OSPF – Filtering routes from a specfic source


In OSPF we can filter routes from a specific source through the use of a route-map. In this example we have 3 routers (R1, R2 and R3) on a common subnet. R2 and R3 are both advertising the same route 50.50.50.50/32. In this example we will filter 50.50.50.50/32 coming from R3 leaving the route from R2 in the routing table.

R1(config-router)#do sh ip route osp
50.0.0.0/32 is subnetted, 1 subnets
O       50.50.50.50 [110/11] via 10.1.1.3, 00:03:27, FastEthernet0/0
[110/11] via 10.1.1.2, 00:03:27, FastEthernet0/0

To configure

1) Create a prefix-list to match on the route

R1(config)#ip prefix-list Lo1 permit 50.50.50.50/32

2) Create an ACL match the next hop of the route.

R1(config)#access-list 3 permit host 10.1.1.3

3) Create a route-map that matches on the route in the prefix list plus matches on the IP next-hop of the route and denies them. Add an extra permit to the route-map to allow the rest of the routes.

R1(config)#route-map Filter deny 10

R1(config-route-map)#match ip add prefix-list Lo1

R1(config-route-map)#match ip next-hop 3

R1(config-route-map)#route-map Filter permit 90

4) Configure the distribute-list with the route-map statement under the OSPF process

R1(config-route-map)#router os 1

1(config-router)#distribute-list route-map Filter in

When we check the routing table we see that the route from R3 is now filtered

     R1(config-router)#do sh ip route osp
50.0.0.0/32 is subnetted, 1 subnets
O       50.50.50.50 [110/11] via 10.1.1.2, 00:00:08, FastEthernet0/0

 

Nice and easy filtering !

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About ccie4all
Hello, and welcome to the first post of my CCIE blog This blog has got one simple goal and that is to improve our skills in Cisco Networking field so we can become best engineers on a job market. Wordpress Blog https://ccie4all.wordpress.com/ information about the changes made to Gns3 BGP , MPLS and R&S CCIE labs. In order to access and download all provided materials and receive important updates from Gns3 BGP , MPLS and R&S CCIE labs under GNS3 tab in the main header please go ahead and subscribe to https://ccie4all.wordpress.com/ ! All other posts have not been affected and can be accessed at any given time. Enjoy ! Tom

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