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Community Member

Asyemmetric Routing

Hi There,

I have 2 internet links each from a different ISP, do you recommend me to connect each link to a separate router on the perimeter, or to the same router by having 2 interfaces in the outside?

What is the pros and cons for each setup if there are any?

If you are with the option of going with one router in the perimter what model of router do you recommend me to go with?

Thanks,

Haitham

6 REPLIES
Purple

Re: Asyemmetric Routing

Hi Haitham,

If money is not an issue, go for two routers. There are obvious advantages to doing so - you eliminate the single point of failure you have when using one router.

However, if you want to go with a single router, some of the factors you need to consider are:

- what sort of interfaces do you require to your ISP

- how much traffic do you expect to push through the router

Hope that helps - pls do remember to rate posts that help.

Paresh

Community Member

Re: Asyemmetric Routing

Hi Paresh,

We are expecting like 16 Mbps from each ISP and each is a serial.. I see your point with the single point of failure, but actually I'm concerned about the complexity of the setup with 2 routers; my concerns here are:

1- Is their a possibility that the traffic that enters the network from one link can go out from the other link?

2- I'm sure with this scenario that I'd need to deploy my FW pair in Active-Active mode, but with PIX, the Active-Active setup is not mature and I'm concerned that it will cause me problems since my understanding that Active-Active in PIX is originally designed to feed separate internal subnets and not the same internal subnet (i.e. in most of the cases you'll receive in PIX errors related to duplicate static and NAT should you have configured the NAT to be the same on the different security contexts).

I'm attaching with this post a draft design for how the network with 2 routers in the perimter and the rest of the components connected together will look like; please have a look and advise taking into account my concerns mentioned above. Please also let me know based on the link BW mentioned what router models you'd recommend me to go with in case I chose to go with 1 router.

Appreciate your feedback.

Regards,

Haitham

Hall of Fame Super Gold

Re: Asyemmetric Routing

Haitham

You are correct that the safety of having two routers and eliminating the single point of failure comes with more complexity. With two routers you will need to configure EBGP on each router to the ISP peer and will need to configure IBGP between your two routers. It will make it slightly more complicated if you want to load balance traffic (how do you get traffic from the internal routers to the "right" external router for the particular destination). But I agree with Paresh that if the additional cost of two routers is not an issue that the design with two routers is probably better.

1) There is certainly a possibility that traffic could come in one link and go out the other link. This is a possibility whether you have a single router or have two routers.

2) The firewall issue can get more complicated when there are two routers, so you are correct here.

HTH

Rick

Community Member

Re: Asyemmetric Routing

Thanks Rick, your post was really good to me.

In case I chose to go with one router, what router model do you recommend me to go with?

Thanks,

Haitham

Re: Asyemmetric Routing

Your design scenario with 2 permiter routers with 2 firewalls is not an uncommon setup. You can load balance traffic out by running OSPF on your PIX and permiter routers.

Perimeter routers do not have to be high end routers. One thing you need to make sure is, if you are running BGP and learning full/partial internet routing table, you should have enough memory to handle the routes.

Assuming you are using BGP, you have many options to influence traffic coming into your network and that depends on whether you want incoming traffic load balanced or use one as a primary ISP and the 2nd one as redundant ISP.

Pls. rate all helpful posts.

HTH,

Sundar

Community Member

Re: Asyemmetric Routing

Thnaks Sundar for you have added, is their any similar articles in Cisco or others that talk about similar setup and gives configuration example to my case?

If you have something like that, it would be exteremely helpful to me.

Thanks,

Haitham

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