BGP - Two ISP failover

[jeffkim.cisco]

f I want to set up Two ISP for a company with Full BGP set up.....

ISP A - 60.1.1.0/24

ISP B - 120.0.0.0/24

Do I need work with ISP B and inform or ask them to service 60.1.1.0/24 network?

If ISP A goes down.....how does ISP B will allow 60.1.1.0 network to pass through their network???

Or I don't need to contact ISP at all and just configure iBGP internally?

I am wondering how ISP B will service 60.1.1.0/24 IP address block although it is not their IP address block.

[Richard Burts]

Normally ISP B would not forward traffic for the IP addresses that belong to ISP A. But you can negotiate with them and request that they process those addresses. I have worked with customers who have done this successfully. So it can work.

HTH

Rick

[jeffkim.cisco]

Once the ISP B agrees to forward ISP A address block, do I need to configure anything on my end?

Or ISP B will handle it for me?

[Richard Burts]

When I did this with customers we advertised the address block to the other ISP. But in negotiating with the ISP you could ask if they would statically route it.

HTH

Rick

[jeffkim.cisco]

Please correct me if I am wrong.

I would need to work with both ISP A and B.

I would have ASN from ISP A. Let's say AS# 100.

So I would need to ask ISP B to advertise BGP AS#100 for me.

BGP ASN 100 (60.1.1.0/24) will be advertised to the public by both ISP A and B.

Then, how does others on the public decide to reach my server (60.1.1.100) via ISP A or B?

What if I want, for 60.1.1.100 constantly comes into ISP A and comes into ISP only if ISP A fails?

Thank you very much.

[Richard Burts · Accepted solution]

What we are discussing does get quite complicated.  For most of the questions that you ask the real answer is "it depends". But for your last question there is a real answer. If you want traffic to 60.1.1 to normally come to you using ISP A and only use ISP B when there is a problem with ISP A then you would use BGP conditional advertisement. I configured BGP conditional advertisement for my customer and it worked well. IP packets to the address block of the primary ISP used the primary ISP and not the backup. If there was a problem with the primary ISP then  the conditional advertisement did advertise that address block to ISP B and traffic to those addresses came through ISP B.

HTH

Rick