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Understanding Dropped Packets and Untransmitted Traffic Using show Commands

 

Starting with Junos OS Release 14.2, packets that need to be forwarded to the adjacent network element or a neighboring device along a routing path might be dropped by a router owing to several factors. Some of the causes for such a loss of traffic or a block in transmission of data packets include overloaded system conditions, profiles and policies that restrict the bandwidth or priority of traffic, network outages, or disruption with physical cable faults. You can use a number of show commands to determine and analyze the statistical counters and metrics related to any traffic loss and take an appropriate corrective measure. The fields displayed in the output of the show commands help in diagnosing and debugging network performance and traffic-handling efficiency problems.

The following show commands and associated fields applicable for dropped packets enable you to view and analyze some of the system parameters for errors or disruption in transmitted packets:

show interfaces extensive Command

show interfaces extensive—Display input and output packet errors or drops

The following are the output fields related to packet drops:

Input Errors—Input errors on the interface. The following paragraphs explain the counters whose meaning might not be obvious:

  • Errors—Sum of the incoming frame aborts and FCS errors.

  • Drops—Number of packets dropped by the input queue of the I/O Manager ASIC. If the interface is saturated, this number increments once for every packet that is dropped by the ASIC's RED mechanism.

  • Framing errors—Number of packets received with an invalid frame checksum (FCS).

  • Runts—Number of frames received that are smaller than the runt threshold.

  • Policed discards—Number of frames that the incoming packet match code discarded because they were not recognized or not of interest. Usually, this field reports protocols that the Junos OS does not handle.

  • L3 incompletes—Number of incoming packets discarded because they failed Layer 3 (usually IPv4) sanity checks of the header. For example, a frame with less than 20 bytes of available IP header is discarded. L3 incomplete errors can be ignored by configuring the ignore-l3-incompletes statement.

  • L2 channel errors—Number of times the software did not find a valid logical interface for an incoming frame.

  • L2 mismatch timeouts—Number of malformed or short packets that caused the incoming packet handler to discard the frame as unreadable.

  • FIFO errors—Number of FIFO errors in the receive direction that are reported by the ASIC on the PIC. If this value is ever nonzero, the PIC is probably malfunctioning.

  • Resource errors—Sum of transmit drops.

Output Errors—Output errors on the interface. The following paragraphs explain the counters whose meaning might not be obvious:

  • Carrier transitions—Number of times the interface has gone from down to up. This number does not normally increment quickly, increasing only when the cable is unplugged, the far-end system is powered down and then up, or another problem occurs. If the number of carrier transitions increments quickly (perhaps once every 10 seconds), the cable, the far-end system, or the PIC or PIM is malfunctioning.

  • Errors—Sum of the outgoing frame aborts and FCS errors.

  • Drops—Number of packets dropped by the output queue of the I/O Manager ASIC. If the interface is saturated, this number increments once for every packet that is dropped by the ASIC's RED mechanism.

  • Collisions—Number of Ethernet collisions. The Gigabit Ethernet PIC supports only full-duplex operation, so for Gigabit Ethernet PICs, this number should always remain 0. If it is nonzero, there is a software bug.

  • Aged packets—Number of packets that remained in shared packet SDRAM so long that the system automatically purged them. The value in this field should never increment. If it does, it is most likely a software bug or possibly malfunctioning hardware.

  • FIFO errors—Number of FIFO errors in the send direction as reported by the ASIC on the PIC. If this value is ever nonzero, the PIC is probably malfunctioning.

  • HS link CRC errors—Number of errors on the high-speed links between the ASICs responsible for handling the router interfaces.

  • MTU errors—Number of packets whose size exceeded the MTU of the interface.

  • Resource errors—Sum of transmit drops.

Queue counters (Egress)

CoS queue number and its associated user-configured forwarding class name.

  • Queued packets—Number of queued packets.

  • Transmitted packets—Number of transmitted packets.

  • Dropped packets—Number of packets dropped by the ASIC's RED mechanism.

Queue counters (Ingress)

CoS queue number and its associated user-configured forwarding class name. Displayed on IQ2 interfaces.

  • Queued packets—Number of queued packets.

  • Transmitted packets—Number of transmitted packets.

  • Dropped packets—Number of packets dropped by the ASIC's RED mechanism.

show interfaces queue Command

show interfaces queue—Display class-of-service (CoS) queue information for physical interfaces.

The following are the output fields related to packet drops:

Queue counters (Egress)—CoS queue number and its associated user-configured forwarding class name. Displayed on IQ2 interfaces.

  • Queued packets—Number of queued packets.

  • Transmitted packets—Number of transmitted packets.

  • Dropped packets—Number of packets dropped by the ASIC's RED mechanism.

Tail-dropped packets—Number of packets dropped because of tail drop.

RL-dropped packets—Number of packets dropped due to rate limiting. For rate-limited interfaces hosted on MICs, MPCs, and Enhanced Queuing DPCs only, this statistic is not included in the queued traffic statistics.

RL-dropped bytes—Number of bytes dropped due to rate limiting. For rate-limited interfaces hosted on MICs, MPCs, and Enhanced Queuing DPCs only, this statistic is not included in the queued traffic statistics.

RED-dropped packets—Number of packets dropped because of random early detection (RED).

  • (M Series and T Series routers only) On M320 and M120 routers and the T Series routers, the total number of dropped packets is displayed. On all other M Series routers, the output classifies dropped packets into the following categories:

    • Low, non-TCP—Number of low-loss priority non-TCP packets dropped because of RED.

    • Low, TCP—Number of low-loss priority TCP packets dropped because of RED.

    • High, non-TCP—Number of high-loss priority non-TCP packets dropped because of RED.

    • High, TCP—Number of high-loss priority TCP packets dropped because of RED.

    • (J Series routers and MX Series routers with enhanced DPCs, and T Series routers with enhanced FPCs only) The output classifies dropped packets into the following categories:

      • Low—Number of low-loss priority packets dropped because of RED.

      • Medium-low—Number of medium-low loss priority packets dropped because of RED.

      • Medium-high—Number of medium-high loss priority packets dropped because of RED.

      • High—Number of high-loss priority packets dropped because of RED.

RED-dropped bytes—Number of bytes dropped because of RED. The byte counts vary by interface hardware.

  • (M Series and T Series routers only) On M320 and M120 routers and the T Series routers, only the total number of dropped bytes is displayed. On all other M Series routers, the output classifies dropped bytes into the following categories:

    • Low, non-TCP—Number of low-loss priority non-TCP bytes dropped because of RED.

    • Low, TCP—Number of low-loss priority TCP bytes dropped because of RED.

    • High, non-TCP—Number of high-loss priority non-TCP bytes dropped because of RED.

    • High, TCP—Number of high-loss priority TCP bytes dropped because of RED.

  • (J Series routers only) The output classifies dropped bytes into the following categories:

    • Low—Number of low-loss priority bytes dropped because of RED.

    • Medium-low—Number of medium-low loss priority bytes dropped because of RED.

    • Medium-high—Number of medium-high loss priority bytes dropped because of RED.

    • High—Number of high-loss priority bytes dropped because of RED.

show class-of-service fabric statistics summary Command

show class-of-service fabric statistics summary—Display class-of-service (CoS) switch fabric queue drop statistics.

The following are the output fields related to packet drops:

Drop statistics—Fabric queue statistics for dropped traffic:

  • Packets—Dropped packet count for high-priority and low-priority queues.

  • Bytes—Dropped byte count for high-priority and low-priority queues.

  • pps—Dropped packets-per-second count for high-priority and low-priority queues.

  • bps—Dropped bits-per-second count for high-priority and low-priority queues.

show pfe statistics traffic fpc Command

show pfe statistics traffic fpc—Display packet drops related to the entire FPC

The following are the output fields related to packet drops:

Packet Forwarding Engine Hardware Discard statistics—Information about Packet Forwarding Engine hardware discards:

  • Timeout—Number of packets discarded because of timeouts.

  • Truncated key—Number of packets discarded because of truncated keys.

  • Bits to test—Number of bits to test.

  • Data error—Number of packets discarded because of data errors.

  • Stack underflow—Number of packets discarded because of stack underflows.

  • Stack overflow—Number of packets discarded because of stack overflows.

  • Normal discard—Number of packets discarded because of discard routes. Packets are dropped silently without being further processed by the host. Normal discards are reported when packets match a firewall filter term that has an action of discard or when the final result of the route look-up is a next hop of discard.

  • Extended discard—Number of packets discarded because of illegal next hops. Packets are dropped silently but are also sent to the Routing Engine for further processing. Extended discards are reported when packets match a firewall filter term that has an action of discard and an additional action that requires Routing Engine processing, such as log, count, sample, or syslog.

  • Invalid interface—Number of packets discarded because of invalid incoming interfaces.

  • Info cell drops—Number of information cell drops.

  • Fabric drops—Number of fabric drops.

Packet Forwarding Engine Local Traffic statistics—Information about Packet Forwarding Engine local traffic:

  • Local packets input—Number of incoming packets from the local network.

  • Local packets output—Number of outgoing packets dispatched to a host in the local network.

  • Software input high drops—Number of incoming software packets of high-priority, dropped during transmission.

  • Software input medium drops—Number of incoming software packets of medium-priority, dropped during transmission.

  • Software input low drops—Number of incoming software packets of low-priority, dropped during transmission.

  • Software output drops—Number of outgoing software packets that were dropped during transmission.

  • Hardware input drops—Number of incoming hardware packets that were dropped during transmission.

The preceding commands represent only the main parameters that you can use to identify and monitor traffic drops or errors. Depending on your specific deployment scenario and network conditions, you might need to view the output of other relevant show commands to evaluate different factors that might be resulting in traffic transmission losses.