Common OSPF configuration issues

Common OSPF configuration issues
2.3.7 


This page will discuss some configuration issues that could prevent communications between OSPF routers.
An OSPF router must establish a neighbor or adjacency relationship with another OSPF router to exchange routing information. Failure to establish a neighbor relationship is caused by any of the following reasons:

• Hellos are not sent from both neighbors.
• Hello and dead interval timers are not the same.
• Interfaces are on different network types.
• Authentication passwords or keys are different.

In OSPF routing it is also important to ensure the following:

• All interfaces have the correct addresses and subnet mask.
• network area statements have the correct wildcard masks.
• network area statements put interfaces into the correct area.

The next page will discuss some show commands.

Verifying the OSPF configuration
2.3.8 


This page will explain how show commands can be used to troubleshoot OSPF.
To verify the OSPF configuration a number of show commands are available. Figure lists these commands. Figure shows commands useful for troubleshooting OSPF.
This page concludes this lesson. The next page will summarize the main points from this module.

Configuring OSPF timers / OSPF, progagating a default route

Configuring OSPF timers
2.3.5

This page will explain how the hello and dead intervals are configured on an OSPF network.
OSPF routers must have the same hello intervals and the same dead intervals to exchange information. By default, the dead interval is four times the value of the hello interval. This means that a router has four chances to send a hello packet before being declared dead.
On broadcast OSPF networks, the default hello interval is 10 seconds and the default dead interval is 40 seconds. On nonbroadcast networks, the default hello interval is 30 seconds and the default dead interval is 120 seconds. These default values result in efficient OSPF operation and seldom need to be modified.
A network administrator is allowed to choose these timer values. A justification that OSPF network performance will be improved is needed prior to changing the timers. These timers must be configured to match those of any neighboring router.
To configure the hello and dead intervals on an interface, use the following commands:

Router(config-if)#ip ospf hello-intervalseconds

Router(config-if)#ip ospf dead-intervalseconds 

OSPF, propagating a default route

2.3.6

This page will teach students how to configure a default route for an OSPF router.
OSPF routing ensures loop-free paths to every network in the domain. To reach networks outside the domain, either OSPF must know about the network or OSPF must have a default route. To have an entry for every network in the world would require enormous resources for each router.
A practical alternative is to add a default route to the OSPF router connected to the outside network. This route can be redistributed to each router in  the AS through normal OSPF updates.
A configured default route is used by a router to generate a gateway of last resort. The static default route configuration syntax uses the network 0.0.0.0 address and a subnet mask 0.0.0.0:

Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address ]

This is referred to as the quad-zero route, and any network address is matched using the following rule. The network gateway is determined by ANDing the packet destination with the subnet mask.
The following configuration statement will propagate this route to all the routers in a normal OSPF area:

Router(config-router)#default-information originate

All routers in the OSPF area will learn a default route provided that the interface of the border router to the default gateway is active.
The Lab Activities will help students configure an OSFP network and then set up a default route.
The next page will discuss some important considerations for OSPF routers.