Multihoming, BGP - 1 or 2 routers? Pros? Cons?

[brettp]

So, I've been tasked with re-designing our internet connections for "automatic failover." To me, it's a daunting project and I came up with two designs. In theory, it easy, but when considering nuances of the network -- VPNs, ppp links, etc. it turns into a headache. One option is to have both providers come into one active router (using SLA to monitor some internet IP and remove/add default route. I would also have a second standby router in this case.) Or, have one provider come into one router and the other into another, using HSRP on the "inside" interface to switch over when one ISP goes down (using SLA to monitor some internet IP.) Load balancing is not important in any case. Are there any pros or cons for either of these designs? I know there's no answer set in stone, but I'm wondering if any one has anything to add from past experience. I have zero experience.

[brettp]

Makes sense… Thank you for the information! Seems like my headache is getting bigger.

[Richard Burts]

One of the possible issues in advertising your block of provider supplied IP addresses to the other provider is the size of the address block. Many ISP will not accept an advertisement smaller than /24. How big is your block of addresses?

HTH

Rick

[brettp]

We’ve got a /26 from the provider… But this is turning into a bigger headache because our provider has been bought out by Verizon (won’t get into that, haha.)

[Richard Burts]

It does seem like your headache has multiple dimensions. I can speak to the one about the size of the address block. With a /26 assigned to you advertising it to the second ISP is a problem. There might be ways of handling that, depending on whether you have resources in your network that need to be accessible from the Internet (traffic initiated from the Internet coming into your network), or whether all of your traffic is initiated from inside your network and going to the Internet.

I can not speak to the other dimensions of your headache.

HTH

Rick

[brettp]

I have included a screenshot of the plan I came up with (but it does not include current VPNs or future point-to-point links planned.) This was my idea for this project... but again... I have no experience with this. As you can see, I was already going the 1 ISP per router route. The only point of BGP will be for us to advertise our prefix. No floating default route... I'd be using SLA and HSRP to swap routers to fail over to the other standby ISP link. I didn't want to make this a conversation about my idea per say... I was more interested in if two ISP on one router, or 1 ISP on two routers was better. SLA would track an internet IP and would drop the HSRP priority failing over to the other router/ISP.

[Jon Marshall]

I would agree with Rick, one ISP connection per router keeps it more simple. 

Whether or not you need to run IBGP depends on whether you want to manipulate traffic paths etc. 

If you were not running IP SLA then yes you would need IBGP but if you use IP SLA to switch across you may not need to exchange routes between routers if you are just getting a default from both ISPs. 

Difficult to say without the full picture. 

Jon

[Jon Marshall]

Rick just to add to this thread. 

The reason you may want to use IP SLA with BGP is if the default route the provider is sending is generated on the EBGP peer without checking network connectivity to the rest of the provider network which means you can still have a default route on the HSRP active router even if you are isolated from the rest of the provider network. 

I have come across this and the provider answer was to receive full internet routing tables but sometimes you only want the default route. 

One possible solution is to setup IP SLA  and ping an IP within the provider network (beyond the EBGP peer) to ensure you had connectivity and if you didn't switch HSRP across to another router connected to another ISP. 

This was a well known provider as well. 

Jon

[a.alekseev]

as you said
>In theory, it easy, but when considering nuances of the network

What are your nuances?
single point of failure is the main reason for two.

if it's not your case then in most cases you can combine two routers in one.