I need conceptual clarification on OSPF "DISTRIBUTION-LIST" & "FILTER-LIST".
I labbed it and my result is below:
Using DISTRIBUTION-LIST in/out --- Identified prefix is not installed in AREA 0 but advertised to other non-backbone areas
Using FILTER-LIST in/out ---- Identified prefix is installed in AREA 0 but not advertised to other non-backbone areas
What is the real cause of this behaviour.
I read in one of the artcle as below:
" Now one important aspect of this process. The re-generated summary LSAs are generated AFTER applying the OSPF filter associated with the routing-process via the distribute-list in command. Thus, if you filter some of the inter-area routes from entering the RIB, the respective new summary LSAs will NOT get generated. This will stop routing information propagation into the attached non-backbone areas."
But my observation on DISTRIBUTION-LIST contradictly allows the denied prefix to be advertised to non-backbone areas . but not appearing in RIB of area 0 router. The configuration is below
router ospf 1
area 1 range 10.10.0.0 255.255.252.0
area 2 range 172.16.0.0 255.255.252.0
network 192.168.1.0 0.0.0.3 area 1
network 192.168.1.4 0.0.0.3 area 2
distribute-list 1 in
access-list 1 deny 10.10.4.0 0.0.0.255
access-list 1 permit any
Can you share your valuable ideas and shed some light on this
>> Thus I thought RIB information in AREA 0 plays a major role here. Thus I am surprised, how when a route is removed (example 172.16.1.0) from RIB of AREA0 using DISTRIBUTION-LIST, is been sent as LSA 3 to AREA 0.
this is exactly the problem with distribute-list and OSPF.
distribute-list can just influence if an IP prefix is installed in the local routing table but it cannot "filter" the corresponding LSA.
So if you have a router RD in area0 downstream ABR (0,2) and ABR (0,2) has a distribute-list that filters 172.16.1.0/24 the corresponding LSA reaches RD and RD installs the route in its routing table.
I didn't try distribute-list as a way to perform inter-area filtering and I don't recommend it even it would work.
the right tools are the first two methods with area filter list providing the greatest flexibility.
I would suggest you to use books like Jeff Doyle routing TCP vol. I and to look at RFC2328.
Very good but difficult to read is Halabi's introduction to OSPF.
Hope to help