IP Subnetting & BGP Announcement

Unanswered Question
Apr 18th, 2012


We have our own /24 which we want to subnet into smaller blocks i.e. /25 /26's at the same time I want to still announce our /24 to our bgp providers.

Can anyone provide me with some links or documentations to how I can do this please?

the end result I want to take our /24 turn it into smaller IP blocks and then put them into different public vlans.

Kind Regards

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Richard Burts Wed, 04/18/2012 - 11:41

I am not clear whether you intend to use the /24 to create various subnets within your network (this works quite well and you still just advertise the /24 to your BGP upstream) or whether you want to divide the /24 into various subnets which will appear at different places in the Internet. This last alternative is problematic. Can you clarify which it is that you want to do?



veils1de123 Wed, 04/18/2012 - 14:10


thanks for your comments.

To give you a better understanding we have two switches running advanced ip services with 2gb etherchannel running between both switches, isp 1 lands onto switch 1 and isp 2 lands on switch 2 creating a redundant network.

we have a single /24 running which all we want to do is create smaller subnets within our network and still manage to be able to advertise our /24 to both isp's.

hope that helps.

Richard Burts Wed, 04/18/2012 - 14:25

The additional explanation is helpful. You should be able to subnet your /24 in whatever way you want on your switches. You would run BGP with each ISP and would advertise your /24 to each ISP.



veils1de123 Thu, 04/19/2012 - 03:13

Due to the fact we're using two cisco 3750 switches running on advanced ip services, should i be looking at SVI?

Hitesh Vinzoda Thu, 04/19/2012 - 04:57


Create a /24 route on the switches and point them to null0: with AD 254 and then using a network statement under BGP process advertise this route to your ISP's. This will do the job.

ip route x.y.z.0 null0: 254

router bgp xxx

network x.y.z.0 mask


Hitesh Vinzoda

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