Interface Status
Interface Type
Maximum Transmission Unit
Reliability, Txload, Rxload
Communication Channel
   Half Duplex
   Full Duplex
Media Type
QoS Information


We will have a fundamental course today, the interface setting of a Cisco device. I am going to introduce some terms that we always see in network lessons instead of talking too much theory. Let us start from show interface to check what parameters we can configure.

Please click ⬇️  to see details.

SW1#show interfaces fastEthernet 1/0/1 
FastEthernet1/0/1 is up, line protocol is up (connected) 1⃣️ ⬇️ Hardware is Fast Ethernet, address is 0016.468f.c383 (bia 0016.468f.c383) 2⃣️ ⬇️ Description: Management 3⃣️ ⬇️ MTU 1500 bytes 4⃣️ ⬇️, BW 100000 Kbit 5⃣️ ⬇️, DLY 100 usec 6⃣️ ⬇️,
reliability 255/255, txload 1/255, rxload 1/255 7⃣️ ⬇️ Encapsulation ARPA, loopback not set Keepalive set (10 sec) Full-duplex 8⃣️ ⬇️, 100Mb/s 9⃣️ ⬇️, media type is 10/100BaseTX 1⃣️0⃣️ ⬇️ input flow-control is off, output flow-control is unsupported 1⃣️1⃣️ ⬇️ ARP type: ARPA, ARP Timeout 04:00:00
Last input never, output 00:00:00, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo
Output queue: 0/40 (size/max) 5 minute input rate 1000 bits/sec, 2 packets/sec 1⃣️2⃣️ ⬇️ 5 minute output rate 1000 bits/sec, 2 packets/sec 457 packets input, 37444 bytes, 0 no buffer Received 94 broadcasts (0 multicasts) 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog, 69 multicast, 0 pause input 0 input packets with dribble condition detected 1095 packets output, 87569 bytes, 0 underruns 0 output errors, 0 collisions, 1 interface resets 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier, 0 PAUSE output 0 output buffer failures, 0 output buffers swapped out

1⃣️ Interface Status

This the the Layer (L1) and Layer 2 (L2) status of an interface. FastEthernet1/0/1 is up shows L1 is working and line protocol is up shows L2 is working. If both are Up, we can tell the interface is "Up UP". Shutdown command can be used to turn off the Interface and no shutdown command can be used to turn on the Interface.

SW1(config)#int fastEthernet 1/0/15
SW1(config)#int fastEthernet 1/0/15
SW1(config-if)#no shutdown

If L1 is not Up, please check the network cable and the cable plug. If L2 is not Up, it is not easy to confirm the cause. Both network cable and the L2 Protocol setting is needed to check.

2⃣️ Interface Type

It shows what the Interface type is, Fast Ethernet or Gigabit Ethernet? If it is a VLAN Interface, EtherSVI will be shown here. SVI means Switched Virtual Interface. The address shows at the end is the MAC Address of the Interface.

3⃣️ Description

You may put some note on this Interface. A network administrator will always put the Host or Switch that this Interface is connecting to. Use the following command to change the description.

SW1(config)#int gigabitEthernet 1/0/1
SW1(config-if)#description Connect to Internet

SW1#show interfaces gigabitEthernet 1/0/1
GigabitEthernet1/0/1 is administratively down, line protocol is down (disabled)
Hardware is Gigabit Ethernet, address is 0016.468f.c381 (bia 0016.468f.c381)
Description: Connect to Internet
MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255

<--Output Omitted-->

4⃣️ Maximum Transmission Unit

MTU is abbreviation of Maximum Transmission Unit. Many years ago, network connection use shared media, such as, use Hubs to connect the computers. But the operation of Hubs is different to Switch, it only allow one host to transmit at the same time. The transmission speed is low at that time, MTU can avoid a computer dominate the network resource. If a host wants to send data that is larger than MTU, it needs to do the fragmentation to divide the data into pieces that are smaller than the MTU. The default value of MTU is 1500 Bytes. For example, if Host A wants to send 4000 Bytes data, the data will be divided into two 1500 Bytes and one 1000 Bytes frame to transmit. We always keep the MTU as default value.

When we set the MTU too large, if some network devices along the path do not support such large MTU, the data will be dropped. On the other hand, if we set the MTU too small, bandwidth will be wasted for the L2 frame header (destination MAC Address and other information) and frame trailer (Error Check Sum). The total size of header and trailer is 18 Bytes. So, small MTU will increase the number of small frames so that bandwidth will wasted on the 18 Bytes long header and trailer.

If you still want to change the MTU after very careful consideration, use the following command. The setting will be valid after restarting the switch.

SW1(config)#system mtu ?
<1500-1998> MTU size in bytes
jumbo Set Jumbo MTU value for GigabitEthernet or TenGigabitEthernet
routing Set the Routing MTU for the system

SW1(config)#system mtu 1700
Changes to the system MTU will not take effect until the next reload is done
SW1#wri mem
Building configuration...
Proceed with reload? [confirm]

SW1#show interfaces fastEthernet 1/0/1
FastEthernet1/0/1 is up, line protocol is up (connected)
Hardware is Fast Ethernet, address is 0014.1c0e.4383 (bia 0014.1c0e.4383)
MTU 1700 bytes, BW 100000 Kbit, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set

5⃣️ Bandwidth

The bandwidth value of the interface. Use Bandwidth to change the value.

SW1(config-if)#bandwidth ?
<1-10000000> Bandwidth in kilobits
inherit Specify how bandwidth is inherited

SW1(config-if)#bandwidth 50000

But change the bandwidth value of the interface do NOT actually change the transmit bandwidth of an interface. It is only a reference value for Routing Protocol or Spanning Tree Protocol to calculate cost.

6⃣️ Delay

DLY is the delay value of an Interface. It is different to bandwidth but they have relationship. To explain bandwidth and delay, think the network connection as a tunnel of cars. The cars are the network data. Bandwidth is the number of cars that can pass through this tunnel every seconds, it depends on the width of the tunnel. Delay is how long a car is needed to pass through the tunnel. Take Hung Hom Cross Harbour Tunnel in Hong Kong as an example, it has a very small delay (only 1 minute for passing through if there is no traffic jam). But the bandwidth is also very small……😡


Also, change the delay value does NOT speed up the connection. It is used for calculate the cost only. You may use the followed command to change the delay.

SW1(config-if)#delay ?
<1-16777215> Throughput delay (tens of microseconds)

SW1(config-if)#delay 5000

7⃣️ Reliability, Txload, Rxload

Reliability, Txload and Rxload are used for calculating cost. For example, EIGRP uses these values to calculate metric. Visit Enhanced Interior Gateway Routing Protocol (EIGRP) for details.

8⃣️ Communication Channel

An important concept. The 3 Communication Channel are, Simplex, Half Duplex and Full Duplex.


Simplex is a unidirectional transmission. One side send out data and the other side receive. Traditional television channel are simplex. TV company generates signal at the microwave station and the television receive the signal by antenna.

Half Duplex

By using Half Duplex, data can be transmitted and received in both directions. But it only allows one direction transmission at the same time. Think about the walkie-talkie, when you want to talk, press the button to occupy the channel. Then, release the button after finish speaking and waiting for reply.

Full Duplex

Full Duplex supports send and receive at the same time in both directions. Telephone system is Full Duplex. You and your girl friends can speak at the same time on the phone.

Interface of Cisco devices do not support simplex. Auto is the default duplex setting, you may set Half Duplex or Full Duplex manually by using the following command.

SW1(config-if)#duplex ?
auto Enable AUTO duplex configuration
full Force full duplex operation
half Force half-duplex operation

SW1(config-if)#duplex full 

9⃣️ Speed

Speed is limit of how fast the Interface could send the data on to the wire. It may be configured as 10, 100, 1000 or more. The default is auto and it may be changed by using the following command.

SW1(config-if)#speed ?
10 Force 10 Mbps operation
100 Force 100 Mbps operation
auto Enable AUTO speed configuration

SW1(config-if)#speed 10

1⃣️0⃣️ Media Type

There are many choices for media type. Normally, we do not need to configure the media type. But what is the meaning of 100BASE-T, 1000BASE-CX that we always see on the product datasheet?

The number that in front of BASE represents the bandwidth. We can see the following different types.

BandwidthFormal NameExample
10 Ethernet 10BASE-F
100 Fast Ethernet 100BASE-T
1000 Gigabit Ethernet 1000BASE-X
10G 10 Gigabit Ethernet 10GBASE-SR
40G 40 Gigabit Ethernet 40GBASE-T
100G 100 Gigabit Ethernet 100GBASE-ZR

The characters that behind the Base is the most complicated part. The first letters represents the cabling type.

T Twisted-Pair Cable It is the Twisted-pair Network cable that commonly used. Length < 100 meters
K Copper Backplane  
C Balanced Copper Cable  
F Optical Fiber Cable  
B Two wavelengths over a Single Optical Cable  
S Short-range multi-mode Optical Cable Length < 100 meters
L Long-range Single / Multi-mode Optical Cable 100 meters to 10 kilometers
E Extended-range Optical Cable 10 kilometers to 40 kilometers
Z High wavelength over Long-range Single-mode cable  

The second letter is the Coding Scheme. Encryption (or called coding) is needed before a binary data is translated to electrical signal that send to the network cable to ensure that there is no transmission error. Coding Scheme is the method of encryption. Take is easy, it is not the learning scope of a network engineer.

X  4B/5B Block Coding or 8B/10B Block Coding
R  64B/66B Block Coding

1⃣️1⃣️ QoS Information

The QoS (Quality of Service) settings of the Interface. We leave it to the tutorial of QoS in the future.

1⃣️2⃣️ Counters

At last, there is a large amount of numbers. They are counters of the interface. The counters record the statistics of the interface from system booted. When we want to troubleshoot an issue, we may focus on counters like error and CRC. If these counters increase continuously, it may be a L1 problem.

Sometimes, we want to reset the counters. Use the following command.

SW1#clear counters gigabitEthernet 1/0/1
Clear "show interface" counters on this interface [confirm]