| Cisco Systems |
Troubleshooting IP, IPv6, and VLANsTroubleshooting IP Network Connectivity 1. Check the cables to find out if there’s a faulty cable or interface in the mix and verify theinterface’s statistics. 2. Make sure that devices are determining the correct path from the source to thedestination. Manipulate the routing information if needed. 3. Verify that the default gateway is correct. 4. Verify that name resolution settings are correct. 5. Verify that there are no access control lists (ACLs) blocking traffic. There are four steps for checking the PC1 configuration: 1. Test that the local IP stack is working by pinging the loopback address. 2. Test that the local IP stack is talking to the Data Link layer (LAN driver) by pinging thelocal IP address. 3. Test that the host is working on the LAN by pinging the default gateway. 4. Test that the host can get to remote networks by pinging remote Server1. Using IP SLA for Troubleshooting I want to mention one more thing that can help you troubleshoot your IP network, and this is using IP service-level agreements (SLAs), which will allow us to use IP SLA ICMP echo to test far-end devices instead of pinging manually. There are several reasons to use the IP SLA measurements: Edge-to-edge network availability monitoring For example, packet loss statistics Network performance monitoring and network performance visibility For example, network latency and response time Troubleshooting basic network operation For example, end-to-end network connectivity 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, troubleshooting 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. VLAN Troubleshooting A couple of key times to troubleshoot VLANs are when and if you lose connectivity between hosts and when you’re configuring new hosts into a VLAN but they’re not working. Here are the steps we’ll follow to troubleshoot VLANs: 1. Verify the VLAN database on all your switches. 2. Verify your Content Addressable Memory (CAM) table. 3. Verify that your port VLAN assignments are configured correctly. And here’s a list of the commands we’ll be using in the following sections: Show vlan Show mac address-table Show interfaces interface switchport switchport access vlan vlan Trunk Troubleshooting You’ll need to troubleshoot trunk links when you lose connectivity between hosts that are in the same VLAN but are located on different switches. Cisco refers to this as “VLAN leaking.” Seems to me we are leaking VLAN 10 between switches somehow. These are the steps we’ll take to troubleshoot VLANs: 1. Verify that the interface configuration is set to the correct trunk parameters. 2. Verify that the ports are configured correctly. 3. Verify the native VLAN on each switch. And here are the commands we’ll use to perform trunk troubleshooting: Show interfaces trunk Show vlan Show interfaces interface trunk Show interfaces interface switchport Show dtp interface interface switchport mode switchport mode dynamic switchport trunk native vlan vlan |