BGP multihoming with a class C range

[smith.dean]

Hi all. I am looking to implement a BGP configuration across 2 seperate ISPs with a portable class C range across 2 seperate sites. There would be 4 routers and internet feeds all up comprising of :

Site 1 Router 1 - Connection to ISP A to be the primary route for traffic type 1 (a 28 bit subnet of the Class C public range)

Site 1 Router 2 - Connection to ISP B to be the primary route for traffic type 2 (a 28 bit subnet of the Class C public range)

Site 2 Router 1 - Connection to ISP A to be the backup route for traffic type 2 (a 28 bit subnet of the Class C public range)

Site 2 Router 2 - Connection to ISP B to be the backup route for traffic type 1 (a 28 bit subnet of the Class C public range)

I have a grasp of how I can determine the path out of our AS via local preference settings but I'm trying to get my head around how I can influence 2 ISPs on paths inbound. My understanding is that they will not advertise anything less than a class C range and therefore I would not be able to influence which 28 bit subnet of the Class C range gets routed to which site. Is it possible ISP public communities would offer that granularity ? Looking for clarification and suggestions ! Thanks in advance.

[pkhatri]

Hi,

In such a case, one option is to use the MED parameter so that you get a bit of a split of traffic, at least from each ISP. The fact that you will not be able to split the /24 further means that your options are indeed quite limited in the inbound direction.

If you can negotiate with your ISPs to at least accept /28 from you but still advertise just the /24 out to the Internet, you can advertise the /28s with the NO-EXPORT community and that will get you what you want.

Hope that helps - pls rate the post if it does.

Paresh

[smith.dean]

Thanks Paresh. I have come to the conclusion that the only real viable solution would be to get another Class C range and split the traffic types across both of those. I'm not too sure how easy it is to get another Class C range but this will be where I focus my efforts next ! I appreciate the feedback though. I agree MED is definetly viable for /24 masks and less but "negotiating" with carriers may be harder than getting a new range. At least you have given me a fall back plan. Thanks

[vladrac-ccna]

Hello Dean,

I'm not sure I understood it quite well, but for the question:

I'm trying to get my head around how I can influence 2 ISPs on paths inbound

You could you 2 features:

MED

and AS-PREPEND

the "problem"with MED is that is best for using to compare routes on a single AS, so you be multi-homing to a single provider.

If you preprend AS on the routes you advertise to these ISP you can make them more or less desirable.

check the following link:

http://www.cisco.com/univercd/cc/td/doc/product/software/ios124/124cg/hirp_c/ch05/brbext.htm#wp1027195

Also for BGP desing I would suggest Halabi book, quite excelent reading.

Hope this will help.

Vlad.

[smith.dean]

Thanks Vlad. My problem here is in applying MED's or AS-path prepending on only a subset of the /24 bit range. I don't think ISPs are likely to act on them and route one /28 range one way and another /28 another way. I'm going to persue getting another /24 range and split the traffic over both /24, which will then satisfy the /24 mask requirements of the ISP's in their BGP implementations. Appreciate your feedback Vlad. Thanks

[vladrac-ccna]

So, does it mean that you are advertising a /24 out?

because this can be done per route out?

if you are advertising /28 then each can be altered.

Vlad

[pgalligan]

We just got ourselves another C class when in the same situation....

[smith.dean]

Thanks. Think I'll be following suit. Appreciate the feedback.