You want to look at the status of your router's interfaces.
You can look at the current status of any interface using the show interfaces EXEC command. With no arguments, this command shows the status of all interfaces on the router:
You can also look at a particular interface by including its name with the command:
Router1#show interfaces FastEthernet0/1
It is also often useful to look specifically at the IP configuration of one or all of your interfaces by using the show ip interface command:
Router1#show ip interface brief Router1#show ip interface FastEthernet0/1
There is a huge amount of information in the output of the show interfaces command, and the actual content varies from one interface type to another:
Router1#show interfaces FastEthernet0/1 FastEthernet0/1 is up, line protocol is up Hardware is AmdFE, address is 0001.9670.b781 (bia 0001.9670.b781) Internet address is 172.22.1.3/24 MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation ARPA, loopback not set Keepalive set (10 sec) Full-duplex, 100Mb/s, 100BaseTX/FX ARP type: ARPA, ARP Timeout 04:00:00 Last input 00:00:04, 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 0 bits/sec, 0 packets/sec 5 minute output rate 1000 bits/sec, 1 packets/sec 265295 packets input, 21235441 bytes Received 105678 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog 0 input packets with dribble condition detected 1337306 packets output, 125379250 bytes, 0 underruns 0 output errors, 0 collisions, 8 interface resets 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out Router1#show interfaces Serial0/0 Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255 Encapsulation FRAME-RELAY, loopback not set Keepalive set (10 sec) LMI enq sent 108260, LMI stat recvd 108252, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 0 LMI type is ANSI Annex D frame relay DTE Broadcast queue 0/64, broadcasts sent/dropped 306266/2, interface broadcasts 306266 Last input 00:00:04, output 00:00:02, output hang never Last clearing of "show interface" counters 1w5d Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (size/max total/threshold/drops) Conversations 0/3/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) Available Bandwidth 1158 kilobits/sec 5 minute input rate 0 bits/sec, 1 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 934269 packets input, 83226465 bytes, 0 no buffer Received 0 broadcasts, 0 runts, 0 giants, 0 throttles 1 input errors, 0 CRC, 1 frame, 0 overrun, 0 ignored, 0 abort 879200 packets output, 60483145 bytes, 0 underruns 0 output errors, 0 collisions, 4 interface resets 0 output buffer failures, 0 output buffers swapped out 16 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up Router1#
The first line is one of the most important:
FastEthernet0/1 is up, line protocol is up Serial0/0 is up, line protocol is up
This tells you that the interface is operational. There are four main possibilities here. The interface and line protocol can both be up, or they can both be down, or the interface can be up, with the line protocol down. A fourth option is that the interface can be Administratively down, which means that somebody has deliberately disabled it with a shutdown command. There are other options, such as standby and spoofing states, as well.
If the interface is up, this means that the router is receiving the correct Layer 1 physical signaling. For the line protocol to also be up, the router must also see correct Layer 2 information. Clearly, this varies for different media types. In some cases, such as virtual software interfaces, there is no physical carrier. So you will never see a loopback interface in an up/down state. But for other types of interfaces, this can be extremely useful information for debugging problems.
The next line tells you about the interface hardware:
Hardware is AmdFE, address is 0001.9670.b781 (bia 0001.9670.b781) Hardware is PowerQUICC Serial
The interface in the first case is a FastEthernet interface that uses a FastEthernet ASIC made by AMD. This information tends to be useful only when there is a known hardware bug and you want to see if your router is affected. The rest of this line is extremely useful, however, because it tells you the Ethernet MAC address. Note that it lists both the address that the router is using as well as the Burned-In Address (BIA). Most of the time these will be the same, but in Recipe 16.10 we will show how you can make your router use a different Ethernet MAC address. The second interface is a Serial interface, which doesn't have a MAC address, so none is listed.
Next is the IP address, if one is configured:
Internet address is 172.22.1.3/24
In the Serial interface example above, the interface doesn't have any Layer 3 protocol addresses, so nothing is listed here. Note, however, that this command will also display IPX or AppleTalk addresses if they are configured.
Then come two lines that tell you a series of useful pieces of information about the interface's configuration and utilization. Here is the FastEthernet interface:
MTU 1500 bytes, BW 100000 Kbit, DLY 100 usec, reliability 255/255, txload 1/255, rxload 1/255
And here is the corresponding information for the Serial interface:
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec, reliability 255/255, txload 1/255, rxload 1/255
The first field here is the Maximum Transmission Unit (MTU), which is 1,500 bytes for both interfaces. This 1,500-byte MTU size is typical for IP networks, although you can change it relatively easily using the mtu interface configuration command, as we show in Recipe 16.2. Using a variety of different MTU values in your network can cause performance problems due to fragmentation, however.
It is important to remember that this Layer 2 MTU affects all protocols that use the interface, and not just IP. This is the size of the largest Layer 2 packet that the router can send through this interface. Some media can support very large packets; others are more tightly constrained.
The BW field shows the configured bandwidth of the interface. It is important to note that sometimes this is not the actual throughput of the interface. On Serial interfaces, the router will always show the default value here. Even for Data Communications Equipment (DCE) serial interfaces that supply the clock signal and therefore have a good way of estimating the theoretical maximum throughput, this bandwidth value stays at its default value unless you change it manually by using the bandwidth interface configuration command. This parameter is used for calculating routing protocol metrics, as well as for converting raw bit transmission rates into utilization statistics. It has nothing to do with how fast the router will transmit packets.
The reliability field in the next line is no longer commonly used. It was part of the optional metric calculation for IGRP, so it is included for historical reasons. However, the other two values, txload and rxload, are very useful. These represent the traffic utilization for the interface outbound and inbound, respectively. Both of these values are expressed as fractions of 255 rather than percentages. This may seem like an odd value, but a range from 0255 can be conveniently represented by using an 8-bit variable, which is why Cisco does it this way. Each of these values represents a fraction of the total available bandwidth shown in the BW field.
Because these txload and rxload values are rates, the router has to measure them by counting the number of bits sent and received over some finite period of time. By default, this interval is five minutes. However, you can adjust the measurement period with the load-interval interface configuration command:
This command takes a value in seconds as an argument. The number must be a multiple of 30 seconds, with a maximum value of 600.
The next set of lines describes the Layer 2 encapsulation on the interface. For the FastEthernet interface example, there are four lines:
Encapsulation ARPA, loopback not set Keepalive set (10 sec) Full-duplex, 100Mb/s, 100BaseTX/FX ARP type: ARPA, ARP Timeout 04:00:00
In this case, you can see that this interface uses 100 Mbps full-duplex Ethernet on a 100BaseTX or 100BaseFX interface. At Layer 2, it uses ARPA (which stands for the Advanced Research Projects Agency of the U.S. government, the agency that sponsored the initial development of the TCP/IP protocol suite) encapsulation. There are two Layer 2 encapsulation types. The older ARPA encapsulation standard for IP packets is described in RFCs 894 and 895. This standard dates to a time when Ethernet was still an experimental protocol, and there was not yet an IEEE standard.
Some time later, the IEEE officially documented the Ethernet protocol and encapsulation standards. But the IEEE standards differed from the existing ARPA standard, which was already enjoying considerable popularity in IP networks. So, rather than changing, IP continues to use the old standard, while other Layer 3 protocols such as IPX offer a choice of encapsulation types.
The interface shown in this example is only configured for IP, which is why it shows ARPA encapsulation here.
The Serial interface in the example is using the Frame Relay protocol, so the show interface output includes different relevant information:
Encapsulation FRAME-RELAY, loopback not set Keepalive set (10 sec) LMI enq sent 108260, LMI stat recvd 108252, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0 LMI DLCI 0 LMI type is ANSI Annex D frame relay DTE Broadcast queue 0/64, broadcasts sent/dropped 306266/2, interface broadcasts 306266
In both cases, you can see that these interfaces use the default keepalive value of 10 seconds. This means that these interfaces will send out a small packet every 10 seconds just to see if the interface is still working properly. If the keepalive test fails, the router will declare the interface's line protocol to be down.
Both of these examples also include the phrase "loopback not set." If you were to apply an external loopback test to this interface, so that the router gets back all the data that it transmits, this text would change to say "loopback set." And you would also see at the top of the show interface output that the line protocol is up but looped:
Serial0/0 is up, line protocol is up (looped)
Then the next few lines show how long it has been since the router has sent or received a packet on this interface, and how long it has been since you last cleared the statistical counters on this interface:
Last input 00:00:04, output 00:00:02, output hang never Last clearing of "show interface" counters 1w5d
The queue parameters are extremely important. The Serial interface in the example is using weighted fair queueing, while the Ethernet uses First In First Out (FIFO). Please refer to Chapter 11 for more information on different queueing strategies:
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: weighted fair Output queue: 0/1000/64/0 (size/max total/threshold/drops) Conversations 0/3/256 (active/max active/max total) Reserved Conversations 0/0 (allocated/max allocated) Available Bandwidth 1158 kilobits/sec
The show interface output also shows the number of packets currently in each queue (size), the maximum number of packets that the queue can hold (max), and other parameters such as drop thresholds and the number of tail drops, which vary for different queueing strategies.
Next is a large and extremely useful block of performance related information:
5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 1000 bits/sec, 1 packets/sec 265295 packets input, 21235441 bytes Received 105678 broadcasts, 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored 0 watchdog 0 input packets with dribble condition detected 1337306 packets output, 125379250 bytes, 0 underruns 0 output errors, 0 collisions, 8 interface resets 0 babbles, 0 late collision, 0 deferred 0 lost carrier, 0 no carrier 0 output buffer failures, 0 output buffers swapped out
The first two lines of this block show the rates for both sending and receiving data through this interface, measured in both bits per second and packets per second, and averaged over a five-minute period. This is an excellent way of quickly checking to see how much data this interface is handling. You can also get more detailed information from the exact counters that follow.
And, finally, the serial interface output ends with two lines that show you information about its physical state:
16 carrier transitions DCD=up DSR=up DTR=up RTS=up CTS=up
In this case, the interface has gone up and down 16 times since the last time the interface counters were cleared. The last line shows current state of all of the Serial signals. The DCE device raises a voltage on the Data Carrier Detect (DCD) pin to indicate that the link is ready for transmitting data. For other types of media, this line may contain other relevant Layer 1 information.
The DCE device on a Serial medium sends the Data Set Ready (DSR) signal when it is ready to send or receive data. When the Data Terminal Ready (DTR) signal is high, it means that the DTE device is ready to send or receive data. By default, the DTE device will wait until it sees the DSR signal, and the DCE device will wait for the DTR signal by default before sending any packets.
The Request To Send (RTS) signal indicates that the DTE device would like to send data and is checking to make sure that the link and the far end are ready to receive it. If everything is ready, the DCE device responds by raising the Clear To Send (CTS) signal.
So, in the above example, all of the serial signals are high, which tells you immediately that everything is working properly. Sometimes the show interface output will show one or more of these signals in a down state. This may indicate a cabling problem, or it may simply indicate that the far end device is simply busy right now and can't accept packets.
The show ip interface command shows a different set of information about the interfaces. With the keyword brief, this command gives you an extremely useful listing of all of your interfaces:
Router1#show ip interface brief Interface IP-Address OK? Method Status Protocol Async65 unassigned YES NVRAM down down FastEthernet0/0 unassigned YES NVRAM up up FastEthernet0/0.1 172.25.1.5 YES NVRAM up up FastEthernet0/0.2 172.16.2.1 YES NVRAM up up Serial0/0 unassigned YES NVRAM up up Serial0/0.1 172.25.2.1 YES NVRAM up up Serial0/0.2 172.20.1.1 YES manual up up FastEthernet0/1 172.22.1.3 YES NVRAM up up Serial0/1 10.1.1.2 YES NVRAM up up Loopback0 172.25.25.1 YES NVRAM up up Router1#
This output shows you all of your interfaces, their IP addresses, and both the interface and protocol status. The other two columns here are labeled "OK?" and "Method." "OK?" simply refers to whether the router thinks that interface is operating correctly, while "Method" indicates how the interface acquired its IP address.
Notice that the IP addresses of almost all of these interfaces were configured by NVRAM. This simply means that they have not changed since the last reboot. However, one of the interfaces, Serial0/0.2, was manually configured since the last reboot.
You can include a specific interface name in place of the brief keyword to get details on the IP configuration of this interface:
Router1#show ip interface FastEthernet0/1 FastEthernet0/1 is up, line protocol is up Internet address is 172.22.1.3/24 Broadcast address is 255.255.255.255 Address determined by non-volatile memory MTU is 1500 bytes Helper address is not set Directed broadcast forwarding is disabled Multicast reserved groups joined: 18.104.22.168 22.214.171.124 126.96.36.199 188.8.131.52 184.108.40.206 Outgoing access list is not set Inbound access list is not set Proxy ARP is enabled Security level is default Split horizon is enabled ICMP redirects are always sent ICMP unreachables are always sent ICMP mask replies are never sent IP fast switching is enabled IP fast switching on the same interface is disabled IP Flow switching is disabled IP CEF switching is enabled IP CEF Fast switching turbo vector IP multicast fast switching is enabled IP multicast distributed fast switching is disabled IP route-cache flags are Fast, CEF Router Discovery is enabled IP output packet accounting is disabled IP access violation accounting is disabled TCP/IP header compression is disabled RTP/IP header compression is disabled Probe proxy name replies are disabled Policy routing is disabled Network address translation is disabled WCCP Redirect outbound is disabled WCCP Redirect inbound is disabled WCCP Redirect exclude is disabled BGP Policy Mapping is disabled Router1#
In addition to these commands, there are two hidden commands that we find very useful. The first is simply to add the keyword stats to the show interfaces command:
Router1#show interfaces FastEthernet0/1 stats FastEthernet0/1 Switching path Pkts In Chars In Pkts Out Chars Out Processor 294567 18704930 239526 22219870 Route cache 7758 681257 48303 6129834 Total 302325 19386187 287829 28349704 Router1#
This output displays packet switching statistics for this interface. The Processor line shows both how many packets and how many characters the router has switched using process switching, and the Route cache line shows the values for Fast Switching. The Pkts In and Chars In columns show the values for incoming packets, while the other two columns show values for packets transmitted out through this interface.
You can get a more detailed breakdown of this switching information by adding the switching keyword to the show interfaces command:
Router1#show interfaces FastEthernet0/1 switching FastEthernet0/1 Throttle count 0 Drops RP 0 SP 0 SPD Flushes Fast 0 SSE 0 SPD Aggress Fast 0 SPD Priority Inputs 40510 Drops 0 Protocol Path Pkts In Chars In Pkts Out Chars Out Other Process 11562 1022965 18730 1123800 Cache misses 0 Fast 0 0 0 0 Auton/SSE 0 0 0 0 IP Process 102271 8491851 220342 21066444 Cache misses 0 Fast 7758 681257 48304 6129962 Auton/SSE 0 0 0 0 ARP Process 1819 109140 467 31756 Cache misses 0 Fast 0 0 0 0 Auton/SSE 0 0 0 0 Router1#
For more information on Fast Switching and Process Switching, please see Chapter 11.
Chapter 11; Recipe 16.2; Recipe 16.10
Router Configuration and File Management
User Access and Privilege Levels
Handling Queuing and Congestion
Tunnels and VPNs
NTP and Time
Router Interfaces and Media
Simple Network Management Protocol
First Hop Redundancy Protocols
Appendix 1. External Software Packages
Appendix 2. IP Precedence, TOS, and DSCP Classifications