Refer to the exhibit. An engineer must prevent the R6 loopback from getting into Area 2 and Area 3 from Area 0. Which action must the engineer take?
Refer to the exhibit. An engineer must prevent the R6 loopback from getting into Area 2 and Area 3 from Area 0. Which action must the engineer take?
To prevent the R6 loopback address from reaching Area 2 and Area 3 from Area 0 in an OSPF network, you need to apply a filter list inbound on R3 and R7. This method stops the specific route from being installed into the specified areas (Area 2 and Area 3) when it comes from Area 0. The filter list applied inbound ensures that the Type 3 LSAs (which include the R6 loopback address) are not propagated into these areas, thus effectively preventing their advertisement into Area 2 and Area 3.
An Inbound filter list on R3 and R7 would prevent the route from being installed in Area 0, the criteria were to make sure it did not reach Area 2 and 3 only. An outbound filter can be applied on ABR's, so I believe A is the answer.
WRONG. distribute-list out command. This command only works on the routes being redistributed by Autonomous system boundary Router into OSPF https://community.cisco.com/t5/networking-knowledge-base/distribute-list-out-command-in-ospf/ta-p/3120931 The correct answer it´s "C"
What does that command have to do with the question? You apply a filter list using the "filter-list prefix" command, where prefix is a previously defined rule that blocks the network we don't want to reach Area 2 and 3. The outbound filter list applied on R3 and R7 would stop advertising LSA3 out of all it’s other connected areas (including area 0). So it would work.
No, outbound direction is NOT correct. CISCOs definition of that command is: Router(config-router)# area <area-id> filter-list prefix <prefix-list-name> in >> Configures the router to filter interarea routes INTO the specified area. You are arguing with device direction view you know from ACLs, but filter-lists have area direction views, which are different. So INBOUND direction is correct.
You are NOT correct! Outbound for filter lists is not correct. FIlter-lists are related to areas, not devices. So.. area 2 and area 3 from view of R3 unf R7 is INBOUND!
The correct answer it´s "C" Tested this in eve-ng. === ON R7===== ip prefix-list BLOCK seq 5 deny 6.6.6.6/32 ip prefix-list BLOCK seq 10 permit 0.0.0.0/0 le 32 router ospf 1 area 2 filter-list prefix BLOCK in === ON R3===== ip prefix-list BLOCK seq 5 deny 6.6.6.6/32 ip prefix-list BLOCK seq 10 permit 0.0.0.0/0 le 32 router ospf 1 area 3 filter-list prefix BLOCK in
This one is tricky if you don't understand the logic of Filter-Lists in OSPF. In a nutshell, a Filter-List can only be applied at the ABRs to filter TYPE 3 LSAs, so this immediately indicates you can only apply the filter list at R7 and/or R3. Now, the direction of the filter list is very important to understand. Let's take an example configuration: area 2 filter-list prefix DENY [ in | out ] Here we use area 2 as an example, and we have referenced a prefix-list named DENY. Now, we have two options for directions, INBOUND and OUTBOUND. The IN keyword states that we will filter prefixes based on the prefix-list referenced towards the area identified in the command. Meaning that TYPE 3 LSAs going to Area 2 will be filtered based on the prefix-list. Inversely, if we used OUT, it means that TYPE 3 LSAs coming FROM Area 2 will be filtered based on the prefix-list.
very clear explanation from hugodiaz as alwasys !!
on R7 you are referencing Area2 and therefore filter list IN on R3 you are referencing Area3 and therefore filter list IN
It is C, not A. I made the same mistake in the first place. Applying route filtering in OSPF is not like applying ACLs. In fact, it works the other way around. You must think from the ABR perspective that is - what LSAs should I pass from area 0 into area 1 or 3? This is why it has to be IN, not OUT.
To prevent the R6 loopback from getting into Area 2 and Area 3 from Area 0, the engineer should apply a filter list inbound on R3 and R7. This will filter out the routes coming from R6's loopback interface and prevent them from being advertised into Area 2 and Area 3. So, the correct answer is: C. Apply a filter list inbound on R3 and R7.
My mistake. This is incorrect. The answer is A.
Only on ABRs, and only for LSA T3 'in' filters LSA type 3 into an area 'out' filters LSA type 3 out of an area
R7 ip prefix-list FILTRO seq 5 deny 6.6.6.6/32 ip prefix-list FILTRO seq 10 permit 0.0.0.0/0 le 32 router ospf 1 area 2 filter-list prefix FILTRO in R3 ip prefix-list FILTRO seq 5 deny 6.6.6.6/32 ip prefix-list FILTRO seq 10 permit 0.0.0.0/0 le 32 router ospf 1 area 3 filter-list prefix FILTRO in
filtering traffic in OSPF is inverted ACLs: inbound -> TO area outbound -> FROM area so when you apply prefix-list "area 2 filter-list prefix BLOCK in" (@Amoako) it means: filter traffic TO area 2.
thus answer should be C!
From Cisco training source: To configure Type-3 LSA filtering, use the area area-number filter-list prefix prefix-list-name in | out command under OSPF configuration mode. The referenced prefix list is used to match the subnets and masks to be filtered. The area-number and the in | out option of the area filter-list command work together, as follows: When out is configured, IOS filters prefixes coming out of the configured area. When in is configured, IOS filters prefixes going into the configured area.
Outbound will also prevent routers in area 0 to learn LSA type 3 Inbound will prevent only the targeted area to learn that LSA type 3. ! ip prefix-list FILTER_R6_Lo0 deny 6.6.6.6/32 ip prefix-list FILTER_R6_Lo0 permit 0.0.0.0/0 le 32 ! router ospf 1 area 2 filter-list prefix FILTER_R6_Lo0 in area 3 filter-list prefix FILTER_R6_Lo0 in ! So A is the best answer.
Please Ignore
Outbound will also prevent routers in area 0 to learn LSA type 3 Inbound will prevent only the targeted area to learn that LSA type 3. ! ip prefix-list FILTER_R6_Lo0 deny 6.6.6.6/32 ip prefix-list FILTER_R6_Lo0 permit 0.0.0.0/0 le 32 ! router ospf 1 area 2 filter-list prefix FILTER_R6_Lo0 in area 3 filter-list prefix FILTER_R6_Lo0 in ! I meant C is the best answer.
A is the correct answer, Outbound from Area 0 into Area 1 and 2 will be prevented but it will appear in Area 0 and not in Area 1 and 2. Guess this answers the question https://www.youtube.com/watch?v=PUNo0VHzHzI
Correct answer is C. Tested it on GNS3 you need to look from the perspective of the are that the LSA will be propagated, in that case R3 and R7 will take the prefix 6.6.6.6/32 and "put" it in are 2 and 3, which means that you need to filter this prefix from entering the area. like this: ip prefix-list OSPF seq 10 deny 6.6.6.6/32 ip prefix-list OSPF seq 20 permit 0.0.0.0/0 le 32 router ospf 1 area 3 filter-list prefix OSPF in
I'll go with A
i think C is the correct answer
OSPF Route Filtering Inbound Filtering: When you apply a filter list inbound on a router, you prevent specific routes from being accepted into the routing table from a specific neighbor or area. This means the routes will not be learned by that router from its neighbors. Outbound Filtering: When you apply a filter list outbound on a router, you prevent specific routes from being advertised to specific neighbors or areas. This means the routes will not be sent out from that router to its neighbors.
c is correct- as to filter the prefix you either have to do outbound area 0 or inbound area 2 and 3.