Routers 5 and 1 have valid and best routes. (NEXT HOP SELF)

Router 2 and 3 do not have a valid and best route.

Let’s do next hop self.

Now both 2 and 3 have valid and best.  So how can we make 2 and 3 to not know of the existence of the 16, 17, 18 ,19 routes, but we do want the 55 network and all networks added to router 5/advertised in the future.

The first problem is finding where we put the prefix list.

Router 1 is the HUB so the only place to put the filter is router 1.

First write you prefix list.

 The purpose is to deny. Remember the implicit deny that exits as well.

Now we need to put in the prefix list equivalent of permit any.

This rule matches everything other than the denys we’ve created.

Lets now apply this prefix list

Now the routes are filtered.

Routes 16,17,18,19 are all gone but 5 is being advertised