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ospf config



I have come accross a config on one of our routera. Can somone help me understand what the below does


distribute-list prefix ospf-filter in
ip prefix-list ospf-filter seq 1 permit

Is the above saying do not learn any routes coming in? what is permit doing?

interface FastEthernet0/0
 ip address
 speed 100
interface FastEthernet0/1
 ip address
 duplex auto
 speed auto
interface ATM0/1/0
 no ip address
 no atm ilmi-keepalive
 dsl operating-mode auto
router ospf 3778
 network area 0
 network area
 distribute-list prefix ospf-filter in

ip prefix-list ospf-filter seq 1 permit
logging source-interface FastEthernet0/0

VIP Purple

Hellothis distribute-list is


this distribute-list is allowing only a default route into the route table and deny everything else.




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HelloThere are 5 ways to


Just adding some more information to what Mr PAUL has said.

There are 5 ways to perform OSPF Route Filtering

1. DISTRIBUTE LIST - Filters all LSAs from the Routing Table, but they stay in the OSPF Database

!!!distribute-list OUT works on both, routing table and OSPF database, but ONLY on ASBR for LSA5 and 7!!!

In Your case you are matching the exact default route with the help of prefix list and you have used it in the distribute-list.

2. FILTER LIST - Filters only LSA3, so - only on ABR, but filters from OSPF Database.
filter-list can be applied: IN - into the area, OUT - out of the area

3. NOT-ADVERTISE - ONLY filter LSA Types 1 and 2, apply on ABR (filters both, routing table and OSPF Database)
Can be used with both, "area X range" (ABR) and "summary-address" (ASBR) commands

4. DISTANCE - Set the AD of the advertised routes to 255, so that they are UNREACHABLE

5. DATABASE-FILTER - If you want to prevent ANY LSAs from being advertised (can be applied per neighbor or on INT):

you may also refer

Regards Thanveer "Everybody is genius. But if you judge a fish by its ability to climb a tree, it will live its whole life believing that it is a stupid."

Hi, a nice blog but I wouldn



a nice blog but I wouldn't agree on 3.

My understanding is that the area range command suppresses the automatic origination of Type-3 LSAs on ABRs for the component routes matching the configured network. By default (advertise keyword), the ABR will advertise a Type-3 summary instead of those components, and with the not-advertise keyword it does not. So actually it filters Type-3 LSAs.

The area filter-list command is a more sophisticated way for Type-3 LSA filtering, introduced in IOS 12.0(15)S. I guess that before, the area range not-advertise command was the only way to achieve Type-3 LSA filtering (introduced already in IOS 10.0).




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