OSPF Question

[James Hoggard]

I have a few questions in regards to the below.

The prefix list and distribute list are they linked? Is this saying it will only receive a default route and nothing else? Is this to make the routing table small? Is this the best way to filter routes would it not be best to use a total stubby area. Is this poor design??

Second question is the area's these have been put into? on an ABR I thought one had to connect via area 0?? then there is a command that has put 37.0 as an NSSA?

Hope you can help

interface FastEthernet0/0
 ip address 192.168.37.38 255.255.255.0
 speed 100
 full-duplex
!
interface FastEthernet0/1
 ip address 172.19.47.65 255.255.255.192
 duplex auto
 speed auto
!
interface ATM0/1/0
 no ip address
 shutdown
 no atm ilmi-keepalive
 dsl operating-mode auto
!
router ospf 10
 router-id 192.168.37.38
 log-adjacency-changes
 area 192.168.37.0 nssa
 passive-interface default
 no passive-interface FastEthernet0/0
 network 192.168.37.0 0.0.0.255 area 192.168.37.0
 network 172.19.47.64 0.0.0.63 area 192.168.37.0
 distribute-list prefix ospf-filter in
!
!
ip prefix-list ospf-filter seq 1 permit 0.0.0.0/0

Thanks

[Giuseppe Larosa]

Hello James,

>> Is this saying it will only receive a default route and nothing else?

No, OSPF is a link state protocol the distribute list is just a way to control what OSPF prefixes go to the IP routing table, but it does not influence the contents of the OSPF database.

With this configuration only the OSPF 0.0.0.0./0 prefix is installed in the local node IP routing table.

However, a router downstream to this one would install more then the default route if other prefixes are present in the area database!

The 192.168.37.0 area is a not so stubby area NSSA you can see from the line 

area 192.168.37.0 nssa

So yes, in some terms this is a poor design and the appropriate configuration to minimize OSPF database and IP routing table size should be done on the ABR between area 0.0.0.0, and area 192.168.37.0 making the area a totally NSSA (using no-summary at the ABR router)

The use of NSSA would allow to the local router to inject routes in the OSPF domain that can be converted to standard external routes in OSPF ABR node.

In other terms if no router in area 192.168.37.0 is performing redistribution a totally stub area would be a better fit as you noted

Hope to help

Giuseppe