BGP Multihomed with single /24

Unanswered Question
Mar 14th, 2012

If having a single /24 allocated address space, can i split this into smaller subnets so that half of the subnet preferred to ISP1 and half to ISP2 as the primary path respectively? and should there will be link failure, convergence routes via iBGP peer towards the peering ISP?

I have this problem too.
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andrew.prince@m... Wed, 03/14/2012 - 15:13

why do you want to split it???? I have 4 data centres each advertise a single /24 to dual independent isp's works well for me, :o)

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JosephDoherty Wed, 03/14/2012 - 17:28

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For outbound, if your devices support it, have you examined OER/PfR?

darren.g Wed, 03/14/2012 - 18:47

Gerard Gacusan wrote:

If having a single /24 allocated address space, can i split this into smaller subnets so that half of the subnet preferred to ISP1 and half to ISP2 as the primary path respectively? and should there will be link failure, convergence routes via iBGP peer towards the peering ISP?

Most ISP's will not accept anything smaller than a /24 in advertisement - so no, you won't be able to advertise say a /25 out one ISP and another /25 out the other.

However, if you advertise the whole /24 to both uplinks at the same precedence, and without any prepending, then traffic will come in via the 'best" path from its originating point. That's about all you can do with incoming data, really - even if you prepend and give one link a much higher precedence, there will always be *some* traffic which will come in over the second link simply because the "path" to your terminating point is shorter.

If you want to control how your *outbound* data flows, then as Joseph has suggested, you should look into OER/PfR - I've explored this recently for a similar situation, and will be implementing it as soon as I can get a change window approved.

Gerard Gacusan Wed, 03/14/2012 - 18:56

yeah i agree ... i tried prepending and bgp looking glass tells me the whole /24.

i advertised /24 on both ISP's and i have no problem with that.

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Posted March 14, 2012 at 11:09 AM
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