OSPF on CCNA exam.

viper75 · October 2004

I'm currently studying OSPF for the CCNA exam or INTRO.

If I need to setup OSPF, am I required to setup a Loopback interface? or Can you just configure basic OSPF without a Loopback Interface and would it work? When do you need to setup a loopback interface and when not?

Thanks!!!

tunerX · October 2004

You absolutely need a loopback when you are doing ip-unnumbered. Other than that, it is not absolutely necessary. If you do not use a loopback and the interface that has the router ID goes down, the router must resend all of its information out all interfaces that are an OSPF network statement.

viper75 · October 2004

At home I have four 2501 routers configured with OSPF and I do not have a Loopback Interface setup on the routers yet....all 4 routers can see and talk to eachother, all clients can see ping eachother.

How can I test the OSPF setup at home once the Loopback Interfaces have been setup to see how the loopback interfaces work???

2 of the end routers are connected to 2900 switches with 1 client on each end.

Thanks!!!

tunerX · October 2004

You should work with ip-unnumbered.

conf t
int lo0
ip address 10.0.0.1 255.255.255.255
no shut
int serial 0
ip unnum lo0
no shut
int serial 1
ip unnum lo0
no shut
router ospf 1
network 10.0.0.1 0.0.0.0 area 0
end
clear ip ospf

I always use the 10 network for loopbacks. It is recommended that you use and all 1s subnet mask when you work with loopbacks. It saves addresses.

do the above config. But change the ospf process id to the one you use. Also change the last octet of the loopback addresses and also for the network statement.

10.0.0.1
10.0.0.2
10.0.0.3
10.0.0.4

After the clear ip ospf command you should be able to do a show ip ospf neighbor and see the new router ids.

viper75 · October 2004

Ok, thanks man...I'll try that setup out and see how it works.

wildfire · October 2004

You absolutely need a loopback when you are doing ip-unnumbered

No stricty true, you dont have to have a loopback interface, as if you dont OSPF will use the highest IP address as its OSPF address. Also the CCNA normally has you using an ip for serial ports and not IP-unmumberd interfaces.

tunerX · October 2004

with unnumbered you can reference any interface but most articles and references suggest that a loopback interface should be used.

forbesl · October 2004

You absolutely need a loopback when you are doing ip-unnumbered

You can use an ethernet interface for IP unnumbered. Although not recommended, it can be done just as easily.

OSPF on CCNA exam.

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