Cisco Support Community
Community Member

BGP with two ISP's.

I am new to BGP and have a question about redundancy.

It's my understanding that I can get a T1 from two different ISP's and setup BGP to propagate a route to my network to both ISP's core routers.

Hopefully I am correct so far?

My question is our current public IP block was assigned to us by Sprint. If I get a backup T1 through AT&T, can I propagate a route to my Sprint IP block to AT&T's routers so it comes in on the AT&T circuit?

We recently had a Sprint outage and realized we depend to heavily on a single carrier. We want our website to have redundancy via another carrier.

Another question I have is, assuming the above works, should my Sprint T1 go down, how long does it typically take for the core routers to realize it needs to go to AT&T to get to me?




Re: BGP with two ISP's.

If you are multihoming (that means BGP peering to two or more upstream ISP), you need to have your own AS and IP Address block

Here's a nice BGP PPT from Freedman

Hall of Fame Super Blue

Re: BGP with two ISP's.


Without knowing your addressing it's difficult to say but the main problem is that the block of IP addresses assigned to you by Sprint will be part of a bigger block of addressing that Sprint owns.

So Sprint will not be advertising your individual addressing to the rest of the internet rather they will advertise their aggregate block of which you addressing is part.

Even if you could get AT&T to agree to advertise out your Sprint addressing, and they may not as ISP have restrictions on this, your addressing would be advertised out by AT&T as your individual block because it's not part of their aggregate block.

So now all traffic coming to you will go via AT&T because they are advertising a more specific route to your networks.

If you could persuade Sprint to also advertise out your individual block of addressing then you would be okay but ISP's are generally very unwilling to "punch" holes in their aggregate addressing for individual customers.


Community Member

Re: BGP with two ISP's.

Hi Jon,

I've always had blocks assigned to us by ISP's. Where does one go to get a block assigned to a company, ARIN?

Is my thinking at least correct for Internet redundancy by advertising our network via BGP to our ISP's? Is there a different way to do this?

Sprint did assign us a full class c network several years ago, but I see what you are saying about Sprint advertising an even larger block.

Thank you,


Community Member

Re: BGP with two ISP's.


Just went through this. You need to contact ARIN, and get a minimum of a Class C from your ISP to do what you want to do.

The above link has a walk through.

First you get a ORG ID, and then so on.

Once you get an ASN you can talk to both Providers and tell them you want to run BGP and advertise your block over two ISP's. That is where the coordination comes in.

You need to justify why you need a full class C also.

I assume you are talking about incoming redundancy as the primary solution?

Bottom line is it can be done, but takes work with the ISP's and configuring BGP on each router (ideally you have two).

If you search these forums there are a lot of other conversations on this topic. I just posted a question today about getting both Class C blocks to correctly advertise as that is the step I am on.

Hope this helps.

Community Member

Re: BGP with two ISP's.


Thank you so much for this link. To clarify something, did you get this class C from ARIN or one of your two ISP's?

You are correct about incoming redundancy to both our Website and other Internet applications.

Community Member

Re: BGP with two ISP's.


You have the get your Class C from you ISP.

They require you to justify the class C but filling out forms and showing that you will use it. You also have to do some testing to make sure the traffic doesn't come in the other link (not sure if this would cause problems for you) because I have learned that some providers might actually have a shorter route through the second ISP. For me the prepending solution seems to work, though I am still working out the kinks. Hope this helps. I can post here or contact you with other information, as I am in the same process, just a few steps ahead of you. :)

Please rate if possible.

Community Member

Re: BGP with two ISP's.

I guess I'm in luck considering I already have a full class C from Sprint that was issued to us several years ago.

Sounds like you are running into the issue Jon mentioned earlier in this thread with aggregate routing protocol updates causing traffic to come in over the other ISP's link.

Thank you for the link to the ARIN site. I'm off to reading about AS numbers.


CreatePlease to create content