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Configuring OSPF with loopback address

Posted on 2008-10-28
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Last Modified: 2008-10-28
I'm at it again using the simulator.  I'm trying to configure three routers to talk to each other using OSPF.  Each of these routers have loopback address.  Is this the right way to configure the routers (especially the second router with two interfaces?  (all routers in area 0)  I don't know if the wildcard masks should say exact address or 0.0.255.255 since all the routers are in area 0.  Thank you!

Router1 (IP 192.168.1.1/24, Loop 10.10.10.1/24):
R1(config)#router ospf 1
R1(config-router)#network 192.168.1.1  0.0.0.0 area 0
R1(config)int loopback 0
R1(config)ip address 10.10.10.1 255.255.255.0
no shutdown

Router2 (IP 192.168.1.2 & 192.168.2.2, Loop 172.20.100.1):
R1(config)#router ospf 2
R1(config-router)#network 192.168.1.2  0.0.0.0 area 0
R1(config-router)#network 192.168.2.2  0.0.0.0 area 0
R1(config)int loopback 0
R1(config)ip address 172.20.100.1 255.255.255.0
no shutdown

Router3 (IP 192.168.2.1/24, Loop 172.16.100.1/24):
R3(config)#router ospf 3
R3(config-router)#network 192.168.2.1 0.0.0.0 area 0
R3(config)int loopback 0
R3(oncifg)ip address 172.16.100.1 255.255.255.0
no shutdown
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Question by:elly960
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Expert Comment

by:JFrederick29
ID: 22822220
That will work but if you want to be able to communicate between loopbacks, add them to OSPF as well.

R1(config-router)#network 10.10.10.1 0.0.0.0 area 0

R2(config-router)#network 172.20.100.1 0.0.0.0 area 0

R3(config-router)#network 172.16.100.1 0.0.0.0 area 0
0
 
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Expert Comment

by:that1guy15
ID: 22822304
You are right about the wildcard mask. 0.0.255.255 and 0.0.0.0 will both work but 0.0.255.255 will be more efficient.
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Accepted Solution

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JFrederick29 earned 125 total points
ID: 22822409
Better yet, if you want to run OSPF on all interfaces, simply do this:

R1(config)#router ospf 1
R1(config-router)#network 0.0.0.0 255.255.255.255 area 0

R1(config)#router ospf 2
R1(config-router)#network 0.0.0.0 255.255.255.255 area 0

R3(config)#router ospf 3
R3(config-router)#network 0.0.0.0 255.255.255.255 area 0

"0.0.0.0 255.255.255.255" matches all interfaces on the router.
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Author Comment

by:elly960
ID: 22822861
JFrederick,
Using your idea above, s this correct?

R1:
int g1/0
ip address 192.168.1.1 255.255.255.0
no shutdown
int loopback 0
ip address 10.10.10.1 255.255.255.0
no shutdown
R1(config)#router ospf 1
R1(config-router)#network 0.0.0.0 255.255.255.255 area 0

R2:
int g1/0
ip address 192.168.1.2 255.255.255.0
no shutdown
ing g2/0
ip address 192.168.2.2 255.255.255.0
no shutdown
int loopback 0
ip address 172.20.100.1 255.255.255.0
no shutdown
R2(config)#router ospf 2
R2(config-router)#network 0.0.0.0 255.255.255.255 area 0

R3:
int g1/0
ip address 192.168.2.1 255.255.255.0
no shutdown
int loopback 0
ip address 172.16.100.1 255.255.255.0
no shutdown
R3(config)#router ospf 3
R3(config-router)#network 0.0.0.0 255.255.255.255 area 0
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Expert Comment

by:JFrederick29
ID: 22822923
Looks good.  All interfaces will run OSPF.
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Author Comment

by:elly960
ID: 22823538
It worked great!  A quick question... when would I use the IP specific wildcard mask instead of 255.255.255.255?
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Expert Comment

by:JFrederick29
ID: 22823574
Well, this should only be used in instances when you desire running OSPF on "ALL" router interfaces in the "SAME" area.  In a multi-area OSPF setup, this won't work for you since you need to specify different areas per network/interface.
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Author Comment

by:elly960
ID: 22823866
Thank you so much for all your help!
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