Filtering OSPF routes sent to SP

[roger perkin]

I have a challenge currently.

We have a switch running OSPF with is peering with our service provider to get our network routes into their cloud.

Currently the service provider has a redistribution setup on BGP to bring in OSPF routes referencing a prefix list.

This prefix list only permits 10.10.0.0/16

I have another list of networks that I want to bring onto one site and get into the SP OSPF

Method 1:

Get  the SP to add 6 lines to their prefix list referencing the new  networks. This is not a very scaleable solution as I will have to do  this each time I want to add a network.

Method 2:

Get the SP to change the prefix-list to be permit 10.0.0.0/9 - our full internal address space.

Then on the switch connecting to the router, filter the OSPF networks I am sending them, so I have control.

How  do I best achieve this? I started looking at distribute lists but this  only seems to work for stuff coming in, I want to affect stuff I am  sending out?

Thanks for any advice

Roger

[Shahzad Qadir]

Hello

Within an area ospf database need to be same for all routers so you can't do filtering in or out, even when you apply distribute list it only filters routes to go into routing table and routes still exist in database.

What you can do here is assign a different ospf area to your wan link and then use a filter-list command to filter prefixes between your network and your ISP.

[ashok_boin]

Hi,

The SP's generally will not permit flexibility like to accept all networks in 10.0.0.0/9 to avoid security issues as well as to control the routes which they are injecting into external cloud.

You may ask SP to provide you max number of prefixes control configured on the neighbor pointing to you like 6 or 10 so that you have flexibility to advertise up to 6 or 10 prefixes which you can freely advertise.

Regards...

-Ashok.

[roger perkin]

Thank you for your comments, I have not asked the question of SP yet about the 10.0.0.0 but I woudl imagine they would not be too keen!

Could I use a filter list if we could get the WAN router in a different area?

I think I might just leave it at the updated prefix list and stop trying to fix a problem that probably doesn't really exist!

I just lik to know how everything works - I have learnt a lot about distribute-list and ospf today!

Thanks

Roger

[Shahzad Qadir]

Yes you can filter between areas using filter list. Distribute list is more of a distance vector protocol thing, use filter-list instead for ospf.

[ashok_boin · Accepted solution]

Hi Roger,

I am sorry as I have not looked into your question properly.

Yes, you can control your routers advertisement with "area filter-list" command between areas. This is possible if your internal routes are in one area and you are connected to SP in another area. This is a great feature which is not violating the basic OSPF principle to have "Identical AREA topology databases" in OSPF.

You may refer the following link if you would like to check a small lab note with regards to this command.

http://www.networkers-google.com/nwgoogle/Routing/OSPF/How%20Area%20filter%20list%20work1306146737.html

Regards...

-Ashok.

[roger perkin]

Ashok,

That has answered my question fully - thank you

Need to do a bit of re-design now and this will work exactly as I want it to

Roger

[ashok_boin]

Hi Roger,

Glad that it helped you out. Thank you.

-Ashok.