Below line by line we take a look at a typical serial interface and try to define what each line is telling us. Each line corresponds with the table below. Being able to know what your interface is doing at a glance will help you avoid lengthy down time and finger pointing. Working 7 years in a Network Opertaion Center and 3 years with a major telco provider, I've had the chance to be on both sides of the converstation between telco and the customer. Takes only 15 min on the customer's side to make a few simple checks on thier interface to make sure the problem is NOT on thier end.
| Router1#sh int Serial3/0/23:0 | |
| 1 | Serial3/0/23:0 is up, line protocol is up |
| 2 | Hardware is PA-MC-2T3+ |
| 3 | Description: T1 WAN CKT LEV 3 HCGS/375555/NJ |
| 4 | Internet address is 192.168.1.254/30 |
| 5 | MTU 1500 bytes, BW 256 Kbit, DLY 20000 usec, rely 255/255, load 1/255 |
| 6 | Encapsulation PPP, crc 16, loopback not set |
| 7 | Keepalive set (10 sec) |
| 8 | LCP Open |
| 9 | Open: IPCP, CDPCP |
| 10 | Last input 00:00:03, output 00:01:08, output hang never |
| 11 | Last clearing of "show interface" counters never |
| 12 | Input queue: 0/75/0 (size/max/drops); Total output drops: 0 |
| 13 | Queueing strategy: weighted fair |
| 14 | Output queue: 0/1000/64/0 (size/max total/threshold/drops) |
| 15 | Conversations 0/1/256 (active/max active/max total) |
| 16 | Reserved Conversations 0/0 (allocated/max allocated) |
| 17 | 5 minute input rate 0 bits/sec, 0 packets/sec |
| 18 | 5 minute output rate 0 bits/sec, 0 packets/sec |
| 19 | 20950 packets input, 1992090 bytes, 0 no buffer |
| 20 | Received 0 broadcasts, 0 runts, 0 giants, 0 throttles |
| 21 | 1 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 1 abort |
| 23 | 31524 packets output, 10804297 bytes, 0 underruns |
| 24 | 0 output errors, 0 collisions, 0 interface resets |
| 25 | 0 output buffer failures, 0 output buffers swapped out |
| 26 | 0 carrier transitions no alarm present |
| 27 | Timeslot(s) Used: 1-4, subrate: 256Kb/s, transmit delay is 0 flags non-inverted data |
| Router1# | |
| 1 | This status indicates that the interface is functioning properly |
| 2 | This field describes the type of hardware that the interface is connected to. In this case, this Serial interface is part of a channelized T3. |
| 3 | Description line might be the most important line here. Nothing is more frustrating when you have a circuit down and can NOT find the circuit ID. Place the LEC and LONG HAUL should be available. |
| 4 | IP information for this circuit, the lower address will always be odd, and the higher will always be even. So in this example we know the other side is .253 |
| 5 | MTU - Maximum Tranmission Unit. By default, this is 1500 bytes, which describes the largest packet that can be sent through the interface before the packet is fragmented. BW - Bandwidth. This field is defined by the network administrator and has no actual effect on the bandwidth of a line. It is simply used for describing the load on a specific interface. DLY - Delay. Amount of micro seconds of delay. I do not have any more information on this at this time. rely - Reliability. Reliability of the interface as a fraction of 255 (255/255 is 100% reliability), calculated as an exponential average over five minutes (default). load - Load Average. Load on the interface as a fraction of 255 (255/255 is completely saturated), calculated as an exponential average over five minutes (default). |
| 6 | Encapsulation is the type of Data-Link encapsulation. This is commonly either PPP, HDLC (Cisco's proprietary PPP), Frame-Relay, and ATM. Loopback specifies whether the loopback bit is set in the D channel signaling. |
| 7 | Keepalive messages check the health of a link. The keepalive timer is the number of seconds a router waits between sending keepalive messages to a peer. If no response from the peer is received, the local router retransmits the keepalive message up to five times. If no response is received from the peer, the local router brings down the link and withdraws the route from the routing table. |
| 8 | LCP - Link Control Protocol needed for PPP negotiation An LCP state of open means that LCP was successfully completed, while an LCP state of closed indicates an LCP failure. |
| 9 | IPCP (IP Control Protocol This value indicates that IP is the network layer under negotiation in the NCP phase CDPCP (Cisco Discovery Protocol Control Protocol) This message indicates that CDP negotiation occurs in the NCP phase. To turn off CDP on the router, issue the no cdp run command. The Network Control Protocol (NCP) phase in the PPP link connection process is used for establishing and configuring different network-layer protocols such as IP, IPX or AppleTalk. http://www.javvin.com/protocolPPPNCP.html |
| 10 | Last input is the number of hours, minutes, and seconds since the last packet was successfully received by an interface. This is useful for determining when a dead interface. Last output is the number of hours, minutes, and seconds since the last packet was successfully transmitted by an interface. This is useful for determining when a dead interface failed. Output hang is the number of hours, minutes, and seconds (or never) since the interface was last reset because of a transmission that took too long. |
| 11 | This line will tell you when the last time you did a "clear interface Serial3/0/23:0". Great command to use when trying to determine which errors are clocking on your interface. |
| 12 | Number of packets in 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. |
| 13 | Weighted fair queuing (WFQ) is a method of automatically smoothing out the flow of data in packet-switched communication networks by sorting packets to minimize the average latency and prevent exaggerated discrepancies between the transmission efficiency afforded to narrowband versus broadband signals |
| 14 | Number of packets in output 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. |
| 15 | Used in conjunction with COS |
| 16 | Used in conjunction with COS |
| 17 | Average number of bits and packets received and transmitted per second in the last five minutes. 5 min intervals are Cisco's default setting |
| 18 | Average number of bits and packets received and transmitted per second in the last five minutes. 5 min intervals are Cisco's default setting. |
| 19 | Packets input - Total number of error-free packets received. |
| 20 | Broadcasts - Total number of broadcast or multicast packets received. Runts - Number of packets 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. Throttle - This counter indicates the number of times the input buffers of an interface have been cleaned because they have not been serviced fast enough or they are overwhelmed. Typically, an explorer storm can cause the throttles counter to increment. It's important to note that every time you have a throttle; all the packets in the input queue get dropped. This causes very slow performance and may also disrupt existing sessions. |
| 21 | Input Errors - Sum of all errors that prevented the receipt of datagrams. This may not balance with the sum of the enumerated output errors, because some datagrams may have more than one error and others may have errors that do not fall into any of the specific categories. CRC - Cyclic redundancy checksum generated mismatch. CRC errors also are reported when a far-end abort occurs and when the idle flag pattern is corrupted. This makes it possible to get CRC errors even when there is no data traffic. Frame - Number of packets received incorrectly having a CRC error and a non integer number of octets. 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. Abort - Number of packets whose receipt was aborted. |
| 23 | Underruns - Number of times that the far-end router's transmitter has been running faster than the near-end router's receiver can handle. This may never happen (be reported) on some interfaces. |
| 24 | Output Errors - Sum of all errors that prevented the final transmission. Interface Resets - Number of times an interface has been completely reset. Interface Resets - Number of times an interface has been completely reset. |
| 25 | Used with keepalives, some cases it is necessary to configure small buffers to give keepalives a chance to recover. |
| 26 | Carrier Transitions - Number of times the carrier detect signal of a serial interface has changed state. |
| 27 | Looking at this we can see that this is a FRAC -T, we are only using 4 channels out of our 24 for a total of 256k Circuit. |
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