Ccna routing and Switching Complete Study Guide

sh ip sla statistics

IPSLAs Latest Operation Statistics

 

IPSLA operation id: 1

Type of operation: icmp-echo

        Latest RTT: 1 milliseconds

Latest operation start time: *15:27:51.365 UTC Mon Jun 6 2016

Latest operation return code: OK

Number of successes: 38

Number of failures: 0

Operation time to live: Forever

The IP SLA 1 test on R1 has been successfully performed 38 times and the test never failed.

Using SPAN for Troubleshooting

A traffic sniffer can be a valuable tool for monitoring and troubleshooting your network. 

However, since the inception of switches into our networks more than 20 years ago, trou-

bleshooting has become more difficult because we can’t just plug an analyzer into a switch 

port and be able to read all the network traffic. Before we had switches, we used hubs, and 

when a hub received a digital signal on one port, the hub sent that digital signal out on all 

ports except the port it was received on. This allows a traffic sniffer that is connected to a 

hub port to receive all traffic in the network.

Modern local networks are essentially switched networks. After a switch boots, it 

starts to build up a layer 2 forwarding table based on the source MAC addresses of the 

different packets that the switch receives. After the switch builds this forwarding table, it 

forwards traffic that is destined for a MAC address directly to the exit port. By default, 

this prevents a traffic sniffer that is connected to another port from receiving the unicast 

traffic. The SPAN feature was therefore introduced on switches to help solve this problem 

(see Figure 20.2).

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Chapter 20 

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  Troubleshooting IP, IPv6, and VLANs

f I g u r e   2 0 . 2     Using SPAN for troubleshooting

PC1

PC2

SW1

Fa0/1

Fa0/3

Fa0/2

Ingress Traffic

Egress Traffic

Copy of Traffic

Sniffer

The SPAN feature allows you to analyze network traffic passing through the port and 

send a copy of the traffic to another port on the switch that has been connected to a net-

work analyzer or other monitoring device. SPAN copies the traffic that the device receives 

and/or sends on source ports to a destination port for analysis.

For example, if you would like to analyze the traffic flowing from PC1 to PC2, shown in 

Figure 20.2, you need to specify a source port where you want to capture the data. You can 

either configure the interface Fa0/1 to capture the ingress traffic or configure the interface 

Fa0/3 to capture the egress traffic—your choice! Next, specify the destination port inter-

face where the sniffer is connected and will capture the data, in this example Fa0/2. The 

traffic flowing from PC1 to PC2 will then be copied to that interface, and you will be able 

to analyze it with a traffic sniffer.

Step 1: Associate a SPAN session number with the source port of what you want to 

monitor.

S1(config)#