ISP only provided one IP, redundancy question

[jamesallen36]

Hi all,

We are installing a new fiber circuit from Comcast which they are delivering to us with one ethernet connection. Initially, I was under the impression that they were giving us two public addresses so I could put one on each of our routers for redundancy configuration. However, I now found out that they are giving us one one public IP for a router so I am not seeing how I can configure our routers in a redundant way.

To be clear, we are getting more IPs for actual addressing for services which are in a seperate network...I am only talking about the external interface IPs within the ISP routing range. Does this make sense?

Also, we will be putting in another ISP for redundany internet connection later so I guess we could have one router setup to one ISP and the second router setup to the secondary ISP and then configure failover only if the primary goes down.

What does everyone think? Should I ask comcast for another IP or is there a way to get this to work? Maybe a layer 3 switch in the front can assume that one IP and then pass the traffic back to the two routers then using IPs from the customer usable pool?

[jamesallen36]

I think I am onto something. I was watching a video on youtube regarding BGP configuration with peering setup between the two routers. So in theory, I would configure one router to one ISP and then other to the other ISP and then the BGP would route traffic out as desired via configuration. The single IP Comcast gave me would reside on the one router, an IP from the other router would reside on the second router and then the peering interface between the routers would be on a different network.

Then my customer usable IPs would be configured on my firewalls on the backend which I would advertise via BGP through both ISP connections.

Does this sound pretty solid or more like the ramblings of a madman?

[Reza Sharifi · Accepted solution]

For all these connections you need 3 /30 subnets 2 to connect to your ISPs and one between your routers. In this case r5 and r7 are ISP routers. You can use one of you own subnet for the connection between your routers. Peering between you and the ISP should be EBGP and Peering  between your routers should be IBGP.

[jamesallen36]

Thank you so much for your response Reza. So just to be clear, R5 and R7 are ISP routers which can be from two different providers entirely. I guess the answer to that question would also answer my next question also which would be...the IPs to the seperate ISP routers R5 and R7 would not need to be reside in the same network then, correct?

This diagram is very helpful in grasping the concept behind this.

[Reza Sharifi]

James,

That is correct.  You will need 2 different /30 subnets, one for each ISP.

Good luck,

Reza