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Ospf neighborship troubleshooting

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Ospf neighborship troubleshooting

Ospf neighborship troubleshooting

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  • 1. OSPF Neighborship TroubleshootingWhen setting up dynamic routing protocols, there are certainly a number of thingsthat need to be configured correctly for everything to end up working as planned. Ontop of this, each of the different routing protocols has different elements that theyexpect to be configured first for each of them to operate correctly. This article takes alook at these requirements from the perspective of OSPF and shows the differentcommands that can be used to ensure properOSPF neighborship configuration andcommunications between devices.OSPF Neighborship RequirementsFrom the perspective of OSPF, there are a couple of things that must match for aOSPF neighborship to establish; these include: 1. The devices must be in the same area 2. The devices must have the same authentication configuration 3. The devices must be on the same subnet 4. The devices hello and dead intervals must match 5. The devices must have matching stub flagsOSPF Neighborship Configuration Verification and TroubleshootingStarting from the top of the list, the interfaces connecting devices must be on thesame area. To display the various commands and what to look for, Figure 1 shows asimple lab has been setup with two devices that are connected together via anEthernet connection.Figure 1 - Simple LabMismatched AreasThe first thing that is going to be checked by the OSPF device is whether the remotedevice is in the same area. No other processing will occur on the device until bothdevices have been configured with the same area. Figure 2 shows an example of themessage given by thedebug ipospfadj command when the remote device is not inthe same area. As can be seen from the figure, the remote device is configured intoarea 1 (notated as 0.0.0.1) while the local device is configured for area 2 (notated as0.0.0.2).http://blog.router-switch.com/
  • 2. Figure 2 - Mismatched AreasAuthentication MismatchThe second entry on the list was that each device must have matching authenticationconfiguration; before any other information is exchanged between the devices theymust agree on an authentication type (if any is configured). If the parameters are notthe same on both sides, the neighborship process will never progress. Figure 3 showsthe error message that will be displayed (from the debug ipospfadj command) whenan authentication mismatch occurs.Figure 3 - Authentication MismatchSubnet MismatchAnother valuable command to use when troubleshooting OSPF is debug ipospf hello.As the name suggests, this command shows debugging information forhttp://blog.router-switch.com/
  • 3. the hello event s between devices. Since a neighborship starts with a hello exchange,this is a valuable command to use.The third entry on the list was that each device must be in the same subnet. Figure4 shows an example of a message that shows the engineer that a mismatch existsbetween devices and upon closer inspection it shows that the subnet mask isconfigured differently on each device. In this case the remote device was configuredin the 10.10.10.0/25 subnet while the local (connected) device was configured in the10.10.10.0/24 subnet.Figure 4 - Subnet MismatchHello and Dead interval mismatchThe fourth entry on the list is matching hello and dead intervals. When configuringthe hellointerval on an interface, the device (With Cisco equipment) willautomatically adjust the deadinterval; Figure 5 shows a case where the remotedevice was configured with a hello interval of 8 (Seconds) while the local device usesthe default setting of 10.http://blog.router-switch.com/
  • 4. Figure 5 - OSPF hello and dead interval mismatchStub Flag MismatchThe final entry on the list was that the device must agree on whether the area is astub or not. Figure 6 shows an example of a message where the devices disagree onwhether the area is a stub area or not.Figure 6 - Stub Flag MismatchWhile it can be often overlooked, a neighborship is the first thing that must beestablished before any communication will happen between devices. Each of thedifferent routing protocols has their own requirements that must be met before thisneighborship will establish. This article takes a look at the elements that must matchfor OSPF neighborships to establish and what commands to use to troubleshootwhich misconfiguration potentially exists. Hopefully, the information in this article,http://blog.router-switch.com/
  • 5. when committed to memory, will help in future OSPF configuration endeavors.More Related Networking Tips:OSPF, How to Configure OSPF in the Cisco IOS?http://blog.router-switch.com/