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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 device 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—Display input and output packet errors or drops. Following are some of the show interfaces extensive input counters and their definitions.

Following are definitions for some of the output counters for show interfaces extensive:

Following are definitions for some of the Queue counters for show interfaces extensive (both outbound and inbound). This includes CoS queue number and its associated user-configured forwarding class name, and is displayed on IQ2 interfaces.

Errors

Sum of the incoming frame terminates 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.

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 terminates 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.

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—Display class-of-service (CoS) queue information for physical interfaces. Following are some of the show interfaces queue output fields and their definitions.

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.

RED-dropped packets

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

On M320 and M120 routers and most T Series routers, just the total number of dropped packets is displayed. For other M Series routers, as well as MX Series routers with enhanced DPCs, T Series routers with enhanced FPCs, and all J Series routers, the output classifies dropped packets into the following catetories:

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

show class-of-service fabric statistics summary—Display class-of-service (CoS) switch fabric queue drop statistics. Following are the 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—Display packet drops related to the entire FPC. Following are the FPC-level statistics for Packet Forwarding Engine hardware discards:

The following statistics are related to Packet Forwarding Engine local traffic for show pfe statistics traffic fpc:

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.

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.

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.