RIP Filtering
There are three commonly used command for Distance Vector routing protocol filtering, especially for RIP Filtering.
distribute-list
offset-list
distance
Before going to details, we need an access-list. From IEWB Lab2, Lab4 and Lab6, there is a common statement that: Routes learned from Back Bone n that have an { odd | even} {1st | 2nd | 3rd | 4th} octet should be {accept | not accept | metric of #}. We all know that the last significant digital determine odd or even. “1″ is odd and “0″ is even. The wildcast in access-list will be 254 (care bits). For example, if request that R3 should not accept any routes from BB3 that have an odd number in the firest octet, the access-list will be:
ip access-list standard FIRST_OCTET_ODD
permit 1.0.0.0 254.255.255.255
offset-list is to change the route metric of distance vector prefix. In RIP, the maximum metric is “16″, which means infinite. So, we can use “offset-list FIRST_OCTET_ODD in 16 fa1/1″ to invalidate the route from routing table. Or, we can reassign metric value to the route. “offset-list FIRST_COTET_ODD in 9 vlan33″ will set the route metric to “10″.
distance command will revise administrative distance. By altering the administrative distance of these routes to infinite (255), they will be removed from the IP routing table. “distance 255 204.12.1.254 0.0.0.0 FIRST_OCTET_ODD” statement means that prefixes learned from the neighbor 204.12.1.254 that are matched in access-list will have their distances changed to 255.
Of course, the most common way to filter off a routing prefix in a distance vector protocol is to use the distribute-list command. A distribute-list is a way to apply an access-list to routing protocol updates. A routing prefix may also be filtered out by poisoning the metric or distance of the route.
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