OSPF WAN Routing Question

[jc84_]

Hi All,

Just wanted to hear some feedback on a design question.

Scenario

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I will be getting a WAN connection to a few offices and I have a need to control routes recieved and advertised to/from them.  The service provider will be placing a CPE device on-site and will support OSPF with my edge router; in this case a Cisco 2821.  That 2821 router will ideally be configured with OSPF routing toward my two core switches.

Routing Requirements

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-> C2821 to NOT have the full routing table from the Core switches

-> Only needs knowledge of two routes from the Core switches and routes from remote offices.

-> Controlled routing advertisements.  I do not control the remote offices and would like to ensure they do not accidentally advertise routes into my enviroment that could create a conflict.

I'm assuming the Service Provider will be running BGP on their CPE router, which will mean that the OSPF routes recieved by my Cisco 2821 edge router will be OSPF E2 routes.  So if thats the case the 2821 would need to advertise E2 routes.

I'm not sure if I should be configuring the 2821 in Area 0...because its meant to be a WAN edge router; but if I configure it in another area...say 200...the Service Provider may configure his CPE router in Area 0...which I'm guessing would pose a problem as the 2821 would be lodged in between two area 0s?

From the reading I've done it sounds like I could use NSSA...but I'm not sure if this is the best design.  So I'm seeking some insights from the group ...and I've attached a logical diagram in an effort to help.

[Mahesh Gohil]

Hello Jeff,

Few suggestion

> To make sure remote office do not advt. routes which you do not want you can create two ospf process
  and redistribute only those routes to the process of your interest. Not scalable solution but a robust
  one. But those routes will be available as E2 routes.

> If SP is running bgp you will get E2 routes but if you ask them to run ospf at their cpe (need confirmation from provider) then you will have chance to get interarea routes

> I advise to configure area 0 at boundary router i am sure SP will agree on this otherwise you have to configure virtual link to cross area0

Regards

Mahesh

[jc84_]

Hi Mahesh,

I don't like the idea of running two OSPF processes.  To my that sounds like added complexity on the Core switches and the feeling I have is doing that starts me down the wrong path.

BGP is the most common routing protocol for the PE-CPE design; I don't know if the SP will change this...more so, I don't know if making the SP change something on his network to allow me to make something work is a good thing either.

[Adrian Lazar]

Hi Jeff,

You could simply filter what you receive via BGP from the provider in order for your branch offices not to advertise some overlapping routes and then redistribute in OSPF. The NSSA area is fully viable for you situation (actually it was created especially for this kind of topology) so from my point of view there should not be any issues for you.

Best regards,

Adrian

[jc84_]

Hi Adrian,

Are you suggesting that I run BGP with the Providers CPE router?  and an OSPF process facing my Core switches?

[Adrian Lazar]

Yes, this is pretty much what I'm suggesting. Run BGP with the provider, filter the routes you receive and then redistribute into OSPF NSSA area.

Best regards,

Adrian