ospf problem

[highland8]

Hello

on router 2501 i have:
#sh ru
interface Ethernet0
ip address 223.0.0.98 255.255.255.252
no ip directed-broadcast
ip ospf cost 100
ip ospf priority 0
!
interface Ethernet1
ip address 223.0.0.100 255.255.255.224
no ip directed-broadcast
ip ospf cost 100

and i wanted to enable ospf on both interfaces:
Radmin(config)#router ospf 1
Radmin(config-router)#network 223.0.0.98 0.0.0.3 area 100
Radmin(config-router)#network 223.0.0.100 0.0.0.31 area 100
Radmin(config-router)#exit

but when i checked:
Radmin#sh ip protocols
Routing Protocol is "ospf 1"
Sending updates every 90 seconds, next due in 10 seconds
Invalid after 30 seconds, hold down 0, flushed after 60
Outgoing update filter list for all interfaces is
Incoming update filter list for all interfaces is
Redistributing: ospf 1
Routing for Networks:
223.0.0.96 0.0.0.3 area 100
Routing Information Sources:
Gateway Distance Last Update
223.0.0.98 110 00:00:03
Distance: (default is 110)

i see that routing for networks: is only for 223.0.0.96/30. Why ?

Thanx
Michal

 

[jneiberger]

You have your addressing misconfigured. The IP address and subnet you have configured on Ethernet1 conflicts with Ethernet0.

In other words, 223.0.0.98/30 resides inside 223.0.0.100/27.

Fix your addressing and this problem will probably go away.

Another word of advice: always use a wildcard mask of 0.0.0.0 in your OSPF network statements. It makes troubleshooting this sort of thing much easier and it makes your wildcard mask substantially less complicated. Of course, this means you have to add a network statement for each interface but the small amount of additional work is worth the effort later.

 

[highland8]

Thanx, i've made a mistake in addressing, after correction it's working fine

 

[cyberspace]

jneiberger - You said "In other words, 223.0.0.98/30 resides inside 223.0.0.100/27"

I am unable to work out why - Any chance you could explain why that is the case?

many thanks!

 

[jneiberger]

Work out the ranges:

223.0.0.100/27 covers 223.0.0.96 through 223.0.0.127.

223.0.0.98/30 covers 223.0.0.96 through 223.0.0.99.

As you can see, both ranges begin at 96. Therefore, the smaller range is entirely encompassed within the larger range. This is an example of overlapped addressing and you must avoid it it when possible.