues are referred to as virtual because the queues physically exist on the ingress Packet Forwarding Engine only when the line card actually has packets enqueued to it.

Figure 1 shows three ingress Packet Forwarding Engines—PFE0, PFE1, and PFE2. Each ingress Packet Forwarding Engine provides up to eight virtual output queues (PFEn.e0.q0 through PFEn.e0.q7) for the single egress port 0. The egress Packet Forwarding Engine PFEn distributes the bandwidth to each ingress VOQ in a round-robin fashion; therefore they will receive equal treatment regardless of their presented load.

For example, egress PFEn's VOQ e0.q0 has 10 Gbps of bandwidth available to it. PFE0 has an offered load of 10 Gbps to e0.qo, whereas PFE1 and PFE2 have an offered load of 1Gbps to e0.q0. The result is that PFE1 and PFE2 get 100 percent of their traffic through, wheras PFE0 gets only 80 percent of its traffic through.

Figure 1: Virtual Output Queuing on PTX Series RoutersVirtual Output Queuing on PTX Series Routers

When congestion occurs because of the load on the egress output queue, the ingress VOQs corresponding to the egress output queue contain RED dropped packets.

Note:

For more information about VOQ, see Understanding Virtual Output Queues on PTX Series Packet Transport Routers.

Using the following procedure, you can identify the ingress Packet Forward Engine (in terms of ingress traffic) that is contributing to the egress congestion.

To determine which ingress Packet Forwarding Engine is contributing to the RED dropped packets:

  1. Determine whether there are RED dropped packets on the egress link.

    1. Run the show interfaces queue interface-name command on the egress interface.

    2. In the show output, determine whether the interface is experiencing RED dropped packets, by locating the RED-dropped packets field and checking whether its value is greater than zero.

      The following example shows RED-dropped statistics for the egress Ethernet interface configured on port 0 of PIC 0, located on the FPC in slot 7.

  2. If the interface is experiencing RED dropped packets, run the show interface voq interface-name command on the egress interface that is experiencing the RED dropped packets.
    Tip:

    When using the show interfaces voq command, you can use command filters to help locate the exact queue. For command usage, see show interfaces voq.

  3. In the show output, determine whether the interface is experiencing RED dropped packets.

    The following example shows the count of the ingress RED-dropped packets for the egress Ethernet interface configured on port 0 of PIC 0, located on the FPC in slot 7.

    The sample output shows that the cause of the congestion is the ingress Packet Forwarding Engine PFE 0, on FPC number 4, and the ingress Packet Forwarding Engine PFE 0 on FPC number 6, as denoted by the count of RED-dropped packets.

Note:

For an aggregate interface, follow the same steps, but you must run the show interface queue command on each child link of the aggregate interface to determine which child egress link is experiencing the congestion. Then run the show interface voq command on that child link to determine which ingress Packet Forward Engine is contributing to the congestion.

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