BGP peering with two ISPs

[Richard Dumag]

We have a dual homed internet connections with Zayo and Comcast.  Zayo is the primary internet access using /24 IP address range they had assigned.  

We would like to setup BGP peering with Zayo and Comcast and have Comcast route the /24 IP address range should Zayo have an outage.

Zayo and Comcast will be enabling BGP on the circuits.

Besides setting up BGP with our ARIN assigned ASN on our router and advertising the /24 IP address range, what additional configuration do we need to do on the router enable for this to be successful.

Any information or comment you can provide will be greatly appreciated.  Thank you!

Richard

[Joseph W. Doherty]

Would suggest you confirm with Zayo and Comcast how this all should work and their BGP peering expectations with you.

One side effect, if this is done as Giuseppe describes, you may not need to use conditional BGP advertisment.  If all traffic will transit Zayo, they can prefer the direct connection to you.

Your egress may prefer the path directly via Zayo if it is one less AS hop.

[Richard Dumag]

Hi Joseph.  Definitely agree with you.  We are still in the planning stages with a lot of discussions with Zayo and Comcast to make sure we sure we have all the information and configurations needed to make this all work properly and seamlessly.  We will probably hire a consultant to assist with the implementation.

Richard

[paul driver]

Hello
You dont mention anything regards running any IGP internally, if you are then you would need to adverse that default being received from the ISPs into it unless that is you statically assign one

Review attached file for possible solution.,

[Richard Dumag]

Thank you Paul for your response.  We have IGP internally but don't plan on advertising that.  This is just for internet access only.  So if Zayo goes down, the /24 traffic will reroute to Comcast so users in the office or vpn can access the internet and our online services remain up.  The traffic will route back to Zayo when it's back up.

Richard

[paul driver]

Hello

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@Richard Dumag wrote:

Thank you Paul for your response.  We have IGP internally but don't plan on advertising that.  This is just for internet access only. 

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Then attached file should be applicable, obviously you'll need to apply the nat domains (outside/inside) to their relative interfaces

[Richard Dumag]

Thanks again Paul.  I'll review the file and also setup a lab in GNS3 and test.

[paul driver · Accepted solution]

Hello @Richard Dumag 
Reading the very good responses from @Richard Burts  and @Joseph W. Doherty I have amended the previous attach CFG file to include a bgp conditional advertisement example based on a failure to ISP ZAYO  and if that condition is met and true your rtr will ONLY then advertise a specific prefix to the COMCAST ISP.

[Richard Dumag]

Thank you Paul!  I will review the file and do some testing once I have lab ready.

Richard

[Richard Dumag]

Hello everyone!  Just a quick update on this project.  So Zayo will change the ASN of the /24 from theirs to our ASN.  They will also handle the routing arbitration.

I had setup a lab using GNS3 and used Paul Driver's suggested solution and it works.  I did not use the NAT commands however since we NAT at the firewall (PaloAlto).  I tried different scenarios by shutting down interfaces on the Zayo router and the ping traffic was redirected to the Comcast router.

I think we are good to go.  The next step is to check if the layer3 device (Cisco 3850) we currently use will have enough resources to handle BGP routing.  We will be replacing it with Cisco 9300 but have not scheduled yet since we have until the end of 2025 before the 3850 is EOS.

Thank you everyone for the responses, comments, and suggested solution!  I will provide another update once we implement this in production.

[Joseph W. Doherty]

Thanks for the update.  Always nice to know how things panned out - especially if it was for the good!

I wouldn't expect a 3850 having any BGP issues if you're only taking (optionally) a BGP default route and advertising just the one /24.

[Richard Dumag]

Joseph, that's what I was thinking too since we are not an ISP and only advertising one /24.

[Richard Dumag]

I do have one more question.  Since we're only advertising one /24, what are the pros and cons of using full route vs. partial route?  I was asked by Comcast about this (if we would like full or partial routing).  And Zayo being the primary have not mentioned or ask me about this.  Appreciate any information anyone can provide.  Thank you.

Richard

[Joseph W. Doherty]

As you (currently, I believe) plan to use only the Zayo link unless it fails, then Comcast.  You have no real need for any routing information from either ISP.

If you were going to use both paths, concurrently, taking routes from the ISPs allow your router to take the "best" path.

As there are so many Internet routes, which makes for a very large route table, often partial routes are taken.  Then the question becomes what to take, which usually depends on the number of AS hops from you.  Minimally, you might take just your ISP AS routes, or those and the next AS hop away, i.e. those with a connection directly connected to one of your ISPs.  You can include further AS hops, but the benefit is often minor compared to the required number of routes you need to take.

Also, in your case, if Comcast only provides a transit back to Zayo, there's probably little to no benefit.

Lastly, LAN switches, like a 3850 or 9300 generally don't have the resources to take huge routing tables.

What you might do, is ask Comcast how many IPv4 and IPv6 routes would be from just their ASN or 1 additional AS hop away.

[Richard Burts]

Richard

I think that you would want only default route for several reasons:

- especially if you will be running BGP on 3850 you want to be careful about the load on the switch. Only default reduces the load.

- the main reason to receive partial routes or full routes is if you want to forward some traffic to one ISP and forward other traffic to the other ISP. My understanding of your post is that you are not wanting to do that but just to have a backup. Default only is sufficient for that and receiving partial routes or full routes increases the complexity of your routing decisions.