Redundant internet connection

[Thura Htoon]

Hello CLN,

I have questions about redundant internet setup connection.  I am sure some of you have that kind of setup already.

IP SLA and BGP come into my mind when I think of redundant internet connection.

I prefer IP SLA over BGP as it is so simple to set it up. However, I am not sure the impact on inbound communication and NAT (both static and dynamic) when failover occurs with IP SLA.  How do you guys tackle inbound traffic especially exchange emails and static NAT mappings?

When should we use BGP? Is BGP the answer to redundant internet connection? I am just looking for active/ standby internet connection. No plan for load sharing between 2 ISP circuits.

Can you share your experience and thoughts on this subject?

Thank you.

[Pawan Raut]

Are you using ISP Public subnet for NAT or it is own by you?

what is the Public subnet you have at site like /24

[Thura Htoon]

Hello Pawan,

I own the public /24 block . I am currently finding a second ISP for back up connection. 

Thank you. 

[Pawan Raut]

It is Pretty simple.

1) Advertised your /24 Public IP subnet to both ISP using BGP.

2) Create route-map and set higher local preference than default 100 and apply on Primary ISP BGP neighbor at inbound direction so that route receiving from Primary ISP will Prefer.

3) Create route-map and set as prepend and apply on secondary ISP BGP neighbor at outbound direction so that route can advertised from both ISP but from secondary ISP it is with longer AS path hence internet world would prefer Primary ISP path.

Please let me know if you need any help for exact configuration.

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[Thura Htoon]

Hello Pawan,

Thank you for your reply. I understand your explanations and i think i can handle that.

I am not quite clear on how BGP handles inbound traffic. Here is my understanding,

When i advertise my public network ( for example, 20.20.20.x/24) network to both ISP via BGP, both isp has a route back to our domain 20.20.20.x/24. And i manipulate the routes in a way that ISP 1 is prefered.  Now, ISP 1 fails and ISP 2 still knows how to get back to 20.20.20.x/24. Am i correct?

So, now i have a website of www.mysite.com that is mapped to 20.20.20.20. The rest of the internet will still recognize 20.20.20.20 and forward it to ISP2. Then ISP2 forwards it to us. Am i correct?

Is that how it works?

Thank you again. I want to have a clear picture. 

[Pawan Raut]

Yes you are right. Both your ISP know how to route the traffic for your own Public IP subnet both ISP advertised your Public subnet to Internet world but on the basis of BGP attribute (AS Path) ISP1 is preferred but ISP2 two is also consideration as backup path incase ISP1 path not available to internet world.

[Thura Htoon]

Hello there,

I have another question for you. As you know, we own /24 public subnet. We want to run redundant internet setup. 

Do i need to have a public ASN? Do i need 2 seperate ASN for each ISP?

Thank you. 

[Pawan Raut · Accepted solution]

No you can use one Public ASN for redundant setup.

[Thura Htoon]

Thank you Pawan.

[Thura Htoon]

Hello Pawan and Nitesh,

I have anther question on multihoming. With the help of BGP , i can advertise our routes to internet and both isp 1 and isp1 and the rest of the world know how to route. My question is 

First scenerio,

The public subnet owned by us

www.example.com is mapped to 20.20.20.20 ( Assume that we own the address space and we advertise the space to both isps)

In this case, isp 2 knows how to route to 20.20.20.20 and our domain name and url does not change. 

Second scenerio,

We do not own the subnet. www.example.com is still mapped to 20.20.20.20 which is owned by isp1 and now isp1 is down. 

In that case, how would a user be able to connect to www.example.com when the address 20.20.20.20 is not accessible. 

We may have bgp running and the rest of the world know www.example.com. How would www.example.com be resolved. 

I am confused. Please shed some lights. 

Thank you. 

[nitesh.tomar1]

Hi ,

  First thing if you are going to take the redundant internet link from the same ISP then you can go in the way Pawan has mention and can use local preference for the outgoing traffic and AS prepend for the incoming traffic from the internet and you will get the optimal result

But if you are going to use the redundant link from the other ISP then I would suggest you to use the conditional advertisement or summarization for the incoming traffic from the internet as you can't be sure how much value you can use to prepend the traffic. For outgoing you can use the Local preference.

 Please let me know if any further help is required.

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