BGP Multiple Default routes, specific for each region and backup of each other

Answered Question
Mar 5th, 2012

I have a BGP/routing/design question.

Here is the example: we have remote offices in the US, Asia and Africa that go into MPLS cloud, we run BGP with our ISP. We also have 3 data centers in each region, that have ineternet exit points.   Right now we have only 1 default route that is advertised into the MPLS cloud so every office accesses Internet from one location, Asia.

My question is, what can I do to setup 3 default routes, 1 for each region.  Where, US will be primary only for US offices, Africa default route only for AF offices and same for the Asia , but the Asia will be the backup for both?

We use different AS# for each locations.

Thanks,

I have this problem too.
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Correct Answer by RAIS AHMAD about 2 years 1 month ago

I agree with Milan that each CE router will receive only one best route advertised by the PE router. I believe that PE should choose the closest route [locally advertised in the region] as the best route should we start advertising default from all three locations, should it not?

Data Centers can use local exit point and rely on MPLS provided backup.

Thanks.

Correct Answer by milan.kulik about 2 years 1 month ago

Hi,

that's not so easy, as each CE router will receive only one best route advertised by the PE router.

IMHO, you need to ask your MPLS provider  to tag the default prefixes received from each data centre (using communities, e.g.) and configure PE routers in each region to choose the proper default route prefix based on those tags.

Or generally: do you need a default route to access the Internet?

Isn't it safer to use a proxy in each data centre?

HTH,

Milan

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RAIS AHMAD Mon, 03/05/2012 - 13:09

You should simply start advertising these defaults into the network. Depending upon BGP attributes, offices will pick the best exit route + provide redundancy automatically.

Rais.

matarasov Mon, 03/05/2012 - 13:22

How can I make sure 172.17.0.0/16 will pick Africa default route, 172.18.0.0/16 will pick Asia and 172.19.0.0/16 will pick US.

RAIS AHMAD Tue, 03/06/2012 - 07:31

As mentioned, you have exit points in each region. A local route will be preferred over distant route. Whether you are using iBGP or IGP in your network, your metrics for each route would change with each hop and should reflect preference for a given default route.

What is making you think it wouldn't work?

Thanks.

Correct Answer
milan.kulik Tue, 03/06/2012 - 08:53

Hi,

that's not so easy, as each CE router will receive only one best route advertised by the PE router.

IMHO, you need to ask your MPLS provider  to tag the default prefixes received from each data centre (using communities, e.g.) and configure PE routers in each region to choose the proper default route prefix based on those tags.

Or generally: do you need a default route to access the Internet?

Isn't it safer to use a proxy in each data centre?

HTH,

Milan

Correct Answer
RAIS AHMAD Tue, 03/06/2012 - 10:24

I agree with Milan that each CE router will receive only one best route advertised by the PE router. I believe that PE should choose the closest route [locally advertised in the region] as the best route should we start advertising default from all three locations, should it not?

Data Centers can use local exit point and rely on MPLS provided backup.

Thanks.

milan.kulik Tue, 03/06/2012 - 23:21

Hi,

I believe that PE should choose the closest route [locally advertised in the region] as the best route

Not necessarilly.

I remember a very similar case when a US site used a default route advertised from London instead of the one advertised from New Jersey :-(

Don't forget PE router is chosing the best path based on BGP attributes.

Which depend on the ISP infrastructure sometimes - the lenght of the AS_PATH might be increased in some regions, e.g.

should we start advertising default from all three locations, should it not?

Well, you can try to advertise some dumb subnet from all data centres as a test.

You would see which path would each remote site use to route to the dumb subnet.

And probably the same path would be chosen for the default route when advertised.

But without the provider involved you are still not sure it will not change with some possible infrastructure change.

And also the second original request

US will be primary only for US offices, Africa default route only for AF offices and same for the Asia , but the Asia will be the backup for both.

can't be fulfilled without the provider involved, I'm afraid.

BR,

Milan

matarasov Tue, 03/06/2012 - 13:49

Thanks, it makes sense to involve the ISP, just want to know if we could avoid that or if anyone did that already.

Proxy wont work for our setup.

Thanks,

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Posted March 5, 2012 at 12:01 PM
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