BGP default / default + backbone options for redundancy

[MauryJ]

Hello all,

I've read several threads on the various routing options available when using BGP, but, am still not clear on which option available to us would allow us to do what we're looking for, with the equipment that we have.

We're currently using a 2821 router with 256MB of memory, and have just one ISP now.  We're looking to get service from a second ISP, who will be providing a /24 block of IP's to use for BGP.   Our goal is to allow for redundant connectivity from the internet, to our hosted services.  However, there are some inbound (and outbound) connections services that we would prefer to use one ISP over the other.

Given our router's specs, it doesn't look like we can handle the full routing table.   But if we were to just receive the default routes, would this be sufficient for what we're trying to accomplish?

The full options that we have are:

Default Route

Default & Backbone Routes

Default & Full Routes

Default Route

Full Routes

ISP Backbone Routes

(I just noticed that we can upgrade this router to 1GB RAM with 2x 512MB DIMMs)

Thanks for any insight!

Message was edited by: Maurice Jeter

[amabdelh · Accepted solution]

Yes I beleive the default route is very good option, you will need to use more specific routes if you want to prefer one path over the other for specific destiniation or you can play with your config.

before you lease the /24 subent, check with the two ISPs if they can advertize this subnet, some ISPs put restricitons, an example some ISPs dont advertise less than /22. You may need to use a private AS, check wth the two AS if they are willing to do this, if not possible then you will need to get a public AS but I think no need for that.

regards

[MauryJ]

Thanks Amjad, this is very helpful!

[rfalconer.sffcu]

Something that I've done in the past that works to balance outbound traffic better than just defaults is to accept full routes from the ISPs and then write a regular expression to permit only routes with an AS path of 4 or less into the routing table. The thought behind that is that if it's longer than 4 AS paths away, then it doesn't matter which link it will take.

This helped quite a bit with balancing our outbound traffic.

Have you thought about a 2nd router to increase the redundancy?

Amjad makes a very important point about the IP addresses you use. You may want to apply for your own public space so that you can definitely have them advertised through both/any ISPs you use. It also makes portability to a new provider much simpler if/when you ever change providers in the future. If you have a lot of public IP addresses, it can be challenging to migrate to new IP space if you need to.

In my experience, a /24 or less is all an ISP will advertise.

[MauryJ]

Hello Robert,

That does sound like an ideal setup (for balancing outbound traffic).  My only concern with that, is the CPU power of our internet router -- a 2821.  I don't think that it would be able to handle full routes from both of our ISP's.   It may be time for us to start looking at newer routers though.

We do have a second 2821 that I've thought about setting up for redundancy -- thank you for the tip.

We originally did plan to get an independent /24 block, but, we decided in the end to get it from one of our two ISP's instead.  

Thanks!