OSPF

OSPF route filtering

OSPF route filtering commands .

  1. intra-area routes 
    1. area range + not advertise
    2. distribute-list in ( affect the routing table only )
    3. filter-list
  2. inter-area routes 
    1. filter-list
    2. distribute-list in ( affect the routing table +flooding)
  3. external routes
    1. distribute-list out
    2. summary-address + not advertise
    3. redistribute + filter (route-map)


intra area routes

1. area range + not advertise  

  • will only work for directly connected networks.
  • this command will create a summary route, but it will not be injected to area 0.
R1(config-router)#area 1 range 172.16.0.0 255.252.0.0 not-advertise

2. distribute-list in 

  • be careful because this command behavior change with inter routes.
  • this command will only block the route from the routing table but the route will still be flooded to other areas.
    • the reason is that the route will be summarized before the distribute-list
    • in easy word, type 3 will created and flooded before the distribute-list in
  • this command will create blackhole. because the flooded route will be unreachable .
Please use this command with intra routes unless you know what you are doing 
R1(config-router)#distribute-list prefix myrange in
R1(config)# ip prefix-list block172 seq 1 deny 172.16.0.0/14 ge 24 le 24

3. Filter-list

  • will be explained in next section.

 


Inter area routes

1. Filter-list  ( intra + inter)

  • can be used at any ABR.
  • this will just prevent the LSA that received/created by a router from being inject to an area.
  • this command work with intrainter and  summarized routes .
  • this command work with routes that is going IN or coming OUT from an area
    • filter-list out will include all areas that an ABR connected to, so if you have 1 area connected to that ABR it wont make a different, but if you have two areas and want to choose then use IN
R1(config-router)#area 0 filter-list prefix block172 in  (intra)
or
R1(config-router)#area 1 filter-list prefix block172 out (intra)
or
R2(config-router)#area 0 filter-list prefix block172 out (inter)
or
R2(config-router)#area 2 filter-list prefix block172 in  (inter)
or
R2(config-router)#area 3 filter-list prefix block172 in  (inter)

2. distribute-list in

    • what this command will do is preventing the routes from entering the routing table, and as a result of that:the ABR is re-generating a type-3 lsa, the Route must be in the routing table,so this command remove a route from routing table so type-3 will not be created.
R2(config-router)#distribute-list prefix block172 in

now i think you got the idea about they work.


the question will be where to add them, here or there and why. 

check this diagram : i have added all possible points to use the above commands, you will only use one of them and that will depend on the situation.

 

on point 2 :

distribute-list in : remove the route from routing table + it will not be sent to area 2 or 3

filter-list out : filter a route from being sent to any area ( this route can be intra or inter or a summarized )

filter-list in   : will filter a route going to a specific area.

on point 1 :

distribute-list in : remove the route from routing table but the route will continue to be flooded (blackhole)

area range     : will create a summary but this summary will not be advertised to any area

filter-list out : will filter the router from being sent to any area , imagine that R1 is connected area 4 + area 0

filter-list in   : this will prevent the route to be send to specific area’s

 


external routes 

 

1.Summary-address + not advertise

  • so this command will create external summary, but this summary will not be injected.
R2(config-router)#summary-address 172.16.0.0 255.252.0.0 not-advertise

2.Distribute-list out

R3(config-router)#distribute-list prefix block200 out

3. redistribute + route map 

R3(config-router)#redistribute eigrp 1 route-map block200
R3(config)#route-map block200 permit 1
R3(config-route-map)#match ip address prefix-list block200
Standard

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