Filtering route for OSPF

[cisco]

Hi,

      I was going through this URL , according this filters cant be applied to OSPF updates. is it true ?

regards

Neo

[mtsb]

Hi Neo,

Yes it is true. Read again below lines...

"On the other hand, routers running link state protocols determine their routes based on information in their link state database, rather than on the advertised route entries of its neighbors."

EIGRP, RIP distance vector protocols will send the actual routes in the routing update to its neighbors so route filtering makes sense on them.

But in OSPF, LSAs are being advertised to its neighbors and not the routes. Routes are being computed using these LSAs received from neighbors. So we cannot do route filtering whereas it is achieved by LSA filtering on ABRs and ASBRs.

Thanks,

Madhu

[cisco]

Hi Madhu,

                   Thanks for the explanation, just querious to know LSA filtering will be done using distribute-list only or there is some other way ?

regards

Neo

[mtsb]

Hi Neo,

In general 3 types of filtering can be done with OSPF.

1. Type 3 LSA filtering on ABR

2. Type 5 LSA filtering on ASBR

3. Filtering a route to be added into its own IP routing able on a

single route.

Case 1 is done with "area number filter-list prefix name in | out"

command under "router ospf" configuration mode.

In Case 2, you can either prevent few external routes being

redistributed into OSPF area using route-maps as a parameter along with

"redistribute" command. There is no way to filer one lsa selectively and

there is a closed bug on this CSCdw49150 which is visible only inside

Cisco ddts tool.

Case 3 can be done using "distribute-list" command under "router ospf"

configuration mode.

You can refer to CCNP ROUTE official exam certification guide if you

want a solid foundation on routing concepts. Particularly Chapter 7

focuses on OSPF route filtering and summarization concepts in which

Wendell Odom has explained so clearly which has been an eye opener for

me in this topic.

Please rate all replies if you feel it was useful to you.

Thanks,

Madhu

[Tihomir Yosifov]

Hi,

you can filter routing updates only on ABR and ASBR. This is because in link state routing protocols, the link-state database should be the same for all routers in one area. So you have only 1 area, you have no chance to filter routing updates.

But you can filter the routes that are added to the local routing table with distribute-list [number] in command.

Regards,

Tihomir