Mutlihoming BGP

Unanswered Question
Sep 12th, 2010

Hi Guys,

Need some advice on the best way to implement the attached.

So R1A and R1B will be advertising their local networks (10.0.10.0/24 and 10.0.20.0/24) There is a 100Mb interconnect between R2A and R2B and then somewhere within the MPLS cloud there is another 30Mb link which is the backup

Now I want to run my IGP as OSPF, R2A and R2B will be sending out a default route via OSPF, the routers within the "cloud" should pick up their default gateways automatically via metrics, weights etc. However the problem that I have is that if R2A dies, R1A will still be advertising 10.0.10.0/24. So traffic will go out via R1B but try to get back in via R1A. How can I get around this problem?

The second problem is that I do not know the best way to connect R1A with R2A, should I expand my OSPF area 0 to R1A or use a default route with something like IP SLA?

Any help will be much appreciated,

Loopback99.

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ashok_boin Mon, 09/13/2010 - 01:42

Hi,

How did you configure R1A advertisment of 10.0.10.0/24 (network command, redistribute etc)?

And you can utilize BGP conditional advertisement feature at R1A (for eg) and can start/stop advertising specific subnets depends upon the reachability to R2A.

Please refer the follownig link for details.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094309.shtml

Regards...

-Ashok.

Loopback99 Mon, 09/13/2010 - 04:52

Ah OK, I see where you are going with that. At the moment its just network x.x.x.x mask x.x.x.x but I guess I could redistribute my OSPF subnets into BGP and filter them with route-maps...would that work? If it would then what relationship do you think R1A and R2A should have? Is the above also vulnerable to route flapping to my upstream provider?

I should also mention that my 2 upstream transit providers provide me with a default route only.

Thanks, L99.

ashok_boin Mon, 09/13/2010 - 22:13

Yes, redistributing OSPF subnets with careful filtering with route-maps will work. This will directly translate the advertisement or withdrawal of your internal networks based upon OSPF advertisements. You may tune BGP dampening parameters as well.

Otherwise, you can keep the existing configuration (network xxx) as it is and utilize the BGP conditional advertisement of 10.x.x.x by tracking the reachability to R2A.

Regards...

-Ashok.

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