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Switch and Router Troubleshooting Tips

Switch and Router Troubleshooting Tips. Warren Toomey. Two Basic Strategies. #1 Try to isolate the problem Confirm which parts of the system work Don’t try to test everything at once Example: don’t ping across 3 routers, start with a ping between two directly connected devices

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Switch and Router Troubleshooting Tips

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  1. Switch and Router Troubleshooting Tips Warren Toomey

  2. Two Basic Strategies • #1 Try to isolate the problem • Confirm which parts of the system work • Don’t try to test everything at once • Example: don’t ping across 3 routers, start with a ping between two directly connected devices • #2 Work up the ISO layers • Layer 1: physical • Layer 2: data link • Layer 3: basic IP addressing • Layer 3: IP routing

  3. Layer 1: Physical Layer Check your cables: straight-through, crossover, are they damaged? Are the cables plugged in to the correct ports? Is power applied to the device? What are the link lights showing you? Red lights: no link Amber lights: spanning tree not yet done

  4. Layer 2: Data Link • Switches: check your mac-address table • Are you seeing the right addresses on the right ports? • You may need to clear your mac-address table • If you moved devices around • You may need to clear your arp table also • If you moved devices around • or if you changed IP addresses on a device

  5. Layer 2: Interfaces • Are your interfaces up? No shutdown? • Show interface fa0/1 • Administratively down: do no shutdown • Down and down: physical problem • Link is up, line protocol down: • Encapsulation is wrong • Example: one side is access, other is trunking • Ensure both sides using same encapsulation

  6. Layer 2 WAN Interfaces • The DCE side needs a clock rate • clock rate 64000 • Both sides need the same encapsulation • HDLC, PPP or Frame Relay • Otherwise line protocol is down • Are both ends in the same IP subnet? • The two usable IP addresses in a /30 subnet • Can they ping each other?

  7. Layer 2: VLANs and Access Ports • Are the access ports in access mode? • Is each access port on the right VLAN? • show vlan brief • Does every switch know about the VLANs? • Do vlan num, vlan num, on every switch • Show vlan brief should show the same VLAN numbers on all switches

  8. Layer 2: Trunk Lines • Are the ports on both ends configured in trunk mode? • Do not rely on one side automatically turning trunking on • show run, show interface trunk • Are you permitting only a list of VLANs across the trunk? • Why? And is the list correct?

  9. Layer 3: IP Addresses • Do end devices have the correct IP address, netmask and default gateway address? • Can you ping between all the devices on the same subnet? • If not, check addressing • If not, check they are on the same VLAN • Which means checking the VLAN numbering, access ports and trunk lines again • Can you ping your default gateway?

  10. Layer 3: Router Interfaces • Some routers will use physical interfaces, not sub-interfaces • Have you done no shutdown on the interface? • Have you set up a correct IP address, mask on that interface? • What information is given by doing: • show interface fa0/0 • show ip interface brief • Can you ping all router interface addresses from machines on the same subnet?

  11. Layer 3: Router Subinterfaces • Have you removed the IP address on the interface? • Have you set up sub-interfaces on the router for each VLAN? • Check consistency with: • Sub-interface number, VLAN number • IP address and correct mask on that VLAN • Can you ping all router sub-interface addresses from machines on the same subnet?

  12. Layer 3: Directly Connected Networks • Can you ping between subnets? • Does the router show all directly connected subnets? • show ip route • Are the network ids and masks correct? • Static routes: check the routing table • Are the static routes correct?

  13. Routing • Is routing turned on? • Are you using RIP version 2? • Are you doing no auto-summary for RIP or EIGRP? Otherwise, it will be in classful mode • What networks are you advertising? • Do you have any passive interfaces • Have you learned any dynamic routes from other routers? • Can you ping other routers?

  14. Routing Table show ip route Check for all directly connected networks and correct masks Check that you have learned about all networks in your design, including WAN links

  15. Routing Protocol • show ip protocol • Shows you what routing protocol(s) you are running • And also what networks you are advertising • Are they correct? • Is passive interface causing any problems?

  16. EIGRP and OSPF • Are you using a consistent EIGRP autonomous system number? • Are you in OSPF area 0 process 1? • Have you got neighbours? • show ip eigrp neighbors • show ip eigrp topology • show ip ospf neighbor

  17. Pings and Traceroute • Ping within one network (VLAN). Check it’s OK • Ping across one router to the next network • Ditto • Repeat the process until you reach the destination or something fails • Problem is isolated somewhat • Use traceroute/tracert to do the same thing

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