Route Filting

This phenomenon,in which a route is advertised in the wrong direction across a redistributing router,is called route feedback.

Any time a router is performing multual redistribution-the multual sharing of routes between two or more routing protocols-route filters should be used to ensure that routes are advertised in only one direction.

Route filters work by regulating the routes that are entered into,or advertised out of ,the route table.It have no effect on link state advertisements or the link state database.

Case Study: Filtering Specific Routes

router rip

 version 2

 network 192.168.75.0

 distribute-list 1 in serial1  distirbute-list 2 out serial1

!

ip classless

access-list 1 permit 192.168.73.0 access-list 2 deny 0.0.0.0 access-list 2 permit any

When the distribute-list command is configured under link state protocols such as OSPF,the ‘out’ keyword cnanot be used in association with an interface.Because link state protocols do not advertise routes from their routing table as do distance vector prococols,ther are no updates to filter.

Case Study: Route Filtering and Redistribution

Any time a router performs mutual redistribution,the potencial for route feedback exists.

At redistribution points we can fileter by route process or by interface.

When filtering by route process,only the ‘out’ keyword is allowed.After all,it makes no sense to specify something like

distribute-list 10 in rip

under OSPF.The route has already been entered into the route table by RIP,and OSPF either advertises it (‘out’) or it does not.

Filter by interface:

router ospf 25

 redistribute rip metric 100

 network 172.16.1.254 0.0.0.0 area 25

 network 172.16.8.254 0.0.0.0 area 25

 network 172.16.50.254 0.0.0.0 area 25

 distribute-list 3 in e0/0  distribute-list 3 in e0/1  distribute-list 3 in e0/2

router rip

 redistribute ospf 25 metric 5

 passive-interface e0/0

 passive-interface e0/1

 passive-interface e0/2

 network 172.16.0.0

 distribute-list 1 in e0/3  distribute-list 1 in e2/0  distribute-list 1 in e2/1

access-list 1 permit 172.16.128.0 0.0.127.255

access-list 3 permit 172.16.0.0 0.0.127.255

Filter by route process.  distribute-list 1 out rip

 passive-interface e0/3

 passive-interface e2/0

 passive-interface e2/1

 distribute-list 3 out ospf 25 Note:

To prevent route feedback,routes must be filtered as they are incoming on an interface,before they are entered into the route tale.

Case Study: A Protocol Migration

(1) Lower the administrative distance of the old routing protocol on all routers.

(2) Add the new routing protocol, and make sure its administrative distance is higher than old

one.Then because of the AD,so no routers will prefer the new routing protocol.

   This approach gives the network administrator time to re-examine the new configurations in each router for accuracy before the conversion.

(3) When the new routing protocol had finished its conversion,changing the old routing protocol back to the higher one.After the old routing protocol’s all timers expired,the new routing protocol will instead of it.

(4) Migration finished.

One final note on using the dual-protocol procedure to migrate to a new routing protocol: Be Sure you understand the behavior of both protocols.

For example,some protocols,such as EIGRP,do not age out their route entries.Therefore,if EIGRP is being replaced,an additional step of the conversion process is to clear the route tables with the clear ip route * command after the distances have been changed.

Case Study: Multiple Redistribution Points Method -- 1 Router-1

router ospf 1

 network 192.168.3.1 0.0.0.0 area 0

 distance 130

 distance 110 0.0.255.255 1                //wildcard mask and acl number

 redistribute ospf 1 metric 2

 network 192.168.2.0

 distance 120 192.168.2.1 0.0.0.2

access-list 1 permit 192.168.4.0

access-list 1 permit 192.168.5.0

access-list 2 permit 192.168.1.0

access-list 2 permit 192.168.6.0

Router-2

 network 192.168.5.1 0.0.0.0 area 0

 distance 110 0.0.0.0 255.255.255.255 1          //wildcard mask and acl number

 network 192.168.6.0

 distance 120 192.168.6.1 0.0.0.2

access-list 1 permit 192.168.3.0

access-list 2 permit 192.168.2.0

Method --- 2

Router 1

 redistribute rip metric 100 route-map rip_ospf

 redistribute ospf 1 metric 2 route-map ospf_rip

route-map rip_ospf deny 10

 match tag 1

route-map rip_ospf permit 20

 set tag 2

route-map ospf_rip deny 10

 match tag 2

route-map ospf_rip permit 20

 set tag 1