{"id":3714,"date":"2012-02-23T20:04:15","date_gmt":"2012-02-23T20:04:15","guid":{"rendered":"http:\/\/www.wikisee.info\/?p=3714"},"modified":"2016-08-09T12:07:29","modified_gmt":"2016-08-09T12:07:29","slug":"ospf-route-filtering-within-area","status":"publish","type":"post","link":"https:\/\/wiki-see.info\/wp\/2012\/02\/23\/ospf-route-filtering-within-area\/","title":{"rendered":"OSPF ROUTE FILTERING WITHIN AREA"},"content":{"rendered":"

There are two points at which OSPF<\/span> routes can be filtered: within an area, or between areas on an<\/span>
\narea border router (<\/span>ABR<\/span>).<\/span><\/p>\n

This article discusses the differences between the two and the considerations which<\/span>
\n should be made when\u00a0implementing OSPF<\/span> filtering.<\/span><\/p>\n

The following topology is provided for illustration of both cases:<\/span><\/p>\n

\"\"<\/p>\n

####<\/span> AREA<\/span>0<\/span>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0 \u00a0####\u00a0<\/span>AREA1<\/span><\/p>\n

The 172.16.7.0\/24<\/span> network has been implemented on R3 for testing. The route is intended only to be<\/span>
\npropagated throughout the local area 1<\/span>, but is currently being advertised to the entire<\/span> OSPF<\/span> domain.<\/span><\/p>\n

(as indicated by the green arrows in the topology):<\/span><\/p>\n

R3# show ip route<\/span><\/strong><\/p>\n

C 172.16.4.0\/24 is directly connected, FastEthernet0\/1<\/span><\/em>
\n C 172.16.5.0\/24 is directly connected, Loopback0<\/span><\/em>
\n C 172.16.7.0\/24 is directly connected, Loopback1<\/span><\/em>
\n O IA 172.16.1.1\/32 [110\/20] via 172.16.4.1, 00:11:06, FastEthernet0\/1<\/span><\/em>
\n O IA 172.16.3.1\/32 [110\/21] via 172.16.4.1, 00:06:33, FastEthernet0\/1<\/span><\/em>
\n O IA 172.16.2.0\/24 [110\/21] via 172.16.4.1, 00:06:33, FastEthernet0\/1<\/span><\/em><\/p>\n

We can implement inter-area filtering (filtering between areas) on R2<\/span> to prevent the route from being<\/span>
\nadvertised outside of area 1<\/span>. First, we define a prefix list on R2<\/span> to deny the<\/span> 172.16.7.1\/24<\/span> prefix and allow all others:<\/span><\/p>\n

ip prefix-list FILTER<\/span> seq 5 deny 172.16.7.1\/32<\/span><\/em>
\n ip prefix-list FILTER<\/span> seq 10 permit 0.0.0.0\/0 le 32<\/span><\/em>
\n!
\n<\/span>
\nR2 :<\/span><\/strong><\/p>\n

router ospf 1<\/span>
\n log-adjacency-changes<\/span>
\n area 1 filter-list prefix<\/span> FILTER<\/span> out<\/span><\/span>
\n network 172.16.2.2 0.0.0.0 area 0<\/span>
\n network 172.16.3.1 0.0.0.0 area 0<\/span>
\n network 172.16.4.1 0.0.0.0 area 1<\/span>
\n!<\/span>
\nAppending le 32<\/span> to the first prefix list entry ensures that any more-specific routes within 172.16.7.0\/24<\/span> are denied as well (as opposed to only the exact \/24<\/span> route).<\/span><\/p>\n

Complete Configuration :<\/span><\/p>\n

R1 :<\/span><\/strong><\/p>\n

interface Loopback0<\/span>
\n ip address 172.16.1.1 255.255.255.0<\/span>
\n!<\/span>
\ninterface FastEthernet0\/0<\/span>
\n ip address 172.16.2.1 255.255.255.0<\/span>
\n!
\nrouter ospf 1<\/span>
\n log-adjacency-changes<\/span>
\n network 172.16.1.1 0.0.0.0 area 0<\/span>
\n network 172.16.2.1 0.0.0.0 area 0<\/span>
\n!<\/span>
\n!<\/span><\/p>\n

R2 :<\/span><\/strong><\/p>\n

interface Loopback0<\/span>
\n ip address 172.16.3.1 255.255.255.0<\/span>
\n!<\/span>
\ninterface FastEthernet0\/0<\/span>
\n ip address 172.16.2.2 255.255.255.0<\/span>
\n!<\/span>
\ninterface FastEthernet0\/1<\/span>
\n ip address 172.16.4.1 255.255.255.0<\/span>
\n!<\/span>
\n!<\/span>
\nrouter ospf 1<\/span>
\n log-adjacency-changes<\/span>
\narea 1 filter-list prefix<\/span> FILTER<\/span> out<\/span>
\nnetwork 172.16.2.2 0.0.0.0 area 0<\/span>
\n network 172.16.3.1 0.0.0.0 area 0<\/span>
\n network 172.16.4.1 0.0.0.0 area 1<\/span>
\n!<\/span>
\n!<\/span><\/p>\n

R3 :<\/span><\/strong><\/p>\n

interface Loopback0<\/span>
\n ip address 172.16.5.1 255.255.255.0<\/span>
\n!<\/span>
\ninterface Loopback1<\/span>
\n ip address 172.16.7.1 255.255.255.0<\/span>
\n!<\/span>
\ninterface FastEthernet0\/1<\/span>
\n ip address 172.16.4.2 255.255.255.0<\/span>
\n!<\/span>
\nrouter ospf 1<\/span>
\n log-adjacency-changes<\/span>
\n network 172.16.4.2 0.0.0.0 area 1<\/span>
\n network 172.16.5.1 0.0.0.0 area 1<\/span>
\n network 172.16.7.1 0.0.0.0 area 1<\/span><\/p>\n

 <\/p>\n

Distribute-lists<\/span> do not work for outbound OSPF<\/span> filtering (even though the CLI may accept the command) <\/span><\/em>
\nas OSPF<\/span> is a link-state protocol and thus all routers within an area must flood all LSAs.<\/span><\/em><\/p>\n","protected":false},"excerpt":{"rendered":"

There are two points at which OSPF routes can be filtered: within an area, or between areas on an area border router (ABR). This article discusses the differences between the two and the considerations which should be made when\u00a0implementing OSPF filtering. The following topology is provided for illustration of both cases: #### AREA0\u00a0 \u00a0 \u00a0 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[526],"class_list":["post-3714","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-ospf-filtering"],"_links":{"self":[{"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/posts\/3714","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/comments?post=3714"}],"version-history":[{"count":1,"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/posts\/3714\/revisions"}],"predecessor-version":[{"id":4970,"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/posts\/3714\/revisions\/4970"}],"wp:attachment":[{"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/media?parent=3714"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/categories?post=3714"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wiki-see.info\/wp\/wp-json\/wp\/v2\/tags?post=3714"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}