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Serial Port FAQ

Understanding the SHOW INTERFACE SERIAL stats by hexn
Posted: 30 Nov 01 (Edited 31 May 02)

Serial0 is up
Indicates whether the interface hardware is currently active (whether carrier detect is present) or if it has been taken down by an administrator.

line protocol is up
Indicates whether the software processes that handle the line protocol consider the line usable (that is, whether keepalives are successful) or if it has been taken down by an administrator.

Hardware is HD64570
Specifies the hardware type.

Internet address is x.x.x.x/x
Specifies the Internet address and subnet mask.

MTU xxxx bytes
Maximum transmission unit of the interface.

BW xxxx Kbit
Indicates the value of the bandwidth parameter that has been configured for the interface (in kilobits per second). The bandwidth parameter is used to compute IGRP metrics only. If the interface is attached to a serial line with a line speed that does not match the default (1536 or 1544 for T1 and 56 for a standard synchronous serial line), use the bandwidth command to specify the correct line speed for this serial line.

DLY xxxxx usec
Delay of the interface in microseconds.

reliablility 255/255
Reliability of the interface as a fraction of 255 (255/255 is 100% reliability), calculated as an exponential average over 5 minutes.

txload 1/255, rxload 1/255
Load on the interface as a fraction of 255 (255/255 is completely saturated), calculated as an exponential average over 5 minutes.

Encapsulation HDLC
Encapsulation method assigned to interface.

loopback not set
Indicates whether loopback is set or not.

Keepalive set (10 sec)
Indicates whether keepalives are set or not.

Last input 00:00:00
Number of hours, minutes, and seconds since the last packet was successfully received by an interface. Useful for knowing when a dead interface failed.

output 00:00:00
Number of hours, minutes, and seconds since the last packet was successfully transmitted by an interface.

output hang never
Number of hours, minutes, and seconds (or never) since the interface was last reset because of a transmission that took too long. When the number of hours in any of the "last" fields exceeds 24 hours, the number of days and hours is printed. If that field overflows, asterisks are printed.

Input queue: 0/75/0 (size/max/drops); Total output drops: 0
Number of packets in output and input queues. Each number is followed by a slash, the maximum size of the queue, and the number of packets dropped due to a full queue.

Queueing strategy: weighted fair
Weighted fair queuing strategy (other queueing strategies you might see are priority-list, custom-list, and fifo).

Output queue: 0/1000/64/0 (size/max total/threshold/drops)
Number of packets in output and input queues. Each number is followed by a slash, the maximum size of the queue, and the number of packets dropped due to a full queue.
Congestive-discard threshold. Number of messages in the queue after which new messages for high-bandwidth conversations are dropped.

5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec

Average number of bits and packets received/transmitted per second in the last 5 minutes.

The 5-minute input and output rates should be used only as an approximation of traffic per second during a given 5-minute period. These rates are exponentially weighted averages with a time constant of 5 minutes. A period of four time constants must pass before the average will be within two percent of the instantaneous rate of a uniform stream of traffic over that period.

packets input
Total number of error-free packets received by the system.

bytes
Total number of bytes, including data and MAC encapsulation, in the error-free packets received by the system.

no buffer
Number of received packets discarded because there was no buffer space in the main system. Compare with ignored count. Broadcast storms on Ethernet networks and bursts of noise on serial lines are often responsible for no input buffer events.

Received x broadcasts
Total number of broadcast or multicast packets received by the interface.

runts
Number of packets that are discarded because they are smaller than the medium's minimum packet size.

giants
Number of packets that are discarded because they exceed the medium's maximum packet size.

throttles
Number of times the receiver on the port was disabled, possibly due to buffer or processor overload.

input errors
Total number of no buffer, runts, giants, CRCs, frame, overrun, ignored, and abort counts. Other input-related errors can also increment the count, so that this sum might not balance with the other counts.

CRC
Cyclic redundancy checksum generated by the originating station or far-end device does not match the checksum calculated from the data received. On a serial link, CRCs usually indicate noise, gain hits, or other transmission problems on the data link.

frame
Number of packets received incorrectly having a CRC error and a noninteger number of octets. On a serial line, this is usually the result of noise or other transmission problems.

overrun
Number of times the serial receiver hardware was unable to hand received data to a hardware buffer because the input rate exceeded the receiver's ability to handle the data.

ignored
Number of received packets ignored by the interface because the interface hardware ran low on internal buffers. Broadcast storms and bursts of noise can cause the ignored count to be increased.

abort
Illegal sequence of one bits on a serial interface. This usually indicates a clocking problem between the serial interface and the data link equipment.

packets output
Total number of messages transmitted by the system.

bytes
Total number of bytes, including data and MAC encapsulation, transmitted by the system.

underruns
Number of times that the transmitter has been running faster than the router can handle. This might never be reported on some interfaces.

output errors
Sum of all errors that prevented the final transmission of datagrams out of the interface being examined. Note that this might not balance with the sum of the enumerated output errors, as some datagrams can have more than one error, and others can have errors that do not fall into any of the specifically tabulated categories.

collisions
Number of messages retransmitted due to an Ethernet collision. This usually is the result of an overextended LAN (Ethernet or transceiver cable too long, more than two repeaters between stations, or too many cascaded multiport transceivers). Some collisions are normal. However, if your collision rate climbs to around 4 or 5%, you should consider verifying that there is no faulty equipment on the segment and/or moving some existing stations to a new segment. A packet that collides is counted only once in output packets.

interface resets
Number of times an interface has been completely reset. This can happen if packets queued for transmission were not sent within several seconds' time. On a serial line, this can be caused by a malfunctioning modem that is not supplying the transmit clock signal, or by a cable problem. If the system notices that the carrier detect line of a serial interface is up, but the line protocol is down, it periodically resets the interface in an effort to restart it. Interface resets can also occur when an interface is looped back or shut down.

output buffer failures, output buffers swapped out
Number of failed buffers and number of buffers swapped out (packets swapped to DRAM).

carrier transitions
Number of times the carrier detect signal of a serial interface has changed state. For example, if data carrier detect (DCD) goes down and comes up, the carrier transition counter will increment two times. Indicates modem or line problems if the carrier detect line is changing state often.

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